Dimensions of Arousal Across Four Brain Types

How arousal actually behaves in the Neurotypical, ADHD, Autistic and AuDHD nervous system

How to read this

There is one arousal-performance curve — the inverted-U (Yerkes-Dodson) — and every brain lives on it. What differs is where each brain sits by default and how it moves along the curve. These 16 dimensions are the distinct axes on which that difference shows up: the hardware (baseline, reactivity, the autonomic accelerator and brake), the drive system (reward, boredom, novelty, movement), the senses (overload, filtering), attention under arousal (vigilance, hyperfocus, state-dependence), the inner and emotional layers (interoception, emotion), and the long-run cost (chronic load and burnout).

Two honesty rules run throughout. ADHD arousal is dysregulation, not a fixed low battery (autonomic studies split ~44% hypo / ~43% null / ~13% hyper). Autism arousal is heterogeneous — the over-aroused, sensory-overloaded profile is the most common and clinically visible, but genuine hypo-aroused and sensory-seeking subgroups exist. And the AuDHD breakdowns are largely a principled composition of the two literatures plus consistent clinical and lived-experience report — a well-motivated synthesis, not yet an independently measured phenotype. Where a claim is inference rather than settled data, the text says so.

At a glance

A plain-language snapshot of each dimension across the four brain types — read any block on its own. The full evidence-backed breakdown for each follows below.

1. Baseline / tonic arousal — how “switched on” the brain is at rest

• Neurotypical: sits right where it works best — no need to hunt for stimulation or fend it off.

• ADHD: runs under-charged, so it has to pull in stimulation just to reach everyone else’s normal.

• Autism: for sensory input, often runs over-charged and close to overwhelm (but this varies a lot).

• AuDHD: both at once — under-charged for the task, over-charged by the room.

2. Phasic reactivity — how sharply it reacts to something sudden or important

• Neurotypical: clean, well-timed spikes of attention when something matters.

• ADHD: reactions are inconsistent — attention keeps dropping out and having to restart.

• Autism: reacts to the “wrong” things — some events hit too hard, others don’t register.

• AuDHD: unreliable and idiosyncratic — misses the cue, then locks onto something random.

3. The “accelerator” — the fight-or-flight (sympathetic) system

• Neurotypical: presses when needed, eases off when the moment passes.

• ADHD: a weak accelerator — measurably under-fires, so effortful tasks feel flat.

• Autism: mixed — one subgroup runs hot at rest, others under-respond.

• AuDHD: the weak ADHD accelerator wins out — under-powered for the task.

4. The “brake” — the calming (parasympathetic) system

• Neurotypical: a flexible brake — calms down and recovers reliably after stress.

• ADHD: a weak brake too — slower to settle once wound up.

• Autism: the brake lets go under pressure, right when it’s needed most.

• AuDHD: weak accelerator and weak brake — hard to get going, hard to calm down.

5. Reward — does a promised payoff pull as hard as one in hand?

• Neurotypical: a future reward feels motivating now, so waiting is bearable.

• ADHD: a promised reward barely registers; only the reward right now lands — hence deadlines.

• Autism: social rewards (praise, a smile) pull less; personal interests pull hard.

• AuDHD: neither the future nor social approval motivates — only immediate, personal interest does.

6. Boredom — how much it hurts to have too little going on

• Neurotypical: dull, but tolerable — you can just wait.

• ADHD: genuinely painful — understimulation feels like an emergency to escape.

• Autism: often a relief (quiet = safe), though some actively seek input.

• AuDHD: bored and needing quiet at the same time — the cruel double bind.

7. Novelty vs. routine — appetite for the new or the familiar

• Neurotypical: comfortable with both, switches easily.

• ADHD: chases novelty — the new thing supplies the arousal a boring task can’t.

• Autism: needs sameness and predictability — surprises are draining and aversive.

• AuDHD: craves novelty and needs predictability — pulled in both directions.

8. Movement & self-stimulation — fidgeting, pacing, stimming

• Neurotypical: minor and dispensable — easy to sit still.

• ADHD: fidgeting is self-medication — movement raises arousal to help focus.

• Autism: stimming is self-soothing — it discharges or calms an overloaded system.

• AuDHD: uses both at once — so “sit still” removes two coping tools at the same time.

9. Sensory overload — how fast an ordinary room becomes “too much”

• Neurotypical: lots of headroom — everyday places sit well below overwhelm.

• ADHD: usually the opposite problem (under-stimulated); overwhelm is mostly poor filtering.

• Autism: a low threshold — ordinary light, noise and texture can already be unbearable.

• AuDHD: overloads fast, yet is still craving stimulation — a very narrow comfortable band.

10. Filtering — tuning out background and repeated noise

• Neurotypical: filters both effortlessly; the hum and the clothing tag fade in seconds.

• ADHD: a leaky filter — irrelevant things keep grabbing attention (distractibility).

• Autism: things never fade — a repeated noise stays at full volume all day.

• AuDHD: the least-filtered of all — can’t tune out the new or switch off the constant.

11. Staying focused over time — how fast attention drains

• Neurotypical: drains slowly and recovers with a short break.

• ADHD: drains fast — attention slips early, especially on dull tasks.

• Autism: deep and lasting on an interest; drops off quickly on imposed, boring tasks.

• AuDHD: brilliant on the interesting, collapses fast on the dull — little in between.

12. Hyperfocus & flow — locking on, and being able to let go

• Neurotypical: can get absorbed but still notices hunger, time, and stops when it should.

• ADHD: hyperfocus overrides the “stop” signal — blows past everything, then crashes.

• Autism: deep, sustained focus on a special interest; being interrupted is costly.

• AuDHD: locked in hard by focus, then thrown out violently by a sensory overload.

13. Conditions — how much output depends on urgency, stakes and noise

• Neurotypical: steady — works fine whether the task is urgent or dull.

• ADHD: hugely condition-dependent — urgency, stakes, even background noise switch it on.

• Autism: depends on predictability and low sensory load, not on pressure.

• AuDHD: needs excitement and calm at once — no single environment satisfies both.

14. Interoception — reading your own body’s signals (hunger, tension, overload)

• Neurotypical: a fairly accurate internal dashboard — notices and adjusts early.

• ADHD: a poor dashboard — misses hunger and fatigue until it crashes.

• Autism: miscalibrated — signals are missed or overwhelming, and hard to name.

• AuDHD: doubly unreliable — crashes and overloads arrive with almost no warning.

15. Emotions — how intense they get and how fast they settle

• Neurotypical: proportionate, and recovers in good time.

• ADHD: fast, intense, hard to bring back down (incl. sharp pain at rejection).

• Autism: easily triggered and slow to recover — meltdowns (out) or shutdowns (in).

• AuDHD: the most volatile — fast intensity plus slow recovery, stacked together.

16. The long-run cost — running on stress, and burning out

• Neurotypical: stress switches on, does its job, and switches cleanly off.

• ADHD: reaches “normal” by borrowing arousal from stress — a loan repaid as burnout.

• Autism: “autistic burnout” — exhaustion and lost skills from constant coping and masking.

• AuDHD: burns out fastest — it pays both bills at once.

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1. Baseline / tonic arousal — where the brain idles on the curve at rest

How aroused the nervous system is by default, at rest, before any specific event demands a response.

Neurotypical. The Yerkes–Dodson inverted-U remains the organising fact of arousal physiology — performance rises with arousal to a moderate optimum and then declines — and the neurotypical brain, on the whole, idles near that optimum. The locus coeruleus–noradrenaline (LC-NE) system behaves like a well-calibrated thermostat, holding a moderate tonic firing rate that keeps cortex responsive without tipping it into noise. Aston-Jones and Cohen’s adaptive-gain framework describes this as an intermediate tonic mode that leaves ample headroom for task-locked phasic bursts. On EEG-based vigilance measures such as VIGALL, neurotypical adults tend to hold higher, more stable vigilance stages before drifting downward at rest. Autonomic readouts — skin conductance, heart-rate variability — cluster around normative middles rather than at extremes. The upshot is a nervous system that neither has to hunt for stimulation nor defend against it, which is precisely why “typical” set-points are hard to notice until you contrast them with brains that sit elsewhere. The default set-point, not the shape of the curve, is what differs across brain types.

• The inverted-U (Yerkes–Dodson, 1908) is treated as universal; what varies between groups is where the resting idle sits on it.

• A moderate tonic LC-NE mode preserves headroom for crisp phasic responses — the signature of well-regulated attention.

• Neurotypical vigilance regulation on VIGALL is comparatively stable, declining gradually rather than collapsing at rest.

ADHD. ADHD is best characterised as idling below the optimum — a generalised hypo-aroused state (as framed in a 2023 Frontiers in Psychiatry review), with reduced sympathetic activation that becomes most visible during effortful, low-stimulation tasks. On EEG vigilance measures, ADHD is associated with unstable regulation: vigilance drops rapidly to lower-arousal stages at rest rather than being held, which is thought to drive compensatory stimulation-seeking — fidgeting, novelty-chasing, talking — as an attempt to climb back up the curve. This reframes hyperactivity not as excess arousal but as self-administered arousal therapy for an under-driven system, and it fits the paradoxical calming effect of stimulant medication, which raises tonic catecholamine tone toward the optimum. Crucially, the “low battery” picture must be qualified: Bellato and colleagues’ review found autonomic studies split roughly 44% hypo-arousal, 43% null, and 13% hyper-arousal. The honest reading is dysregulation — an unstable, poorly-defended set-point — rather than a fixed low idle. Arousal in ADHD wanders; it does not simply sit low.

• Framed as a generalised hypo-aroused state with reduced sympathetic drive under effortful conditions (Frontiers in Psychiatry, 2023).

• VIGALL-type measures show rapid, unstable descent to low-arousal stages at rest — plausibly the engine of stimulation-seeking.

• Bellato’s ~44% hypo / ~43% null / ~13% hyper split means this is arousal dysregulation, not a uniform low battery.

Autism. Autism frequently idles above the optimum on the sensory channel — the Intense World Theory (Markram & Markram) proposes cortical (particularly local microcircuit) hyperexcitability and hyper-reactivity, such that ordinary sensory input lands as excessive and the system is pushed past its comfortable operating point. This aligns with the lived phenomenology of sensory overwhelm and the drive toward predictable, low-entropy environments as a way of keeping input beneath threshold. But the evidence is genuinely heterogeneous, and it is a mistake to treat “autistic = over-aroused” as a rule. Skin-conductance studies identify both a high-tonic subgroup, consistent with chronic over-arousal, and a low-tonic subgroup that looks under-aroused at rest — sometimes within the same samples. Some of this heterogeneity tracks co-occurring anxiety, alexithymia, and measurement context rather than autism per se. So the fair summary is an elevated sensory set-point in many autistic people, held with wide inter-individual variance rather than as a single characteristic value. Above the optimum for sensory input — but far from uniformly so.

• Intense World Theory posits cortical hyperexcitability, predicting an elevated resting set-point for sensory input.

• Electrodermal work reveals distinct high-tonic (over-aroused) and low-tonic (under-aroused) subgroups — the group is not monolithic.

• Environmental predictability and stimming read as set-point management: keeping input below an easily-breached threshold.

AuDHD. The autism-plus-ADHD brain is not an average of the two but a superimposition — under-aroused on the task/cognitive channel and over-aroused on the sensory channel at the same time. The ADHD component pulls the cognitive idle below the optimum (hypo-arousal, stimulation-seeking, unstable vigilance regulation), while the autistic component pushes the sensory idle above it (hyperexcitability, overwhelm), producing the characteristic double bind of feeling simultaneously bored and bombarded. This channel-split helps explain why single-lever strategies so often fail: raising global arousal to satisfy the cognitive deficit can breach the already-elevated sensory ceiling, while damping sensory input to protect against overwhelm starves the cognitive system further. Direct physiology on AuDHD specifically remains thin — much is inferred from additivity of the two literatures rather than measured in co-occurring samples — so the mechanistic story is more assembled than demonstrated. What co-occurring data exist (e.g. within Bellato’s groupings) are consistent with ADHD-linked autonomic features persisting when autism is also present. The pragmatic model is two set-points on two channels, pathologically far apart. Under-aroused where it needs to engage, over-aroused where it needs to filter — at once.

• Best modelled as channel-specific: cognitive idle below optimum (ADHD), sensory idle above it (autism), concurrently.

• The split predicts why global arousal interventions backfire — the lever that helps one channel harms the other.

• Direct AuDHD physiology is sparse; the picture is largely inferred from additive combination of the two evidence bases.

2. Phasic reactivity — the size of the response to a salient or novel event

How sharply the system spikes above its own baseline when something important or unexpected happens.

Neurotypical. Aston-Jones and Cohen’s adaptive-gain theory distinguishes tonic (slow, background) from phasic (fast, event-locked) LC-NE firing, and pupil dilation is the standard non-invasive readout of the phasic burst. In the neurotypical brain the two modes are cleanly separated: a moderate, quiet tonic baseline provides the contrast against which crisp, strong phasic bursts stand out, yielding sharp “exploitative” attention that locks onto task-relevant targets. Because the baseline is stable, the signal-to-noise of each event-locked response is high — the spike is legible precisely because the background is calm. This is the physiological basis of orienting to novelty, the P300 to oddball stimuli, and the pupil’s reliable dilation to salient events. Phasic responses also carry anticipatory structure, ramping ahead of expected significant events rather than only reacting after them. The neurotypical profile is thus one of well-timed, well-scaled bursts against a low-noise floor. Strong phasic signal on a quiet baseline is what makes attention feel sharp.

• Adaptive-gain theory: tonic vs phasic LC-NE modes, with pupil dilation as the canonical readout of the phasic burst.

• A quiet tonic baseline maximises the salience contrast of each event-locked response — high signal-to-noise orienting.

• Phasic bursts are partly anticipatory, ramping ahead of expected salient events rather than merely reacting.

ADHD. ADHD shows blunted and variable phasic responses, and — most robustly — high moment-to-moment reaction-time variability, one of the field’s most replicated ADHD findings (often modelled via ex-Gaussian tau, the long slow tail of lapses). The intra-individual variability is read as unstable arousal: attention that repeatedly drops out and must be re-recruited, rather than a steady stream punctuated by clean spikes. Anticipatory phasic signalling is weak — the ramp toward expected events is muted — which dovetails with the cognitive-energetic and state-regulation models of ADHD in which effort/activation allocation is the core deficit rather than attention per se. Against an already low and drifting tonic baseline, phasic bursts lose the contrast that would make them effective, so even present responses are less behaviourally sharp. Reward and novelty can transiently rescue phasic responding, which is why high-stimulation or gamified contexts normalise performance. The through-line is inconsistency: not a fixed small response, but an unreliable one. Variability, not a uniformly weak spike, is the ADHD signature.

• Elevated reaction-time variability (ex-Gaussian tau) is among the most replicated ADHD findings — a marker of attentional lapses.

• Anticipatory phasic signals are weak, consistent with cognitive-energetic / state-regulation deficit models.

• Novelty and reward transiently restore phasic responding, explaining context-dependent normalisation of performance.

Autism. Phasic responses in autism are best described as atypical in dynamics and orienting rather than uniformly high or low. ERP and pupillometry studies report altered orienting to novelty and salience — differences in P300 amplitude and latency, atypical pupillary light-reflex and task-evoked dilation dynamics — but the direction is inconsistent across studies and paradigms. A recurring theme is altered temporal profile: responses that habituate abnormally (either failing to habituate to repeated stimuli, or over-habituating) rather than a simple gain change. Predictive-coding accounts frame this as aberrant precision-weighting of prediction error, so that the “surprise” assigned to an event is mis-scaled — some events over-drive the system, others fail to register as salient. Social versus non-social salience is often dissociated, with reduced orienting to social cues alongside preserved or heightened responses to non-social stimuli of interest. The consensus is qualitative difference in the phasic response, not a single scalar shift. Atypical dynamics and orienting — mis-scaled surprise — rather than uniformly more or less.

• ERP/pupillometry show altered orienting and habituation, but the direction varies across studies and paradigms.

• Predictive-coding models cast this as aberrant precision-weighting of prediction error — surprise assigned to the wrong events.

• Social vs non-social salience frequently dissociates, sparing (or heightening) interest-driven responses while dampening social orienting.

AuDHD. In AuDHD the phasic profile is mixed and variable — combining ADHD’s unstable, lapse-prone reactivity with autism’s atypical orienting dynamics, and inheriting the reduced baseline contrast from the ADHD side. One plausible reading is that the ADHD component adds moment-to-moment inconsistency (the drifting baseline erodes phasic salience) while the autistic component adds mis-scaled precision (the wrong events capture the burst), so responding is both unreliable in timing and idiosyncratic in what triggers it. This can look, behaviourally, like attention that neither locks on when it should nor filters out what it shouldn’t — captured by a special-interest stimulus, missing a task-relevant cue. Because the two conditions can push phasic parameters in opposing directions, group-level averages in co-occurring samples may cancel toward null, masking real individual dysregulation. Direct pupillometric or ERP work isolating AuDHD is scarce, so this remains largely an inference from the combined literatures rather than a measured phenotype. The safe characterisation is heightened variance and idiosyncrasy rather than any single direction. Unreliable in timing, idiosyncratic in trigger — an interaction that can average to a deceptive null.

• Modelled as ADHD-type inconsistency layered on autism-type mis-scaled orienting — variable timing and idiosyncratic triggers.

• Opposing pushes on phasic parameters can cancel in group averages, hiding genuine individual-level dysregulation.

• Isolated AuDHD electrophysiology is scarce; the profile is inferred from combining the two literatures.

3. Sympathetic drive — the “accelerator” (electrodermal & cardiac)

How strongly and readily the fight-or-flight branch engages — the physiological accelerator pedal.

Neurotypical. Sympathetic drive is read most cleanly through electrodermal activity — skin conductance is a near-pure sympathetic measure, because eccrine sweat glands are innervated almost exclusively by sympathetic cholinergic fibres with no parasympathetic counterpart — and complemented by cardiac sympathetic indices derived from pre-ejection period or from HRV decomposition. The neurotypical profile is a responsive, well-modulated accelerator: skin-conductance responses appear reliably to salient and arousing events, tonic skin-conductance level tracks task demand, and cardiac sympathetic output scales up under challenge and settles afterwards. The key property is proportionality — the accelerator is pressed in relation to demand and released when demand passes, keeping arousal matched to the situation. This modulation, rather than raw magnitude, is what marks typical sympathetic regulation. It provides the mobilising energy for phasic attention and effortful engagement without running the system hot. A responsive, proportionate accelerator — pressed to match demand and released when it passes.

• Skin conductance is effectively a pure sympathetic readout — eccrine glands have no parasympathetic innervation.

• Cardiac sympathetic drive (pre-ejection period / sympathetic index) scales up under challenge and recovers afterwards.

• The neurotypical hallmark is proportional modulation, not raw magnitude — arousal matched to demand.

ADHD. ADHD is associated with a weak accelerator: reduced sympathetic drive on both electrodermal and cardiac measures. Bellato and colleagues reported a significantly lower cardiac sympathetic index in ADHD than in non-ADHD children — F(1,69) = 8.687, p = 0.004 — and the electrodermal literature broadly reports lower tonic skin-conductance level and fewer or smaller skin-conductance responses, i.e. an under-driven sympathetic system. This under-activation is most pronounced under effortful, low-arousal task conditions, the very situations that demand mobilised energy, and it coheres with the hypo-arousal / state-regulation account: too little accelerator to hold engagement, prompting stimulation-seeking to compensate. Stimulant medication, which increases catecholaminergic tone, tends to normalise these readouts — consistent with a genuinely under-pressed pedal rather than a measurement artefact. As with baseline arousal, though, heterogeneity is real and not every study finds hypo-activation. The dominant signal, nonetheless, is a sympathetic accelerator that engages too little. A weak accelerator — significantly reduced cardiac sympathetic drive and blunted electrodermal responding.

• Bellato: significantly lower cardiac sympathetic index in ADHD, F(1,69) = 8.687, p = 0.004.

• Electrodermal work broadly shows lower tonic SCL and fewer/smaller responses — under-driven sympathetic output.

• Under-activation is worst under effortful low-arousal demand and tends to normalise on stimulant medication.

Autism. Sympathetic drive in autism is variable rather than uniformly high or low, but a well-documented pattern is an over-aroused, high-tonic subgroup: individuals with elevated resting skin-conductance level and heightened electrodermal reactivity, often linked to co-occurring anxiety. This fits the Intense World / hyperexcitability picture on the sensory side — a system running the accelerator warm even at rest. Yet other autistic individuals show blunted or atypical sympathetic responses, particularly reduced or delayed skin-conductance responses to social stimuli, so the group again resolves into subgroups rather than a single value. Some studies report a dissociation between self-reported arousal and physiological arousal (interoceptive/alexithymic factors), complicating interpretation of any single measure. Much of the sympathetic variance appears bound up with anxiety and sensory sensitivity rather than being a core autistic constant. The honest summary is atypical and heterogeneous sympathetic arousal, with a recognisable over-aroused high-SCL subgroup at one pole. Atypical and split — a high-tonic, over-aroused subgroup alongside blunted-response profiles.

• A high-tonic skin-conductance-level subgroup shows over-arousal, frequently tied to co-occurring anxiety.

• Other individuals show blunted/delayed SCRs, especially to social stimuli — the group resolves into subgroups.

• Physiological and self-reported arousal can dissociate (interoception/alexithymia), muddying single-measure readings.

AuDHD. In AuDHD the sympathetic accelerator tends to track the ADHD component — low. In Bellato’s data the reduced cardiac sympathetic index was present in both the ADHD-only and the autism-plus-ADHD groups, indicating that co-occurring autism does not rescue the ADHD-linked sympathetic hypo-activation; the weak accelerator persists. This matters because the autistic sensory over-arousal that might be assumed to “cancel” the ADHD hypo-drive operates on a different channel — sensory reactivity — and does not translate into a strong mobilising sympathetic accelerator for effortful engagement. The lived consequence is a system that can feel sensorily flooded while remaining physiologically under-mobilised for the task at hand — the accelerator stays soft even as sensory input overwhelms. Direct sympathetic measurement isolating AuDHD is limited largely to studies such as Bellato’s that included the co-occurring group, so confidence is moderate. The consistent finding is that the ADHD sympathetic signature carries through. The weak accelerator carries through — ADHD-linked sympathetic hypo-drive persists when autism is also present.

• Bellato: reduced cardiac sympathetic index in both ADHD-only and autism+ADHD groups — autism doesn’t rescue it.

• Autistic sensory over-arousal is a different channel and doesn’t supply mobilising sympathetic drive for effortful tasks.

• Evidence is limited to the few studies including co-occurring groups; the ADHD sympathetic signature nonetheless carries through.

4. Parasympathetic tone — the “brake” (vagal / heart-rate variability)

How well the calming, restorative branch can slow the system down and recover — the physiological brake.

Neurotypical. The parasympathetic (vagal) brake is indexed by heart-rate variability — particularly high-frequency HRV / respiratory sinus arrhythmia and derived cardiac vagal indices — where higher resting HRV signals a more flexible capacity to down-regulate arousal. The neurotypical pattern is good vagal flexibility: the brake can be applied to calm the system and released to mobilise, and — critically — vagal tone is appropriately withdrawn to permit engagement during challenge and then restored for recovery afterwards. Porges’ polyvagal framing and Thayer’s neurovisceral-integration model both tie this flexible vagal control to prefrontal regulation, emotion regulation, and adaptive attentional deployment. The hallmark is context-appropriate braking: down-regulation is available when needed and recovery follows reliably once demand passes. This gives the neurotypical system its capacity to settle after stress rather than staying wound up. A flexible brake — applied to calm, released to engage, and reliably restored to recover.

• High-frequency HRV / cardiac vagal index reads the parasympathetic brake; higher resting HRV means more flexible down-regulation.

• Neurovisceral-integration (Thayer) links flexible vagal control to prefrontal regulation and emotion/attention control.

• The hallmark is context-appropriate braking with reliable post-challenge recovery.

ADHD. ADHD is associated with reduced HRV and poorer vagal regulation — a less flexible brake alongside the weak accelerator, so both autonomic branches are implicated. Lower resting high-frequency HRV in ADHD is reported across several studies (though, as ever, with heterogeneity and some null findings), and it maps onto the broader picture of emotion-regulation difficulty and low distress tolerance that accompanies the disorder. Reduced vagal flexibility means slower, less reliable recovery after arousal spikes — the system stays activated (or dysregulated) longer than it should once stressed. Within Thayer’s framework this connects to the prefrontal-regulatory weaknesses central to ADHD, tying cardiac vagal control to executive and self-regulatory function. The combined autonomic picture — soft accelerator, weak brake — is one of poorly damped, poorly recovered arousal. The evidence base is somewhat thinner and noisier than the sympathetic findings, so this is a trend rather than a hard constant. A weak brake to match the weak accelerator — reduced HRV and slower recovery.

• Reduced resting high-frequency HRV and poorer vagal regulation are reported in ADHD (with heterogeneity / some nulls).

• Lower vagal flexibility means slower, less reliable recovery after arousal spikes — arousal stays elevated longer.

• Fits Thayer’s linkage of cardiac vagal control to the prefrontal-regulatory weaknesses central to ADHD.

Autism. The distinctive autistic finding is parasympathetic withdrawal under demand — the brake releasing precisely when regulation is most needed. Bellato and colleagues found a reduced cardiac vagal index throughout an active response-conflict task in autism — F(1,52) = 4.895, p = 0.031 — meaning vagal tone was pulled back across the demanding task rather than being available to modulate arousal. Functionally this is the opposite of adaptive braking: instead of flexible down-regulation, the calming branch is withdrawn under load, leaving arousal poorly contained just when a conflict-laden situation calls for control. Consistent with the field’s heterogeneity theme, parasympathetic response profiles vary substantially between individuals, and this variation has been shown to predict social functioning — greater vagal flexibility associating with better social outcomes. This links autonomic regulation directly to core autistic phenotype rather than treating it as an incidental correlate. The headline is a brake that lets go under pressure. Parasympathetic withdrawal under demand — the brake released exactly when regulation is most needed.

• Bellato: reduced cardiac vagal index throughout an active response-conflict task, F(1,52) = 4.895, p = 0.031.

• Under load the vagal brake is withdrawn rather than modulated — the opposite of adaptive down-regulation.

• Individual parasympathetic profiles vary and predict social functioning, tying vagal control to core phenotype.

AuDHD. AuDHD is the defining double-deficit — a weak accelerator (low sympathetic drive, from the ADHD component) and a weak brake (vagal withdrawal under demand, from the autistic component) operating at once. The consequence is a nervous system with neither strong mobilisation for effortful engagement nor reliable down-regulation for recovery: it is, in the memorable framing, “starved for the right stimulation while flooded by the wrong kind” — under-driven where it needs to activate, and unbraked where it needs to calm. Bellato’s data support each half in the relevant co-occurring or component groups (ADHD-linked low sympathetic index; autism-linked vagal withdrawal), which is what makes the additive picture more than speculation, even if a single study directly measuring both deficits simultaneously in one AuDHD sample remains the gap. Behaviourally this predicts a system that struggles both to get going and to settle down — poor sustained mobilisation and poor recovery, with arousal that is simultaneously insufficient and uncontained. The two branches fail in complementary directions, which is why AuDHD self-regulation is so effortful and so easily exhausted. Weak accelerator and weak brake at once — the autonomic signature of the combination.

• Combines ADHD-linked sympathetic hypo-drive (weak accelerator) with autism-linked vagal withdrawal (weak brake) concurrently.

• Bellato supports each half in the component groups; a single sample measuring both deficits together is still the evidence gap.

• Predicts a system that neither mobilises well nor recovers well — arousal simultaneously insufficient and uncontained.

5. Reward response — anticipation vs delivery, motivation and delay

How a brain values a promised payoff versus the payoff in hand — and whether the future can pull on the present.

Neurotypical. The mesolimbic dopamine system in the neurotypical brain generates a robust anticipatory signal — the ventral striatum ramps up when a reward is merely cued, well before it arrives, in the paradigm made canonical by Knutson’s Monetary Incentive Delay task. This anticipatory ramp is functionally a bridge across the delay: it converts a distant, abstract payoff into a present motivational pull, so that a neurotypical person can grind through unrewarding intermediate steps because the striatum is already, in effect, tasting the reward to come. Delivery of the reward then produces a comparatively modest, well-calibrated response, because much of the “work” of motivation has already been done in anticipation. This is the neural substrate of ordinary delay tolerance — the capacity to defer gratification without the wait feeling like deprivation. It is not that neurotypical people never discount the future, but that their discount curve is shallow enough for tomorrow’s reward to shape today’s effort. The system is, in the healthy case, self-priming: expecting good things is itself motivating.

• Knutson’s Monetary Incentive Delay task isolates the anticipation phase, where the neurotypical ventral striatum reliably ramps up to reward cues.

• The anticipatory signal functions as temporal glue, letting delayed rewards motivate present behaviour and underpinning ordinary delay tolerance.

• Reward delivery produces a comparatively muted response — the motivational work is front-loaded into anticipation, not the payoff itself.

ADHD. The ADHD reward profile is close to a photographic negative of the neurotypical one: blunted anticipation, intact-to-exaggerated delivery. Furukawa and colleagues found that controls showed the expected anticipatory striatal BOLD response “but not in the ADHD group,” while at the moment of delivery the ADHD group showed “significantly greater BOLD in the ventral striatum bilaterally” — a brain that under-reacts to the promise and over-reacts to the thing itself. The anticipation deficit is one of the more robustly replicated findings in the field, meta-analysed by Plichta and Scheres at an effect size of roughly 0.48 (p<0.001), which is substantial for a neuroimaging phenotype. Mechanistically this is often tied to Volkow’s PET work showing reduced D2/D3 receptor and dopamine-transporter availability in the reward pathway, with motivation scores tracking those measures — though this correlation is not proof of causation, and the transporter story is genuinely contested, since Fusar-Poli’s meta-analysis argued that increased DAT findings were largely driven by prior stimulant exposure rather than the disorder itself. The downstream behaviour is the familiar cluster: delay aversion (the wait is not neutral but actively aversive), and the “dopamine transfer deficit” in which the reward signal fails to migrate back onto the predictive cue. The “Reward Deficiency Syndrome” frame popularised by Blum is best treated as a contested organising metaphor, not settled biology. The practical upshot is that distant rewards simply do not reach into the present — the bridge is out.

• Furukawa: anticipatory striatal signal absent in ADHD, yet “significantly greater BOLD in ventral striatum bilaterally” at reward delivery.

• The anticipation deficit is meta-analytically replicated (Plichta & Scheres, ES≈0.48), but the D2/D3 and DAT story is correlational and directionally disputed (Fusar-Poli: stimulant-driven).

• Delay aversion and the dopamine-transfer deficit mean future rewards fail to motivate present effort — “now” massively outweighs “later.”

Autism. In autism the reward story is best read as domain-specific rather than globally blunted: the Social Motivation Theory (Chevallier, Dawson) holds that it is the salience and reward-value of social stimuli — faces, praise, shared attention — that is selectively reduced, while non-social and restricted-interest rewards are often intact or heightened. Dichter’s fMRI work is frequently cited here, showing attenuated ventral-striatal response to social rewards alongside preserved or elevated response to monetary or interest-congruent rewards, though the picture is heterogeneous and later replications have been mixed. This reframes a great deal of autistic behaviour: a child who will work tirelessly for time with trains but not for a smile is not “unmotivated” but differently weighted. The theory is genuinely contested — critics note it can pathologise autistic preference and that reduced social reward may be a consequence of years of aversive social experience rather than a primary deficit. Crucially, the intense, self-sustaining pull of a restricted interest suggests the reward machinery itself is functional; it is the tuning of what counts as rewarding that diverges. Anticipation and delivery may both operate normally within the domains that the autistic brain actually values.

• Social Motivation Theory (Chevallier, Dawson): selectively reduced salience/reward-value of social stimuli, not a global reward deficit.

• Dichter’s fMRI: attenuated striatal response to social rewards with preserved/heightened response to non-social and restricted-interest rewards — though replications are mixed.

• The intensity of restricted-interest reward shows the machinery is intact; what diverges is the tuning of what registers as rewarding — and the deficit framing is contested.

AuDHD. The AuDHD reward profile stacks two distinct distortions on the same axis: the ADHD delay-aversion / blunted-anticipation pattern and the autistic narrowing of social-reward salience. The result is a motivational system that is doubly hard to recruit through conventional levers — neither the promise of a future reward (undercut by ADHD anticipation failure) nor the pull of social reward and approval (undercut by reduced social salience) reliably works, which strips out the two channels most institutions rely on to motivate people. What tends to remain is the intersection of what does still fire: immediate, high-salience, non-social, often interest-congruent reward — the restricted-interest deep-dive that also delivers the phasic novelty the ADHD side craves. There is very little dedicated imaging on the co-occurring profile specifically, so this is largely a principled composition of the two literatures rather than a directly evidenced phenotype, and the two effects need not simply add — they may interact or partly mask one another. Clinically the pattern presents as someone who appears “motivated only by their own things,” which is legible once you see that both the delayed lever and the social lever have been removed at once.

• Combines ADHD’s blunted anticipation / delay aversion with autism’s reduced social-reward salience — the two motivational levers institutions rely on are both weakened.

• What remains recruitable is immediate, non-social, interest-congruent reward, which satisfies both the phasic-dopamine and the restricted-interest pull at once.

• Direct imaging of the co-occurring profile is essentially absent — this is a composition of two literatures, and the effects may interact rather than simply sum.

6. Boredom & understimulation tolerance — how aversive low stimulation is

How much it costs a brain to sit with too little input.

Neurotypical. For the neurotypical brain, low stimulation is a low-cost state: boredom registers as dull, unengaging, mildly unpleasant, but not painful, and it does not compel immediate action. This tolerance rests on a stable tonic arousal baseline that does not sag catastrophically when environmental input drops, so a neurotypical person can sit in a waiting room, endure a slow meeting, or queue without the situation escalating into distress. Boredom here functions more or less as designed — as a gentle signal that current activity is uninformative, nudging (not forcing) a search for something more engaging. Even Wilson’s much-cited “people prefer electric shocks to being alone with their thoughts” study, often read as evidence that everyone hates understimulation, still shows the majority sat quietly without self-administering shocks. The capacity to wait — to remain in an under-stimulated state without importing arousal — is the behavioural signature. Understimulation is tolerated because the brain is not, at baseline, running an arousal deficit that idleness makes acute.

• A stable tonic arousal baseline means dropping input produces dullness, not distress — the state is low-cost.

• Boredom operates as designed: a soft prompt to seek engagement, not a compulsion to act now.

• The signature is the ability to wait — to remain under-stimulated without importing stimulation.

ADHD. In ADHD, understimulation is not merely dull but physiologically aversive — a chronically under-aroused brain experiences low input as an actively uncomfortable, almost intolerable state, generating a felt pressure to import stimulation by any available means: the phone, physical risk, movement, conflict, or manufactured urgency. This is the behavioural face of the state-regulation and optimal-stimulation models (Sergeant’s cognitive-energetic framework; Zentall’s optimal-stimulation theory), in which performance does not degrade uniformly but craters specifically on slow, dull, low-event-rate tasks, while fast, stimulating, high-event-rate tasks can pull performance back toward normal. The self-medicating quality of much ADHD behaviour — seeking noise, novelty, or danger — is legible as an attempt to drag the arousal system back up to its operating point, not as a moral failure of patience. This is why the same person can be paralysed by a tedious form yet fully functional in a crisis: the crisis supplies the arousal the form withholds. The subjective report is telling — many describe boredom as genuinely painful or panicky, a description that maps onto an under-aroused system screaming for input rather than onto simple impatience. Boredom, in short, is a physiological emergency dressed as a mood.

• Understimulation is physiologically aversive, not merely dull — an under-aroused brain feels a compulsion to import stimulation.

• State-regulation / optimal-stimulation models (Sergeant, Zentall): performance craters on slow, low-event-rate tasks and recovers on fast, stimulating ones.

• Risk-seeking, phone-grabbing and manufactured urgency read as arousal self-medication — hence full function in a crisis but paralysis at a tedious form.

Autism. Autism inverts the ADHD picture, but only partially and with important caveats: many autistic people tolerate or actively prefer low-stimulation environments, because for an over-arousable, hair-trigger system a quiet, low-input setting is not deprivation but arousal relief. Low stimulation reduces the flow of surprising, salient, sensory events that a sensitive system must process, so calm is regulating rather than boring. But the tidy story “autism likes calm” is not universal and should be resisted: sensory-processing research (Dunn’s four-quadrant model) identifies a substantial sensory-seeking / low-registration subgroup who are chronically under-stimulated in specific channels and who crave proprioceptive, vestibular or intense sensory input. The heterogeneity is the point — the same diagnostic label spans people who flee stimulation and people who chase it, sometimes the same person across different sensory channels. What is more consistent is that the aversiveness of boredom per se is often lower than in ADHD, because idleness does not threaten an already-elevated arousal baseline. Understimulation, for much of the autistic spectrum, is a safer state than overstimulation — which is precisely the reverse of the ADHD calculus.

• For an over-arousable system, low-stimulation environments deliver arousal relief, so calm is regulating rather than boring.

• Dunn’s model flags a real sensory-seeking / low-registration subgroup — “autism likes calm” is not universal and can flip by sensory channel.

• Boredom is generally less aversive than in ADHD because idleness does not threaten an already-elevated arousal baseline — overstimulation is the greater danger.

AuDHD. AuDHD produces one of the condition’s sharpest internal contradictions: a brain that is bored and under-stimulated on the cognitive/task channel while needing the environment to stay quiet on the sensory channel. The ADHD substrate demands a stream of engaging input to keep task-arousal at its operating point; the autistic substrate demands that the sensory environment stay low and predictable to avoid tipping into overload — so the person craves stimulation their sensory system cannot actually tolerate. The lived result is the loud-restaurant paradox: understimulated and restless because the conversation is dull, yet simultaneously overwhelmed because the room is too loud, too bright, too much. Conventional coping strategies collapse because each one solves half the problem and worsens the other — turning up the input to kill boredom triggers sensory overload, while damping the environment to prevent overload deepens the boredom. This is a compositional inference from the two literatures rather than a directly studied phenotype, and it will vary enormously by which sensory channels are seeking versus avoiding. The regulatory task is therefore not to raise or lower arousal globally but to route stimulation to the starved channel while shielding the flooded one — a genuinely difficult balancing act.

• The paradox: under-stimulated on the cognitive/task channel while needing the sensory environment kept quiet — craving input the sensory system can’t tolerate.

• Standard coping fails because raising input to kill boredom triggers overload, and damping the environment to prevent overload deepens boredom.

• Composed from two literatures, not directly studied, and highly channel-dependent — the real task is routing stimulation to the starved channel while shielding the flooded one.

7. Novelty-seeking vs need for sameness and predictability

Whether a brain’s default appetite is for the new or for the known.

Neurotypical. The neurotypical brain sits, characteristically, in the flexible middle of this axis: it can enjoy novelty and it can tolerate routine, shifting between them without either being especially costly. Novelty produces the ordinary, well-regulated dopaminergic response — a mild lift, an orienting of attention — but it is not needed to sustain function, and its absence does not provoke distress. Equally, routine and predictability are comfortable rather than confining; the neurotypical person can follow the same commute for years without the sameness becoming intolerable. This flexibility is what allows the smooth cognitive set-shifting that executive-function models take as a baseline — the capacity to switch when switching is useful and to persist when persistence is useful. Novelty-seeking exists as a normal personality dimension (Cloninger’s temperament model), but in the neurotypical case it is a preference, dialled up or down by disposition, rather than an arousal necessity. The defining feature is range: comfortable across a broad band from novel to familiar, driven to neither pole.

• Novelty gives an ordinary, well-regulated lift but is not required to sustain function or arousal.

• Routine is comfortable rather than confining — the same commute for years does not become intolerable.

• The signature is range and flexibility: comfortable set-shifting across the whole novel-to-familiar band, pulled to neither extreme.

ADHD. ADHD skews hard toward novelty-seeking, and the mechanism ties directly back to the reward and arousal story: novelty and change drive phasic dopamine release, which transiently up-regulates a chronically under-aroused system toward its operating point — so the new is not merely preferred but functionally medicinal. This is the substrate of the familiar behavioural signature: switching jobs, tabs, projects, hobbies and relationships; a magnetic pull toward whatever is fresh; and a specific, grinding difficulty with the familiar-but-boring, the task that is neither new nor urgent. High novelty-seeking scores on Cloninger’s TCI are among the more consistent temperament correlates of ADHD, and the dopaminergic account links this to the same D4/D2 signalling implicated in the reward-anticipation deficit. Importantly, this is not a failure to appreciate the value of persistence — it is that a familiar task supplies no phasic dopamine and so leaves the arousal deficit unrelieved, making sustained engagement feel like running uphill. The new tab is not a distraction from the work; it is an attempt to generate the arousal the work refuses to provide. The cost is real — abandoned projects, novelty chased past its usefulness — but the driver is arousal regulation, not caprice.

• Novelty and change drive phasic dopamine, transiently up-regulating an under-aroused system — the new is functionally medicinal, not merely preferred.

• High Cloninger novelty-seeking is a consistent ADHD temperament correlate, tied to the same dopaminergic signalling as the reward-anticipation deficit.

• The hard case is the familiar-but-boring task — it yields no phasic lift, so persistence feels like running uphill and switching becomes self-medication.

Autism. Autism sits at the opposite pole: a need for sameness and predictability that is codified as a core DSM-5 feature (”insistence on sameness, inflexible adherence to routines”). Read through the arousal lens, this is not rigidity for its own sake but arousal insurance — a predictable environment generates fewer surprising, high-salience events, and for a hair-trigger, easily-over-aroused system, fewer surprises means fewer spikes toward overload. Unpredictability is therefore not neutral but itself arousing and aversive: the unexpected is, almost by definition, a salient event that demands processing, and a system already close to its ceiling experiences the unexpected as a threat. This account is strengthened by the predictive-coding models of autism (Van de Cruys, Pellicano & Burr), which frame the condition as difficulty attenuating prediction errors — every deviation from the expected registers as a loud, costly signal rather than being smoothly discounted. Routines, sameness and rituals then function as active engineering of a low-surprise world, a way of holding prediction error — and therefore arousal — down. The distress caused by disrupted routine is, on this reading, not stubbornness but a genuine arousal emergency triggered by an unmanageable spike in the unexpected.

• Insistence on sameness is a DSM-5 core feature, readable as arousal insurance — a predictable world generates fewer over-arousing surprises.

• Predictive-coding accounts (Van de Cruys, Pellicano & Burr): difficulty attenuating prediction errors makes every deviation a loud, costly, arousing signal.

• Disrupted routine causes genuine distress because unpredictability is itself aversive — an arousal emergency, not mere stubbornness.

AuDHD. AuDHD places the two poles of this axis inside one nervous system, producing a direct and unresolvable internal conflict: the ADHD side craves novelty and change to relieve under-arousal, while the autistic side needs predictability and routine to prevent over-arousal — so the same person is driven and destabilised by change at once. The novelty that medicates the ADHD arousal deficit is precisely the unpredictability that triggers the autistic prediction-error spike, meaning the very thing one channel reaches for is the thing the other channel cannot absorb. This is often experienced as being at war with oneself: setting up a stimulating change, then being thrown into overwhelm by it; building a comforting routine, then being driven to boredom and abandonment of it. A common compromise reported clinically is novelty within sameness — rigidly-structured containers inside which some variety is permitted, or intense but familiar restricted interests that deliver newness (new facts, new depth) without environmental unpredictability. This remains a compositional inference rather than a directly evidenced mechanism, and the balance point differs sharply between individuals, but the structural tension is real: no single setting of the novelty dial satisfies both systems, so AuDHD self-regulation is less about choosing a level than about continuously negotiating a contradiction.

• The two poles collide in one system: the ADHD side craves novelty to relieve under-arousal, the autistic side needs predictability to prevent over-arousal.

• The same change is medicine and threat at once — novelty relieves the arousal deficit while spiking autistic prediction error.

• A frequent compromise is novelty within sameness (structured containers, deep restricted interests) — a negotiated contradiction, not a settled dial, and highly individual.

8. Movement & self-stimulation as arousal regulation — fidgeting, pacing, stimming

Self-generated motor and sensory input recruited to move arousal toward its optimum.

Neurotypical. For the neurotypical brain, self-generated movement plays only a marginal role in arousal regulation: there are small, incidental fidgets — a jiggled foot, a clicked pen — but these are not load-bearing, and suppressing them carries little cognitive or regulatory cost. Because tonic arousal sits at a stable baseline that does not routinely drift far from its operating point, the neurotypical system rarely needs to import motor or sensory input to correct an arousal error. Movement is therefore mostly instrumental (getting somewhere, doing something) rather than regulatory (adjusting internal state), and stillness can be maintained for extended periods without performance or comfort degrading. Where fidgeting does appear it tends to track transient states — mild boredom, mild anxiety — and it resolves when the state passes, rather than being a continuous background process. The key contrast with the other profiles is precisely this dispensability: ask a neurotypical person to sit still and the request is mildly annoying, not regulatorily destabilising.

• Small fidgets exist but are not load-bearing — suppressing them costs little, cognitively or regulatorily.

• A stable arousal baseline means motor/sensory input is rarely needed to correct an arousal error.

• Stillness is sustainable for long periods; fidgeting tracks transient states and fades when they pass.

ADHD. In ADHD, hyperactivity and fidgeting are best understood not as excess to be suppressed but as self-administered up-regulation — Bellato and colleagues describe such movement as “a compensatory mechanism to upregulate arousal,” a way of dragging a chronically under-aroused system toward its operating point. The mechanism is plausibly catecholaminergic: gross motor activity raises noradrenaline and dopamine availability, lifting the under-aroused brain toward the optimum at which attention and control become possible. This reframes the classroom demand to “sit still” as actively counter-productive — it removes a working arousal lever at the very moment the child needs it, and there is suggestive evidence (Sarver; Rapport’s work) that fidgeting increases precisely during demanding working-memory tasks and may support rather than hinder performance in ADHD. The clinical implication reverses the usual instinct: rather than suppressing movement, permitting or channelling it (standing desks, movement breaks, permitted fidgeting) may free cognitive resources otherwise spent on the effortful business of holding still. The behaviour, in short, is not the disorder leaking out but the system’s own compensation for it. Suppression does not fix the arousal deficit; it merely removes the tool the person was using to manage it.

• Bellato: fidgeting is “a compensatory mechanism to upregulate arousal“ — self-medication for an under-aroused system, not surplus energy.

• Movement raises catecholamines (noradrenaline, dopamine), lifting the brain toward its optimal operating point for attention and control.

• Sarver/Rapport: fidgeting rises during demanding working-memory tasks and may aid performance — so “sit still” removes a working arousal lever.

Autism. In autism, self-stimulatory behaviour — stimming — is a genuine self-regulation mechanism: rhythmic, predictable, self-generated sensory or motor input (rocking, hand-flapping, spinning, repeating sounds) that discharges, masks or modulates a more chaotic arousal load, most often down-regulating or stabilising an over-aroused system. The predictability is doing real work: self-generated input is perfectly forecastable and so, in predictive-coding terms, produces no aversive prediction error, giving the system a controllable, non-threatening signal to hold on to amid an unpredictable environment. Autistic self-advocates and a growing research literature (Kapp’s participatory work) are emphatic that stimming is functional and often calming or focusing, which is why the historical clinical instinct to extinguish it — as in older behavioural programmes — is now widely regarded as harmful, since it removes a working regulatory tool and demands effortful, depleting suppression. Stimming can serve several ends — soothing overload, expressing emotion, aiding concentration, or discharging excess arousal — so it is not one behaviour with one function but a flexible regulatory toolkit. Critically it can be recruited in either arousal direction, but its characteristic use is bringing an over-aroused, sensorily-flooded system back toward tolerable ground. The rhythm is the regulation: predictable input against an unpredictable world.

• Stimming is functional self-regulation — rhythmic, predictable, self-generated input that discharges, masks or modulates a chaotic arousal load.

• Its predictability produces no prediction error, giving an over-aroused system a controllable, non-threatening signal — characteristically down-regulating or stabilising.

• Kapp’s participatory research reframes stimming as calming/focusing; suppressing it (older behavioural programmes) removes a working tool and demands depleting effort.

AuDHD. AuDHD recruits both motor strategies at once, because it is regulating two arousal problems on two channels simultaneously: fidgeting and gross movement to up-regulate the under-aroused cognitive/task channel, and stimming to modulate or down-regulate the over-aroused sensory channel — sometimes literally in parallel. The person may pace or jiggle to lift flagging task-arousal while also rocking or repeating a self-soothing motor pattern to hold sensory overload at bay, which to an outside observer looks like a dense, contradictory tangle of movement but is in fact two coherent regulatory processes overlaid. This dual demand makes stillness especially costly — suppressing movement in AuDHD removes two arousal levers at once, disabling both the up-regulator the task channel needs and the stabiliser the sensory channel needs, which helps explain why demands to “sit still and calm down” can be so disproportionately destabilising. As with the other AuDHD dimensions this is a principled composition of the ADHD and autism literatures rather than a directly measured phenotype, and which movements serve which function will vary by individual. The practical reading is that AuDHD movement should be presumed functional until proven otherwise — likely doing regulatory work on one channel or both — rather than treated as noise to be quieted.

• Uses both levers at once: fidget/movement to up-regulate the under-aroused task channel, stimming to modulate the over-aroused sensory channel — sometimes in parallel.

• Apparent contradictory tangles of movement are in fact two coherent regulatory processes overlaid, addressing opposite arousal errors.

• Stillness is doubly costly — it removes two levers at once — so AuDHD movement should be presumed functional until proven otherwise, not quieted as noise.

9. Sensory overload threshold — how fast ordinary input tips into overwhelm

Where the descending limb begins — the point at which more input stops helping and starts breaking performance.

Neurotypical. In the neurotypical brain the overload threshold sits comfortably high: ordinary rooms — the strip-lit office, the murmuring café, the train carriage — fall well below the peak of the Yerkes-Dodson curve, leaving generous headroom before input degrades function. Sensory registration is dampened by efficient early-stage filtering, so a typical environment is experienced as background rather than as a stream of competing demands. Performance therefore collapses only under genuinely extreme load — a klaxon in a crisis, a sleepless week — and recovers quickly once the stimulus recedes. The margin between comfortable functioning and overwhelm is wide enough that most people rarely meet their own ceiling. This is the implicit baseline against which the other three profiles read as atypical thresholds, not atypical stimuli — the room is the same; the tipping point differs.

• The Yerkes-Dodson inverted-U places NT everyday functioning on the ascending or plateau region, with the descending (overload) limb reached only rarely.

• Robust early sensory attenuation means ordinary environments are encoded as low-priority background, preserving cognitive resources for task demands.

• Recovery from transient overload is fast and near-complete, so the ceiling is seldom sustained or damaging.

ADHD. The ADHD picture is genuinely variable, and honesty requires resisting a single story: under combined high cognitive and sensory load — a noisy open-plan office while drafting under deadline — the ADHD brain can indeed be flooded and tip early. Yet the modal problem runs the other way. The characteristic complaint is under-stimulation, not overload — an arousal deficit that leaves ordinary environments feeling flat and effortful, driving stimulation-seeking rather than stimulation-avoidance. What looks like being “overwhelmed” is more often a failure of salience filtering: irrelevant input wins the competition for attention because top-down control is weak, not because the input itself is intolerably intense. This is distractibility, a gating failure, rather than the raw perceptual over-reactivity seen in autism. The threshold is therefore best described as unstable and context-dependent — low headroom under crowded multitasking, but paradoxically a craving for more input at rest.

• The dominant ADHD phenotype is hypoarousal / under-stimulation, consistent with catecholamine (dopamine–noradrenaline) accounts and the therapeutic paradox of stimulant medication.

• Apparent “overwhelm” typically reflects impaired filtering of irrelevant stimuli (a salience problem) rather than intolerable sensory intensity.

• Flooding can nonetheless occur under simultaneous high cognitive and sensory demand, giving a genuinely variable rather than uniformly high or low threshold.

Autism. The autistic overload threshold is characteristically low, and here the mechanism is perceptual rather than merely attentional. The Intense World Theory (Markram & Markram) frames autism as hyper-reactivity and hyper-perception arising from locally hyperconnected, hyperplastic microcircuits; converging work implicates an excitation–inhibition (E/I) imbalance tilted toward excitation, with reduced GABAergic inhibition and cortical hyperexcitability amplifying ordinary input. Sensory over-responsivity is not incidental but a formal DSM-5 diagnostic criterion. As Markram put it, stimuli that are “bearable and normal to a typically developing child may be unbearable to an autistic child” — the strip light, the seam of a sock, the café hum arrive at close to full salience. The practical consequence is that an ordinary room already sits past the peak of the curve, so any additional demand — a question, a transition, a second conversation — pushes the system into meltdown or shutdown. Overload here is a physiological ceiling reached inside environments the NT brain experiences as neutral.

• Sensory over-responsivity is a DSM-5 criterion; the Intense World Theory attributes it to hyper-reactive, locally hyperconnected cortical microcircuits.

• E/I imbalance toward excitation — reduced GABAergic inhibition and cortical hyperexcitability — provides a candidate mechanism, though the evidence is heterogeneous and not uniform across individuals or modalities.

• Because baseline environments already exceed the peak, meltdown and shutdown are best read as overload responses, not behavioural choices.

AuDHD. The AuDHD combination is not an averaging of the two profiles but a collision between them: the autistic low sensory threshold sits alongside the ADHD drive to seek stimulation, so the person reaches overload fast while still craving input. The result is a punishingly narrow operating band — too little input feels intolerably flat and under-aroused, yet the volume of stimulation needed to satisfy the ADHD side rapidly breaches the autistic ceiling. Individuals often describe oscillating between boredom and overwhelm with little stable middle ground, seeking intensity and then being injured by it. This internal contradiction — approach and avoidance wired to the same input — is a recurring theme across AuDHD self-report and the emerging clinical literature, though formal studies remain sparse and the presentation is highly individual. The design implication is stark: environments must be simultaneously rich enough to engage and quiet enough not to wound.

• Low autistic sensory threshold co-occurs with ADHD stimulation-seeking, producing a narrow band between under-arousal and overload.

• Approach (seek input) and avoidance (protect against overload) are driven by the same stimuli, yielding characteristic boredom–overwhelm oscillation.

• Evidence is largely self-report and clinically emergent rather than well-powered experimental work; presentations are markedly heterogeneous.

10. Sensory gating & habituation — filtering repeated or irrelevant input

Two jobs — suppressing what’s irrelevant and switching off what’s constant-and-safe — and how cleanly each brain does them.

Neurotypical. The neurotypical system performs both filtering functions efficiently and largely without conscious effort. Gating suppresses irrelevant or competing input — the classic index is P50 sensory gating, where the cortical response to the second of two paired clicks is sharply attenuated — while habituation dials down responding to a constant, non-threatening stimulus. The upshot is that the fluorescent hum, the waistband, the clothing tag, the distant traffic fade from awareness within seconds and stay faded. This is not that the stimuli are absent but that a healthy nervous system judges them safe and stops spending on them, freeing capacity for whatever is novel or relevant. The two mechanisms work in concert: gating keeps the irrelevant out at entry, habituation retires the persistent once admitted. The felt result is a world with a quiet, recedent background.

• Efficient P50 gating attenuates the neural response to repeated or paired stimuli, indexing intact sensory filtering.

• Rapid habituation removes constant safe stimuli (tags, hums) from awareness within seconds, conserving attentional resources.

• Gating (entry-stage suppression) and habituation (response decline over time) operate together to keep the sensory background quiet.

ADHD. ADHD gating is best characterised as leaky and variable — the difficulty is filtering irrelevant stimuli, which is the mechanistic heart of distractibility. Studies of P50 and prepulse inhibition in ADHD are mixed but broadly consistent with weaker or inconsistent suppression of task-irrelevant input, and the catecholaminergic dynamics that govern the processing of irrelevant stimuli are precisely those implicated in the disorder. Crucially, this is a filtering and salience problem rather than one of sensory intensity: the ADHD brain does not necessarily register the clothing tag as physically more intense, it simply fails to reliably exclude it from the competition for attention. When top-down control lapses, whatever is most salient in the moment — a notification, a passing conversation — captures processing. Habituation to constant safe input is comparatively less disrupted than in autism; the ADHD signature is the porous gate, not the cortex that never lets go.

• The core deficit is weak/variable suppression of irrelevant input — a gating and salience failure that presents behaviourally as distractibility.

• Catecholamine (dopamine–noradrenaline) dynamics governing irrelevant-stimulus processing map onto ADHD neurobiology and stimulant response.

• The problem is one of filtering rather than sensory intensity, distinguishing it from the impaired-habituation profile of autism.

Autism. Autism is marked less by a leaky gate than by impaired habituation — the cortex keeps registering a repeated stimulus at near-full intensity rather than letting it recede. Neuroimaging and psychophysiological studies report reduced neural habituation in autism, notably a failure of the amygdala and sensory cortices to attenuate their response across repetitions, with GABAergic thalamocortical inhibition repeatedly implicated as the candidate mechanism (dovetailing with the E/I account in dimension 9). Where the NT brain habituates, the autistic cortex does not: the ticking clock, the buzzing light, the scratchy fabric do not fade to background but continue to demand processing, accumulating across a day into exhaustion. This shared habituation-failure mechanism helps explain why persistent, low-level, “harmless” stimuli — the ones others literally stop noticing — become a dominant source of load. The world, in effect, refuses to go quiet.

• Reduced neural habituation — including failure of amygdala/sensory-cortex response to attenuate across repetitions — is a replicated (though heterogeneous) autism finding.

• GABAergic thalamocortical inhibition is the leading candidate mechanism, linking impaired habituation to the broader E/I-imbalance model.

• Persistent “harmless” stimuli remain at near-full salience, so background never fully recedes and load accumulates over time.

AuDHD. AuDHD compounds the two failures into a double filtering deficit: the brain fails to suppress irrelevant input (the ADHD leaky gate) and fails to habituate to persistent input (the autistic cortex that never lets go). Neither channel of relief is reliably available — novel distractors break through the porous gate while constant stimuli refuse to retire into the background — so the sensory field stays crowded from both directions at once. Self-report descriptions of being unable either to tune out a new noise or to stop noticing an old one capture this dual porosity, and it plausibly amplifies the low overload threshold of dimension 9, since more input remains “live” for longer. The evidence base is inferential — combining well-studied ADHD gating findings with well-studied autism habituation findings rather than resting on direct AuDHD studies — but the mechanistic prediction is coherent: the least filtered sensory world of the four profiles.

• Combines a leaky gate (poor suppression of irrelevant input) with impaired habituation (poor retirement of persistent input) — deficits on both filtering axes.

• Predicted to intensify the low overload threshold, as more stimuli remain salient and unretired simultaneously.

• Largely an inferential composite of separate ADHD and autism literatures; direct AuDHD gating/habituation studies remain scarce.

11. Sustained attention & the vigilance decrement — how attention drains over time

Attention treated as a depleting resource — how long the tank lasts and how fast the ceiling arrives.

Neurotypical. In the neurotypical brain, sustained attention drains gradually and predictably. On continuous monitoring or vigilance tasks, performance declines measurably over time — the classic vigilance decrement, often detectable within the first 10–15 minutes as fronto-parietal executive-control activity wanes and mind-wandering rises. Working-memory capacity is a related bottleneck: Cowan’s estimate of roughly four chunks captures how little can be held actively at once, so effortful monitoring competes for a genuinely scarce resource. But the NT decline is graceful — slow enough that a well-motivated person can hold a dull task for a meaningful stretch, and readily restored by a short break or a change of activity. The resource depletes on a shallow slope, and top-down control keeps a low-interest task in play long after intrinsic novelty has gone. This shallow, recoverable curve is the reference against which ADHD and autism read as steeper or more selective.

• The vigilance decrement — declining performance on continuous monitoring — typically emerges within 10–15 minutes as fronto-parietal control wanes and mind-wandering rises.

• Working memory is capacity-limited to roughly four chunks (Cowan), making effortful sustained attention a scarce, contested resource.

• The NT decline is shallow and readily restored by brief breaks, so low-interest tasks can be held for extended periods.

ADHD. ADHD shows a faster and steeper decrement — the ceiling simply arrives sooner. Continuous-performance-task studies consistently report earlier and more frequent lapses, elevated reaction-time variability (a hallmark ADHD signature, often modelled as periodic attentional lapses), and earlier, more spontaneous mind-wandering. The mechanism is competitive: an under-stimulating task cannot hold its own against internally or externally generated alternatives, so attention defects to whatever offers more arousal. Reaction-time variability is diagnostically telling because it reflects not a uniform slowing but intermittent disengagement — the attention flickers rather than fades evenly. The practical reading is that ADHD does not lack the capacity for attention so much as the capacity to sustain it against low-reward demand: the drain is real, early, and tightly coupled to how stimulating the task is. Interest can transiently reset the whole curve — which is precisely the bridge to hyperfocus in dimension 12.

• CPT studies show earlier, more frequent lapses and a steeper vigilance decrement than NT.

• Elevated intra-individual reaction-time variability is a robust ADHD marker, indexing intermittent disengagement rather than uniform slowing.

• Under-stimulating tasks lose the competition for attention sooner; the drain is tightly coupled to task interest and reward.

Autism. The autistic profile is not a global attention deficit but an atypical allocation — narrow, deep, and interest-contingent. On a task that engages a special interest, monotropic focus can sustain attention with a persistence and depth that outstrips the NT norm; on low-interest, externally imposed, effortful tasks, however, a decrement appears and can be pronounced. The picture in the literature is genuinely mixed — sustained-attention findings in autism are inconsistent, partly because outcomes depend so heavily on how interesting and self-directed the task is — but the recurring theme is a channelling of attentional resources into a narrow beam rather than a shallow, even spread. The strength and the vulnerability are two faces of the same allocation style: superb endurance on the chosen, brittle endurance on the assigned. Framing autistic attention as simply “impaired” or simply “superior” both miss the point; it is selectively deployed.

• Monotropic, interest-driven focus can sustain attention exceptionally well, exceeding NT persistence on high-interest tasks.

• A decrement nonetheless appears on low-interest, effortful, externally imposed tasks; the sustained-attention literature is mixed and task-dependent.

• Attention is allocated narrowly and deeply rather than broadly, making endurance strongly contingent on interest and self-direction.

AuDHD. AuDHD compounds the two dynamics: ADHD’s rapid, interest-gated drain meets autism’s narrow, deep allocation, and the outcome is a profile of extremes. On a genuinely engaging, self-chosen task the two strengths can align — narrow autistic channelling plus ADHD interest-driven lock-in — producing formidable, sustained performance. But on an imposed, monotonous task the two vulnerabilities also align: the ADHD ceiling arrives fast and there is little of the broad, flexible, “good-enough” attention that might otherwise carry a dull job along. The result is brilliance on the interesting and unusually fast collapse on the imposed-and-dull, with a wider gap between the two than either condition alone typically shows. The evidence is again largely inferential, extrapolated from the separate ADHD and autism attention literatures rather than direct AuDHD studies, but the pattern is consistently reported in lived-experience accounts: exceptional or nothing, with little serviceable middle.

• ADHD’s fast, interest-gated decrement combines with autism’s narrow allocation to widen the gap between high- and low-interest performance.

• On self-chosen tasks the two strengths can align into formidable sustained focus; on imposed dull tasks the two weaknesses align into rapid collapse.

• Predominantly inferred from separate ADHD and autism literatures plus lived-experience report; direct AuDHD sustained-attention studies are limited.

12. Hyperfocus, flow & the disengagement (”stop”) signal

Locking on is only half the skill — the other half is being able to let go.

Neurotypical. The neurotypical brain can enter flow — Csíkszentmihályi’s state of absorbed, effortless engagement that arises when challenge and skill are well matched — and this state sits near the arousal optimum, where it is self-regulating and even restorative. The defining NT feature, however, is that the disengagement signal survives: even in deep absorption, the person continues to register hunger, fatigue, a full bladder, a waiting appointment, a competing obligation, and can therefore stop when they should. Flow is entered voluntarily-ish and exited cleanly; it enhances performance without incurring a hidden debt. The “stop” mechanism — an intact interoceptive and top-down monitoring loop running underneath the absorption — is what keeps flow adaptive rather than costly. This preserved brake is precisely the faculty that the other three profiles modulate, override, or have violently disrupted.

• Flow (Csíkszentmihályi) arises at matched challenge–skill near the arousal optimum and is self-regulating, even restorative.

• The disengagement/”stop” signal remains intact — interoceptive and top-down monitoring continue to register competing needs during absorption.

• Flow is therefore exited cleanly and incurs no hidden physiological debt.

ADHD. ADHD hyperfocus is a dopamine-driven lock-in on a high-interest task that overrides the stop signal — the same task-interest reset that resolves the vigilance decrement of dimension 11, now taken to its extreme. Where NT flow retains its brake, hyperfocus blows past hunger, fatigue, time, and competing obligations, sometimes for hours, because the interoceptive and monitoring cues that would normally trigger disengagement are drowned out by the reward-driven engagement. The state is not chosen and cannot be summoned to order — it attaches to what is intrinsically rewarding, not to what is important — and it characteristically ends in a crash, the borrowed arousal repaid as depletion. The deep conceptual point is that distractibility and hyperfocus are two faces of the same dysregulated gain-control: a system that cannot reliably modulate attentional gain will both drift off dull tasks and fail to disengage from rewarding ones. One brake, failing in both directions.

• Hyperfocus is a dopamine-mediated lock-in on high-reward tasks that overrides interoceptive stop cues (hunger, fatigue, time).

• It cannot be directed at will and typically ends in a crash — “borrowed” arousal repaid as depletion.

• Distractibility and hyperfocus are two expressions of one dysregulated gain-control system, not opposite traits.

Autism. Autistic deep engagement is best understood through monotropism — the tendency for attention to be pulled into a single, narrow, intensely rewarding channel — expressed most vividly in sustained, high-intensity engagement with special interests. This is an attentional strength: the depth, persistence, and expertise it enables are real and valuable, and the flow-like absorption is often experienced as regulating and pleasurable. The distinctive cost lies not in the lock-in itself but in the transition out of it: interruptions and enforced shifts are experienced as aversive and expensive, imposing a switching cost that can trigger distress or shutdown. The issue is therefore less an overridden stop signal (as in ADHD) than a genuinely high price on disengagement and task-switching. Monotropic focus is a coherent adaptation, not a deficit — but one whose value is fragile at its boundaries, where the world demands a change of channel.

• Monotropism describes attention channelled into a single, narrow, deeply rewarding stream — a genuine attentional strength.

• Special-interest engagement is sustained and intense, often experienced as regulating rather than depleting.

• The characteristic cost is the transition: interruptions and enforced task-switches are aversive and expensive, sometimes precipitating shutdown.

AuDHD. AuDHD produces a uniquely destabilising combination: an intense ADHD dopamine lock-in that can be violently interrupted by autistic sensory overload. The person locks on hard — hyperfocus plus monotropic depth driving formidable, absorbed engagement — and then a single intolerable input (a sudden noise, a light, a texture, a demand) breaches the low sensory threshold of dimension 9 and throws them off the task entirely. The two faculties do not merely coexist; they sabotage each other. The lock-in makes the eventual eviction more jarring, and the low threshold makes eviction more likely, so the state is both harder to leave voluntarily and easier to be forced out of involuntarily — the worst of both exit dynamics. Lived-experience accounts describe this as being wrenched out of deep focus by something the ADHD side barely registered until it became unbearable, followed by difficulty re-entering. The evidence is emergent and largely self-reported rather than experimentally established, but the mechanism is a clean composite: deep, brittle focus — locked in by dopamine, shattered by sensation.

• ADHD hyperfocus/monotropic lock-in can be involuntarily broken by autistic sensory overload breaching a low threshold.

• The two dynamics interact adversely: harder to disengage voluntarily, yet easier to be thrown out involuntarily — and costly to re-enter.

• The account is coherent as a composite mechanism but rests largely on emergent self-report rather than direct experimental evidence.

13. State-dependent performance — the effect of urgency, stakes and noise on output

How far a brain’s output rides on external arousal conditions — deadlines, reward, event-rate, noise — rather than on the task alone.

Neurotypical. For the neurotypical brain, performance is comparatively decoupled from external arousal conditions: the same person produces broadly adequate work whether the task is thrillingly urgent or numbingly dull, because tonic arousal sits near an efficient operating point and top-down control supplies the rest. Sokolov’s classic arousal–performance work and the Yerkes–Dodson inverted-U describe a system that self-corrects toward its own optimum — mild boredom is tolerated, mild pressure absorbed, without either tipping output off a cliff. Event-rate manipulations and reward incentives do move neurotypical performance, but modestly and predictably, along the gentle middle of the curve rather than between rescue and collapse. Crucially, added stimulation such as background noise tends to nudge an already well-arousaled system past its peak, degrading rather than helping. The practical signature is robustness: a neurotypical worker rarely needs a manufactured crisis to begin, and rarely needs perfect quiet to continue. Conditions matter, but they modulate the baseline rather than determine it.

• The inverted-U (Yerkes–Dodson) holds, but the NT sits near its apex, so most real-world condition changes produce shallow, recoverable performance shifts.

• Reward and deadline effects are real but incremental — motivation tunes output rather than switching it on or off.

• Added noise or stimulation usually pushes an optimally-aroused system over the top, mildly worsening accuracy — the mirror image of the ADHD finding below.

ADHD. The ADHD brain is dramatically state-dependent: the identical task can be near-impossible when dull and unstructured yet suddenly fluent under urgency, novelty, competition, reward or a looming deadline, which is why the last-minute all-nighter is so often the only version that gets done. Sergeant’s cognitive-energetic / state-regulation model frames this as a chronically sub-optimal arousal pool that external conditions transiently top up, and the low-arousal / optimal-stimulation accounts (Zentall) describe the compensatory reach for stimulation. The most counterintuitive evidence is stochastic resonance: Söderlund, Sikström and Smart (2007) found that moderate auditory white noise improved memory and cognitive performance in children with ADHD while worsening it in typically developing peers — noise supplies the missing arousal to an under-aroused system yet pushes an already-optimal one past its peak. Event-rate studies converge: slow, low-event tasks collapse ADHD performance, fast ones rescue it. The evidence base is heterogeneous and effect sizes vary, but the direction — performance as a function of state, not just of ability — is robust. It explains both the “can’t start” and the “brilliant under fire” without contradiction.

• Stochastic resonance (Söderlund et al., 2007; moderate-noise benefit) is the signature dissociation — the same input helps ADHD and hurts NT because they sit on opposite sides of the inverted-U.

• The state-regulation model (Sergeant) recasts many “deficits” as effort/activation problems that urgency, reward and high event-rate transiently repair.

• Effect sizes and replications are mixed and moderator-heavy — treat “noise as medicine” as a real but individually-variable phenomenon, not a universal prescription.

Autism. For the autistic brain the decisive external variables are not urgency and reward but predictability and sensory load: performance is protected by structure, routine and low-arousal environments and degraded by unpredictability, unsignalled transitions and sensory-demanding settings. This follows from predictive-coding accounts of autism — the influential “hypo-priors” / HIPPEA framework (Pellicano & Burr; Van de Cruys et al., 2014) casts the autistic system as one for which unexpected input carries excessive precision, so a noisy or shifting environment is not merely distracting but effortful and destabilising. Sensory-processing and intense-world models add that ordinary sensory load can consume the capacity a task needs, so a fluorescent, open-plan, interruption-rich room lowers output before the work even begins. Urgency and incentive help far less here, and can actively harm when they arrive as unpredictability. The optimal condition is environmental safety — same room, same order, low stimulation — under which sustained, precise, detail-faithful performance is often a genuine strength. The literature is heterogeneous across sensory profiles, but the predictability-and-load axis recurs.

• Predictive-coding / HIPPEA accounts (Van de Cruys et al., 2014) explain why unpredictability and transitions are disproportionately costly — surprise is expensive to process.

• Sensory load competes directly for task capacity, so low-stimulation environments raise output independent of motivation or interest.

• Sensory profiles vary widely (hyper- vs hypo-reactive), so “low load” must be individually specified rather than assumed.

AuDHD. The AuDHD brain faces a structural contradiction: the ADHD channel needs urgency, novelty and stimulation to activate, while the autistic channel needs predictability and low sensory load to avoid overload — two optimisation targets that a single environment cannot satisfy at once. The stimulating, high-event, deadline-charged conditions that rescue ADHD output are frequently the same unpredictable, sensory-loud conditions that tip the autistic system toward overwhelm; the quiet, routinised, low-load conditions that protect autistic performance are precisely the under-stimulating ones in which the ADHD system stalls. Because AuDHD is a recent, under-studied convergence rather than a settled phenotype, direct experimental evidence is thin and largely inferred from combining the two literatures plus community and clinical report. The lived signature is the absence of any single “good environment” — what activates one system dysregulates the other. Effective conditions therefore tend to be composite and self-engineered: manufactured stakes for activation paired with sensory control (noise-cancelling, fixed routine, single-tasking) for protection. It is less a set-point than a moving negotiation between opposing demands.

• The core bind: the arousal-raising conditions that switch ADHD on overlap heavily with the sensory-raising conditions that overwhelm the autistic system.

• No single environment optimises both channels, so AuDHD workers typically layer contradictory supports (urgency devices plus sensory dampening) rather than find one ideal setting.

• Direct AuDHD-specific data are sparse — most of this is principled inference from the two source literatures and consistent lived report, not settled experiment.

14. Interoception — how well the brain perceives its own internal arousal state

The internal dashboard — reading heart-rate, tension, hunger and rising overload early enough to correct before the system tips.

Neurotypical. The neurotypical brain has reasonably accurate interoception: it registers the ascending signals of arousal — quickening heart, muscle tension, hunger, fatigue, mounting stress — early and with usable fidelity, so correction can happen before either over- or under-arousal becomes disabling. Craig’s account of interoception as the basis of felt bodily state, and Critchley and Garfinkel’s work dissociating interoceptive accuracy, sensibility and awareness, describe a system in which the objective signal and its subjective reading are decently aligned. That alignment is what lets a neurotypical person notice “I’m getting hungry / wound up / tired” and act — eat, pause, de-escalate — while the adjustment is still cheap. It also underwrites emotion: on Schachter–Singer and later constructionist views, reading bodily arousal accurately feeds proportionate emotional appraisal. None of this is perfect, and interoceptive accuracy varies substantially even among neurotypical people. But the dashboard broadly works, and its readings arrive in time to be useful rather than only as a post-mortem.

• Interoception factorises (Garfinkel & Critchley) into accuracy, sensibility and awareness — the NT profile is characterised by decent alignment among them.

• Early, usable signal is the key asset: drift toward over/under-arousal is felt while correction is still low-cost.

• Even in NTs interoceptive accuracy varies widely, so “accurate” means adequate-and-timely, not uniform or exact.

ADHD. ADHD is associated with poor, noisy interoception: internal signals such as hunger, thirst, fatigue and rising dysregulation are frequently under-registered until they reach an extreme, which is why forgetting to eat, ignoring a full bladder, or “hitting a wall” out of nowhere are such common lived reports. The result is a faulty dashboard whose first reliable reading is often the crash itself — the signal that should have prompted an early, cheap correction is missed, so the correction only happens after collapse. Empirically this is still an emerging and heterogeneous literature: studies report reduced interoceptive accuracy and elevated interoceptive confusion in ADHD, and there is overlap with the well-documented delay-aversion and effort-regulation findings, but sample sizes are modest and results mixed. Mechanistically it plausibly connects to the same tonic under-arousal and noradrenergic/dopaminergic signalling irregularities implicated elsewhere in ADHD — a low-fidelity internal channel matching a low-fidelity external one. The practical upshot is management by external scaffolding: timed meals, alarms and rules substituting for a body-clock that under-reports. It is best stated as a strong, coherent pattern with genuine but not yet decisive evidence.

• The signature failure is late signalling — hunger, fatigue and mounting dysregulation are missed until extreme, so the crash is the first dependable readout.

• Mechanistically consistent with ADHD’s tonic under-arousal and catecholamine signalling irregularities, though the interoception-specific evidence base is still thin and heterogeneous.

• Practical compensation is external: clocks, alarms and rules replace an unreliable internal dashboard rather than sharpening it.

Autism. Autistic interoception is best described as atypical rather than simply reduced: the mapping from bodily signal to recognised state is disrupted, and this can run in either direction — under-registering, so hunger, pain or rising arousal go unnoticed, or over-registering, so ordinary internal sensations (heartbeat, gut, tension) are themselves intrusive and overwhelming. Alexithymia — difficulty identifying and describing one’s own emotional and bodily states — is markedly more common in autistic people, and an influential line of work (Bird & Cook’s “alexithymia hypothesis”) argues that several apparent emotion deficits in autism track alexithymia rather than autism as such. Garfinkel and colleagues have reported interoceptive trait–state discrepancies — a mismatch between how attuned people believe they are and their objective accuracy — linked to anxiety in autistic samples. The through-line is a broken or miscalibrated signal-to-label pathway: the body may be shouting or silent, and either way the interpretation is unreliable. Because sensory and alexithymic profiles differ so much between individuals, the evidence is genuinely heterogeneous. What is consistent is that “read your body and adjust” cannot be assumed to function as it does neurotypically.

• Atypia is bidirectional — the same population contains under-registering (missed hunger/pain) and over-registering (overwhelming heartbeat/gut) profiles.

• Alexithymia is elevated and load-bearing: Bird & Cook’s hypothesis attributes much apparent emotional-processing difficulty to alexithymia rather than to autism per se.

• The disrupted signal-to-label mapping (Garfinkel’s trait–state discrepancy, linked to anxiety) means self-monitoring advice must not presume typical interoception.

AuDHD. In AuDHD the dashboard is doubly unreliable: the ADHD tendency to under-register and notice signals only at the extreme compounds the autistic atypia and elevated alexithymia, so the pathway from internal state to accurate, timely self-knowledge is degraded from both directions. The consequence is that crashes and overloads arrive with little warning — the ADHD channel misses the early rise, and the autistic channel either fails to register it or mislabels it, leaving few of the mid-course cues on which self-regulation depends. This is inference from combining two literatures more than a directly measured AuDHD finding: the interoception evidence is still emerging for ADHD, heterogeneous for autism, and barely studied for their intersection, so confident quantification is not yet warranted. What community and clinical accounts consistently describe, however, is exactly this pattern — sudden hunger, sudden exhaustion, sudden overwhelm with no felt approach. It places unusual weight on external monitoring, because the internal instrument cannot be trusted to warn in time. The honest framing is a coherent, well-motivated prediction awaiting proper study.

• Two failure modes stack: ADHD late-signalling plus autistic atypia/alexithymia leave the mid-range warning band especially sparse.

• The lived signature is warning-less transitions — crashes and overloads that seem to arrive without a felt run-up.

• This is combined-literature inference, not settled AuDHD data; external monitoring is relied upon precisely because the internal dashboard is doubly compromised.

15. Emotional arousal & regulation — intensity, reactivity and recovery

The emotional layer of arousal — how hard the amygdala fires, how intense the response, and how quickly the system returns to baseline.

Neurotypical. In the neurotypical brain emotional arousal is characteristically modulated: responses are broadly proportionate to their triggers, prefrontal circuitry down-regulates limbic activation, and the system recovers toward baseline within a reasonable window. The canonical model here is prefrontal–amygdala regulation — Ochsner and Gross’s work on cognitive reappraisal, and the ventromedial/dorsolateral prefrontal top-down control of amygdala reactivity — describing a brake that engages reliably enough to keep affect within workable bounds. This does not mean neurotypical people feel less; it means the loop from provocation to peak to recovery is comparatively well-governed, so frustration, anger or distress rise, do their signalling job, and subside. Reappraisal and other regulation strategies are available and, importantly, usable under load. The result is emotional weather rather than emotional emergency: real, sometimes strong, but rarely hijacking behaviour or leaving a long tail. It is the implicit benchmark against which the other three profiles’ reactivity and recovery are judged.

• The prefrontal–amygdala regulation loop (Ochsner & Gross reappraisal work) supplies a reliably-engaging brake on limbic arousal.

• Reactions are broadly proportionate and recovery to baseline is timely — affect signals without hijacking.

• Regulation strategies remain accessible under load, so intensity rarely outruns control.

ADHD. Emotional dysregulation is now understood as core to ADHD rather than incidental: responses tend to be fast and intense, frustration tolerance is low, and down-regulation is difficult — the same weak inhibitory brake and impulsive gain-control that drive the motor and cognitive symptoms operate on affect too. Barkley has long argued that deficient emotional self-regulation belongs in the conceptual heart of ADHD, and Shaw and colleagues’ 2014 review (”Emotion dysregulation in ADHD”) consolidated the evidence that it is prevalent, impairing and mechanistically tied to the disorder’s fronto-limbic and reward circuitry. Rejection-sensitive dysphoria — extreme, rapid emotional pain in response to perceived criticism or rejection — is a widely-reported clinical descriptor here, though it is a lived-experience construct rather than a formally validated diagnostic entity, and that caveat is worth keeping. The recovery problem compounds the intensity one: quick to spike, slow and effortful to climb down. Emotion in ADHD is thus not a comorbid extra but part of the same impulsive, under-braked control signature. The evidence for its centrality is now strong; specific sub-constructs like RSD remain more clinically described than empirically settled.

• Emotion dysregulation is core, not comorbid (Barkley; Shaw et al., 2014) — fast, intense affect and difficult down-regulation share the disorder’s under-braked control mechanism.

• Rejection-sensitive dysphoria is a widely-used clinical descriptor but a lived-experience construct, not a validated diagnostic category — cite it with that caveat.

• The recovery deficit matters as much as the intensity — quick to spike, slow and effortful to return to baseline.

Autism. Autistic emotional arousal often combines amygdala over-reactivity with altered relevance-detection: too many stimuli are tagged salient or threatening, so intense emotion is triggered more readily and recovery is slower. The intense-world / enhanced-perceptual-load theory (Markram & Markram) and neuroimaging of atypical amygdala response frame a system in which the salience filter is miscalibrated — the world arrives as more charged and less predictable than it should. This underpins the two characteristic overflow states: meltdowns, an outward over-arousal overflow when regulation capacity is exceeded, and shutdowns, an inward over-arousal withdrawal when the same ceiling is hit — both better read as involuntary responses to exceeded load than as behavioural choices. Alexithymia (see dimension 14) complicates regulation further: a state that is hard to identify is hard to reappraise or name-to-tame. Slower return to baseline means the after-effects of a strong episode persist. The literature is heterogeneous — amygdala findings vary with age, task and anxiety comorbidity — but the pattern of heightened reactivity, miscalibrated salience and effortful recovery recurs across accounts.

• Miscalibrated salience plus amygdala over-reactivity (intense-world framing) means more stimuli are tagged threatening, triggering intense affect more readily.

• Meltdowns (over-arousal overflow outward) and shutdowns (over-arousal withdrawal inward) are involuntary responses to exceeded capacity, not chosen behaviour.

• Alexithymia impedes regulation — hard-to-identify states resist reappraisal — and recovery to baseline is characteristically slow; amygdala findings themselves are heterogeneous across studies.

AuDHD. AuDHD emotional arousal tends to be highly volatile because it stacks two amplifiers: ADHD’s fast, intense, under-braked reactivity on top of autism’s over-reactive amygdala, miscalibrated salience and slow recovery. The combination raises both the probability and the cost of dysregulation — a quick, hard spike (ADHD) meets a system already primed to tag more as threatening and slow to climb down (autism), with meltdown and shutdown both live as end-states alongside RSD-type rejection pain. Alexithymia, more common on the autistic side, removes the naming step that might otherwise help either channel regulate. As with the other dimensions, direct AuDHD-specific measurement is scarce and the account is largely composed from the two source literatures plus consistent clinical and community report, so it should be read as a well-motivated synthesis rather than an independently established finding. What that synthesis predicts — and what lived report describes — is a wider dynamic range and a thinner regulatory margin than either profile alone. Intensity arrives fast, recovery comes slow, and the two rarely offset.

• Two amplifiers stack: ADHD fast/intense reactivity meets autistic amygdala over-reactivity and slow recovery, widening the emotional dynamic range.

• Meltdown, shutdown and RSD-type responses are all live end-states, with alexithymia removing the name-to-tame regulatory step.

• Largely combined-literature synthesis plus lived report rather than direct AuDHD measurement — treat the volatility claim as well-motivated, not independently settled.

16. Chronic load & the crash — cortisol, allostatic load, fatigue and burnout

What running on borrowed arousal costs over time — the bill for reaching baseline by way of stress.

Neurotypical. In the neurotypical system acute stress does its job and then switches off cleanly: the HPA axis mobilises cortisol to meet a demand, negative feedback terminates the response once the demand passes, and the body recovers — keeping chronic load low. This is the healthy allostasis McEwen described: stability maintained through change, with the stress response as a time-limited tool rather than a standing state. Sapolsky’s framing — the physiology that saves a zebra from a lion becomes harmful only when chronically switched on — captures why clean shut-off matters: the damage is a function of duration, not activation. Because the neurotypical brain generally reaches its operating baseline without chronically borrowing arousal from stress and urgency, it accrues comparatively little of the wear that follows repeated or unremitting activation. Recovery windows are real and used. The system’s default is mobilise-then-restore, and it is this clean reset — not the absence of stress — that distinguishes the neurotypical chronic-load profile from the three that follow. It is the benchmark against which “borrowed arousal” is a deviation.

• Healthy allostasis (McEwen): the HPA axis mobilises cortisol on demand and terminates cleanly via feedback, keeping chronic load low.

• Sapolsky’s point — stress physiology harms through duration, not activation — so the clean switch-off is the protective feature.

• Baseline is reached without chronically borrowing arousal from stress/urgency, so recovery windows genuinely restore.

ADHD. The ADHD brain often reaches functional baseline by borrowing arousal from stress and urgency — a loan, not income — and the interest on that loan is accumulating allostatic load, McEwen’s “pay now, pay more later” cost of repeatedly mobilising the stress system to do a regulatory job it was not meant to hold open. The mechanism that makes the loan ruinous is Arnsten’s: uncontrollable stress triggers catecholamine release that takes the prefrontal cortex offline (”loss of prefrontal cortical higher cognition”), degrading precisely the executive functions ADHD already runs short on — so the coping strategy erodes the faculty it depends on. ADHD burnout is plausibly mediated by this executive-function depletion, and there is emerging discussion of a hyper- to hypo-cortisol shift over chronic exposure, mirroring patterns seen in other chronic-stress conditions. The honest caveat belongs here: the individual links — allostatic load, PFC-offlining under stress, cortisol dysregulation — are each well-established as principle, but the full chain assembled into an ADHD-specific clinical burnout is supported by emerging evidence and coherent mechanism rather than by settled, replicated ADHD data. Cortisol findings in ADHD are notably mixed and moderator-heavy. The framework is strong; the ADHD-specific quantification is not yet in.

• Borrowed arousal accrues allostatic load (McEwen, “pay now, pay more later”) — reaching baseline via stress/urgency is a loan with compounding cost.

• Arnsten’s mechanism is the trap: uncontrollable stress takes the PFC offline, degrading the very executive functions ADHD already lacks, so the coping strategy consumes its own basis.

• Caveat: the chain to ADHD-specific clinical burnout is well-supported principle plus emerging evidence, not settled fact — ADHD cortisol data (including any hyper→hypo shift) remain thin and mixed.

Autism. The autistic chronic-load endpoint has a distinct, increasingly evidenced form: autistic burnout, characterised by Raymaker et al. (2020) as chronic exhaustion, loss of skills, and reduced tolerance to stimulus, arising from cumulative life stress and a sustained mismatch between expectations and capacity. What distinguishes it from ordinary burnout is the hidden tax of masking / camouflaging — the continuous effortful suppression of autistic traits to pass in neurotypical settings — which multiple studies (e.g. Hull and colleagues) link to worse mental-health outcomes, exhaustion and identity cost. Two features matter clinically: the skill loss can extend to previously reliable capacities such as speech, executive function or self-care, and — critically — rest alone does not fix it, because the driver is chronic mismatch and camouflage rather than simple fatigue. Raymaker’s is a community-partnered, largely qualitative characterisation; construct-validation and measurement work is still maturing, so autistic burnout is a strongly-described and widely-corroborated phenomenon rather than a fully operationalised clinical diagnosis. The mechanism — chronic stress plus expectation–ability gap plus the metabolic cost of masking — is coherent and consistently reported. Its remedy is load reduction and unmasking, not merely recovery time.

• Autistic burnout (Raymaker et al., 2020): chronic exhaustion, loss of skills and reduced stimulus tolerance from cumulative stress and expectation–ability mismatch.

• Masking/camouflaging is the hidden tax — correlated (Hull et al.) with worse mental health and exhaustion — and rest alone does not resolve it; load reduction and unmasking do.

• Still a community-partnered, largely qualitative construct with maturing measurement — well-corroborated phenomenon rather than a fully operationalised diagnosis.

AuDHD. AuDHD is consistently reported as the fastest to burnout, because it pays two bills at once: the double-bind of up-regulating one channel while down-regulating the other (dimension 13) means the reserve is spent simply holding two opposing arousal demands in balance, before any masking is added on top. Where ADHD borrows arousal from stress and autism pays the camouflage tax, AuDHD does both — manufacturing urgency to activate while suppressing sensory overload and masking traits — so allostatic load accumulates from several directions simultaneously and the recovery margin is thin. The predicted endpoint blends both source syndromes: executive-function depletion and PFC-offlining from the ADHD side, skill-loss and reduced stimulus tolerance from the autistic side, arriving sooner and reported as more severe. As throughout this intersection, direct AuDHD-specific research is scarce and the profiles are heterogeneous; this is synthesis from the two literatures plus a strikingly consistent lived-experience and clinical signal, not independently established epidemiology. The honest position mirrors the ADHD caveat above — mechanism and convergent report are strong, settled AuDHD burnout data are not yet available. What is consistent across accounts is severity: when the reserve is spent servicing contradictory demands, it empties first.

• The double-bind is the accelerant: up-regulating one channel while down-regulating the other spends reserve before masking is even added.

• Loads stack from both syndromes — borrowed arousal and camouflage tax — so allostatic accumulation is multi-source and the recovery margin thin; the endpoint blends EF-depletion with skill-loss.

• Consistently reported as the most severe and fastest crash, but this is combined-literature and lived-report synthesis, heterogeneous and not yet backed by settled AuDHD-specific data.

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Closing note

Sixteen dimensions, four brain types, one underlying curve. The recurring shape is this: the neurotypical brain sits near its arousal optimum with a flexible accelerator and brake, so most of these dimensions read as “well-regulated by default.” The ADHD brain sits below the optimum and spends its life importing arousal — through movement, novelty, urgency, reward-in-the-present — with a weak accelerator, a leaky filter, and a stop-signal that fails in both directions. The autistic brain (in its most common presentation) sits above the optimum for sensory input, with a low overload threshold, impaired habituation, a withdrawn brake, and a deep need for predictability. And the AuDHD brain carries both set-points at once — under-aroused on the task channel, over-aroused on the sensory channel — which is why single-lever strategies backfire and why it reaches the crash first.

Two disciplines hold the whole picture together. First, these are central tendencies, not verdicts on any individual — ADHD arousal is dysregulated rather than fixed-low, autism arousal is genuinely heterogeneous, and the honest move is always to measure the individual (skin conductance, HRV, pupillometry) rather than read them off the label. Second, the AuDHD column is the most inferential — a principled composition of two mature literatures plus a strikingly consistent lived-experience signal, awaiting the direct co-occurring studies that have barely been done. Where the evidence is strong it is named; where it is assembled it is flagged. The map is useful precisely because it tells you which side of the optimum a brain is starting from — and therefore which way its regulation needs to move.