A trained responder blends two systems. A goal-directed system presses only while the outcome is valued; a habit system presses automatically, whatever the outcome is now worth. Set how far the responder relies on habit, then devalue the outcome (feed to satiety). A goal-directed agent stops; a habitual one keeps going — and that residual responding is the laboratory signature of compulsion.
Reliance on the habit system (w)0.30
With habit weight w = 0.30 this responder is mixed control. After the outcome is devalued, responding falls to 0.30 — only the habitual share persists, so the devaluation reveals exactly how much of the behaviour had slipped out of goal-directed control.
The anxiety–compulsion cycle and ERP habituation
A trigger drives subjective distress (SUDS, 0–100) to a first-exposure peak of 80. Under response prevention the compulsion is withheld, the exposure runs to completion, and the peak decays across exposures as Pn = 80(1 − h)n. Turn prevention off and the compulsion is performed each time, aborting the exposure at its peak so distress never habituates.
Habituation fraction per exposure (h)0.25
With prevention and h = 0.25, peak distress runs 80 → 60 → 45 → 33.75… across exposures 0–3. Distress first halves at n = 2.41 (so by the third exposure). Each withheld compulsion teaches that the anxiety subsides on its own, breaking the negative-reinforcement contingency.
One axis, two poles
The dimensional model places impulsivity and compulsivity at opposite ends of a single under- to over-controlled axis. Slide a behaviour from one pole to the other and watch its drivers swap — reward-seeking for harm-avoidance, positive for negative reinforcement — while a shared failure of top-down control stays constant, which is why the "opposite" poles so often co-occur.
Position on the axisimpulsive 0%
At the mid-axis (mixed): the dominant driver is harm-avoidance, the action is rigid / cannot stop, and it is maintained by negative (relief) reinforcement (reward-seeking 50% vs harm-avoidance 50%). The shared failure of top-down cognitive control stays at ~60% the whole way across — the common substrate that lets a behaviour drift from one pole toward the other over the course of a disorder.