Wherefore a Horse Race: Inhibitory Control as Rational Decision Making

Abstract

Humans and animals often face the need to choose among actions with uncertain consequences, and to modify those choices according to ongoing sensory information and changing task demands. The requisite ability to dynamically modify or cancel planned actions, termed inhibitory control, is considered a fundamental element of flexible cognitive control. Experimentally, inhibitory control is often studied using the stop signal paradigm. In this task, subjects’ primary task is to perform detection or discrimination (two-alternative forced choice or 2AFC) on an imperative “go” stimulus. In a small fraction of trials, a stop signal appears after some delay, termed the stop signal delay (SSD), instructing the subject to withhold the go response. Characteristically, subjects’ ability to inhibit the go response decreases as the stop signal arrives later (see also chapters 7 and 11, this volume). The classical model for the stop signal task is the race model, which posits the behavioral output on each trial, go or stop, to be the outcome of two competing, independent go and stop processes, respectively. The go process has a finishing time with stochasticity assumed to be due to noise or other sources independent of the stop process. The stop process has a finishing time of SSD + SSRT, where the stop signal reaction time (SSRT) is a subject-specific stopping latency. A stop trial ends in an error response (stop error, SE) if the go process finishes before the stop process (go RT < SSD + SSRT) or a correct cancellation otherwise (go RT ≥ SSD + SSRT). Thus, the cumulative distribution of go RT, up to SSD + SSRT, determines the error rate at each SSD. Conversely, given the observed go RT distribution and inhibition function, as well as the experimentally imposed SSD, an estimate of SSRT can be formed for each subject. Importantly, it is assumed that the go finishing time distribution is identical across go and stop trials. Given the race model assumptions, the shorter the SSRT, the fewer error responses in stop trials. Consequently, SSRT is often seen as an index of inhibitory ability, and indeed appears to be longer in populations with presumed inhibitory deficits, such as attention-deficit hyperactivity disorder, substance abuse, and obsessive-compulsive disorder. 20