The countermanding task revisited: mimicry of race models.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see The countermanding or stop-signal task involves making a response to a go stimulus , but stopping this response on a subset on trials when a stop signal occurs. As the delay between the go stimulus and the stop signal (stop-signal delay or SSD) increases , stopping becomes more difficult, because the go process has a larger head-start on the stop process. When subjects fail to stop on stop-signal trials, their responses tend to be faster than on trials without a stop signal. These results were explained with the independent race model (Logan and Cowan, 1984), which suggests that a go process, initiated at go stimulus onset, races independently against a stop process that is initiated at the stop signal. If the go process finishes first, subjects complete the go response; if stop finishes first, subjects stop. This model proposes that failed-stop reaction times (RTs) are faster than no-stop-signal RTs because only the fastest go responses escape inhibition. The independent race model (Logan and Cowan, 1984) is formulated very generally to capture finishing time distributions , without any commitment to the underlying neural processes. It allows computation of stop-signal reaction time (SSRT), which can be used to compare inhibitory control across age, patient groups, or experimental conditions. Recent work has attempted to specify the neural underpinnings of stopping. Boucher et al. (2007) proposed the " interactive race model " in which neural inhibition is the mechanism for stopping. Neurophys-iological data from the frontal eye fields and superior colliculus in monkeys performing a saccade stop task (Hanes et al., 1998; Paré and Hanes, 2003) and from premotor cortex of monkeys performing a manual stop task (Mirabella et al., 2011) demonstrated modulation of neural activity arising from a network of mutually inhibitory neurons. The interactive race model involves mutually inhibitory go and stop units. The go process is initiated by the go stimulus, there is a delay for go stimulus encoding, and then the go unit activates. Similarly, the stop process is initiated by the stop signal, there is a delay for stop stimulus encoding, and then the stop unit activates. Model fits …