The functioning of the prefrontal cortex (PFC) is thought to underlie our ability to modulate thoughts, behaviors, and emotions according to our goals or plans. A critical feature of such regulation is the capability to inhibit actions when they are no longer appropriate; for example, stopping yourself from picking up a cup of tea when you realize it is hot. In laboratory experiments, inhibitory control is usually operationalized as the inhibition of an impending motor response, typically in reaction to an external signal (Aron et al., 2014). A common laboratory test is the stop-signal task (Verbruggen and Logan, 2009), in which people make key presses in response to “Go” signals. In a minority of trials, a “Stop” signal is given after the Go signal and participants must attempt to inhibit their impending response. Unlike the latency of Go responses, the latency of stopping cannot be directly observed as successful inhibition results in the absence of a key press. However, this latency can be estimated by varying the interval between the Go and Stop signals, and titrating it until the probability of successfully stopping is 0.5. At this interval, it is assumed that the “race” to completion between the putative Go and Stop processes, initiated by the Go/Stop signals and ending in the generation/inhibition of a response, is on average a tie (Verbruggen and Logan, 2009). This implies that on average the two processes end at a similar time. It then follows that one can estimate the time taken to inhibit a response (stop signal reaction time, SSRT) by subtracting the time of the Stop signal (start of the Stop process) from the average duration of the Go process (from the Go signal to the time of the response, go reaction time).

Functional magnetic resonance imaging (fMRI) research has shown that PFC regions, including the right inferior frontal cortex (rIFC), are more strongly activated during Stop versus Go trials (Garavan et al., 1999; Aron and Poldrack, 2006). This has prompted much research attempting to establish a role for PFC in stopping and has implicated a putative fronto-basal-ganglia–thalamocortical network in such reactive response inhibition (Aron et al., 2014). Although those early fMRI studies also showed that parietal cortex (eg, angular gyrus and temporoparietal junction, TPJ) was strongly activated during response inhibition, the possibility of these areas playing a role in stopping has been largely overlooked. In a recent article in theJournal of Neuroscience, Osada and colleagues examined