Figure 1.
A basal-ganglia model and the Stop-signal paradigm. A, An influential model proposes three pathways through the basal ganglia (direct, indirect, and hyperdirect). SNr, Substantia nigra; THAL, thalamus; STR, striatum. Open arrows are excitatory (glutamatergic); filled arrows are inhibitory (GABAergic). Figure adapted from Nambu et al. (2002). B, The Stop-signal paradigm consists of Go- and Stop-signal trials. On Go trials, the subject has 1 s (the hold period) to make a left or right button press in response to the stimulus. As soon as the subject responds (reaction time), the stimulus is replaced by a blank screen for a variable period of time (1 s − RT = jitter time, where jitter ranges between 0.5 and 4 s, mean of 1 s). On a Stop trial, a tone is played at some delay (SSD) after the arrow stimulus. If the response is inhibited, the arrow remains for 1 s, followed by the blank screen jitter period; if the subject does not inhibit (i.e., responds), then the timing is the same as the Go trial. SSD changes dynamically throughout the experiment to produce a 50% inhibition rate (see Materials and Methods). C, SSRT is estimated using the race model (Logan and Cowan, 1984). This assumes that Go and Stop processes are in a race and are independent of each other. The independence assumption implies that the distribution of Go processes on Stop trials (whether a response is made or not) is the same as the observed distribution of Go responses (when there is no Stop signal). On Stop trials, a tone occurs at some delay, the SSD after the Stop signal. If this delay is short, then P(inhibit) is high and this is likely to be a StopInhibit trial; if the delay is long, then P(inhibit) is low and this is likely to be a StopRespond trial. If SSD is varied so that P(inhibit)=0.5, then SSRT can be estimated by subtracting the SSD from the median value of the Go distribution.