Stop Signal Task

The Stop Signal Task is a laboratory task assessing inhibitory control. In the task, participants were seated in a comfortable desk chair behind a personal computer of which the screen was placed at approximately 60 cm distance. The experimenter was seated next to the participant and held track of performance through a hand computer. The participant held a button box in each hand and was instructed to press buttons with the thumb. The task consisted of 64 practice trials and 256 experimental trials, of which 64 stop-trials and 192 go-trials. On both go and stop-trials, participants saw a fixation point presented for 500 ms. The fixation point was followed by a go-stimulus (a picture of an airplane) that was displayed for 1,000 ms and that was presented in the center of the screen. In response to the go-stimulus, participants were required to press a response button that corresponded to the direction the plane was flying in (left or right). On stop-trials, a white cross was superimposed on the go-stimulus and acted as stop-signal. Participants were instructed not to press any button when a trial contained a stop-signal. Trials were presented in a semi-random fixed order. The longer the delay between go-signal (plane) and stop-signal (white cross; the stop-signal-delay, SSD), the more difficult it was to inhibit the response. To ensure that the percentage of inhibited responses approached 50% for each individual, SSD was systematically varied. If participants inhibited correctly, SSD lengthened by 50 ms; if participants failed to inhibit their response, SSD shortened by 50 ms (see Luman et al., 2004). In order to avoid non-responses or highly delayed responses, the experimenter provided instructions to react as fast as possible after an omission error and after four highly delayed responses on go-trials. Mean reaction time (xcorrect = MRT) over all correct go-trials, Stop Signal Reaction Time (SSRT) and the number of commission and omission errors were assessed.