Stop Signal Task

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.