The stop-signal task (i.e., STOP-IT; Verbruggen, Logan, & Stevens, 2008) was administered to measure response inhibition. An initial

practice session of 32 trials was followed by an experimental phase of four blocks of 64 trials. Each trial began with a 250 ms fixation cross, followed by a circle or square. Participants were asked to press a corresponding “circle” or “square” key, as appropriate. After the participant responded, or 1250 ms had elapsed, the shape disappeared, followed by a 2 s inter-trial interval. selleck compound A 10 s interval separated blocks. Participants were urged to respond as quickly as possible on all trials. However, on 25% of the trials a stop-signal tone (750 Hz, 75 ms) sounded shortly after the shape appeared indicating that participants should withhold their response. At the beginning of the session, the stop signal was delivered at a 250 ms delay after the shape appeared. This stop-signal delay (SSD) was adjusted across trials using an adaptive tracking procedure. When a response was withheld correctly on a stop-signal trial the SSD increased by 50 ms, making it more difficult to withhold their response on the next stop trial; upon failing to withhold their response on a stop trial the SSD decreased

by 50 ms, making it easier to withhold their response. The critical measure in the stop-signal task is stop-signal reaction time (SSRT), which estimates the time it takes to stop an ongoing response. A participant’s SSRT is calculated by subtracting their mean

SSD from their mean RT on go trials. A fast SSRT indicates this website that participants can stop their response quickly, whereas a slow SSRT indicates that participants need additional time to stop. Because of the way in which the STOP-IT program is designed, valid estimates of SSRT can only be obtained when a subject successfully withholds their response on approximately half of the stop-signal trials (Verbruggen et al., 2008). Although the program was designed to ensure that subjects succeed on approximately Thymidine kinase 50% of the trials by dynamically adjusting the SSD in response to each subject’s performance, nine subjects deviated significantly from the 50% criterion, thus precluding valid estimates of SSRT (the criterion range was predetermined by recommendations from Verbruggen et al., 2008). Most of these subjects did not follow the STOP-IT instructions, waiting for the stop signal to sound instead of responding as quickly as possible on each trial. Fortunately, three of these subjects successfully completed STOP-IT in an unrelated experiment, so we were able to use the SSRTs from that study. The remaining six participants, however, had to be excluded. One further subject was removed because they had trouble understanding the STOP-IT task and because their SSRT was 3.4 SDs from the mean. Altogether, data from 125 of the 132 subjects were included.