Alcohol Affects Neuronal Substrates Of Response Inhibition But Not ...

Visual Stop-signal Task (VSST).

Two versions of the VSST were used, modelled on the work by Boehler and colleagues and Sharp and colleagues [34,35,39]. The two versions differed only in the instructions given to participants. In the more classic variant of the task (VSST), participants were instructed to respond to the direction of a frequently occurring green arrow (Go-stimulus) as quickly and as accurately as possible, and to be as accurate as possible at withholding their response when the green arrow turned red (Stop-stimulus). In the “Control” (VSST_C) variant, participants were told to respond to the direction of the Go-stimulus, again as quickly and as accurately as possible, but to carry on, and not withhold their response if it was followed by a Stop-stimulus.

In both variants, each trial began with the presentation of a central fixation cross on a grey background for a jittered duration of 1200-1500ms. The stimulus-display that followed, always began with the presentation of the Go-stimulus on a grey background, which either remained on screen for a total stimulus-display duration (TSD) of 800ms (Go-trials), or was replaced, after a variable stimulus onset asynchrony (SOA), by the Stop-stimulus (Stop-trials; see Figure 1).

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Figure 1. Visual Stop-signal task (VSST): Participants responded to the direction of a central green arrow (“Go-stimulus”), and withheld their response when the arrow turned red (“Stop-stimulus”) in the classic variant of the task.

In the control variant, participants were told to respond as they would normally to the direction of the “Go-stimulus” irrespective of whether it was followed by a “Stop-stimulus”. In both variants of the VSST, each trial began with the presentation of a fixation cross of varied duration. On “Stop-trials”, the “Go-stimulus” was replaced, following a varied Stimulus Onset Asynchrony (SOA), by the “Stop-stimulus”. In the classic variant, the SOA increased or decreased by 50msec as a function of whether participants were correct or not, respectively, at inhibiting their response to the “Stop-stimulus”. In the control variant, the SOA also varied in 50msec increments, but randomly. The total stimulus-display duration (TSD) was 800ms.

https://doi.org/10.1371/journal.pone.0076649.g001

In the VSST, the initial SOA was 200ms and increased by 50ms following successful Stop-trials (i.e. Stop-trials in which participants successfully inhibited their response; SStop), or decreased by 50ms following unsuccessful Stop-trials (i.e. Stop-trials in which participants were not able to inhibit their response; UStop). This staircase procedure resulted in an even number of SStops and UStops [54]. Stop-signal reaction time (SSRT) was calculated by subtracting the mean SOA from the average reaction time to correct Go-trials [55]. Further dependent measures from the VSST included Go-latency (Go_LAT) and Go-accuracy (Go_ACC). Participants completed a total of 160 trials of the VSST (120 Go-trials and 40 Stop-trials) in 8 minutes.

For each participant, in each group, the VSST was always followed by the VSST_C. Here, the initial SOA was the SOA-value from the final VSST Stop-trial. In order to equate the two variants, the SOA of the VSST_C Stop-trials also varied by 50ms steps, but in a random one-up/one-down fashion. Dependent measures included Go-latency (Go_LAT_C), Go-accuracy (Go_ACC_C), Stop-latency (Stop_LAT_C) and Stop-accuracy (Stop_ACC_C). Participants completed a total of 80 trials of the VSST_C (60 Go-trials and 20 Stop-trials) in 4 minutes.

In the initial baseline/task familiarisation session participants completed a baseline run of the classic VSST in order to make sure that the groups were matched on VSST performance prior to the alcohol manipulation.