A critical SSD was computed for each participant that represents the time delay required for the participant to successfully withhold the response in half of the stop trials, following a maximum likelihood procedure. Briefly, SSDs across trials were grouped into runs, with each run being defined as a monotonically increasing or decreasing series. We derived a mid-run estimate by taking the middle SSD (or average of the two middle SSDs when there was an even number of SSDs) of every second run. The critical SSD was computed by taking the mean of all mid-run SSDs. It was reported that, except for experiments with a small number of trials (< 30), the mid-run measure was close to the maximum likelihood estimate of X50 (50% positive response; i.e., 50% stop success in the SST). The stop signal reaction time (SSRT) was computed for each participant by subtracting the critical SSD from the median go trial reaction time.
