Figure 1.
The stop signal paradigm and accumulator model. (a) Stop signal paradigm. In GO trials (approximately 75%, Trial N), participants responded to the go signal (a circle) and had to withhold the response when they saw the stop signal (an X) in STOP trials (approximately 25%, Trial N+1). In both trials, the go signal appeared after a randomized time interval between 1 and 5 s (the fore‐period or FP, uniform distribution) following the appearance of the fixation point. The stop signal followed the go signal by a time delay—the SSD. The SSD was updated according to a staircase procedure, increasing or decreasing by 67 ms following a stop success or stop error trial, respectively. The ITI was 2 s. (b) Accumulator model for the stop signal task. GS: go success trial, in green; SS: stop success trial, in cyan; SE: stop error trial, in red; θ: threshold; Enc.Time1: encoding time after go signal onset; μ G: growth rate after go signal encoding; Enc.Time2: encoding time after stop signal onset; μ R: reduction rate after stop signal encoding; k: leakage; MT: movement time. (c) Simulation results for one participant, whose model fit reached a chi‐square of 0.1072. Green: GS trials; Red: SE trials; Cyan: average of SS trials; Blue line: threshold θ. GS and SE trials were binned at 50 ms. Dotted lines are the average of the GS or SE trials for each bin; solid lines are the average of all GS or SE trials. (d) Histogram of empirical (top panels) and simulated (bottom panels) reaction time distributions of GS and SE trials of the same participant. The median of empirical and simulated values of goRT are 566 ms and 542 ms; stop error reaction time (SERT): 496 ms and 486 ms; go response rate GS%: 98% and 98%; stop success rate SS%: 50% and 50%, respectively. y‐axis is the proportion of trials. [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com.]