Fig. 1.

a Stop signal task in experiments 1 and 2. The go and stop cues differed across the two experiments, but otherwise the task was the same. Clicking on the home pad resulted in the presentation of two potential targets (white squares). After a delay period lasting 1–1.5 s, the go signal was presented for 1.4 s, which was the amount of time participants were allowed to make a response. In experiment 1, the go signals involved one of the target squares changing color from white to green. In stop trials, a stop signal appeared at a variable delay (stop signal delay, SSD), and in experiment 1 the stop signal was a black square that appeared between the two targets. In experiment 2, we changed the go signal so that the appropriate target was indicated by a letter appearing between the two targets, ‘T’ indicated left and ‘X’ indicated right. The purpose was to slow down response times in the primary go task to minimize the chance of the stop signal delay hitting a floor (i.e., the minimum SSD of 33.3 ms). In experiment 2, the stop signal was changed to a red circle, to accommodate the new go signal. b Schematic of response time criteria for go trials and stopping criteria for the home and target staircases on stop trials. Reaction time reflects the time between the go signal and the cursor leaving the home pad. Total response time reflects the time between the go signal and the cursor entering the target. Two different definitions of stop success and failure were used to adjust two different stop signal delay tracking staircases. For the home staircase, a stop was deemed successful if the cursor remained within the home pad, whereas for the target staircase, stopping was successful if the cursor did not enter the target. c Exemplar movement trajectories across different trial types for a participant in experiment 1. Black squares represent the left and right targets. Colored lines represent traces from individual trials shown from the time the cursor left the home pad to when the cursor entered the respective target (Go trials), and to the time of the maximum resultant XY displacement (i.e., when the movement came to a halt; failed stop and successful stop trials). Axes represent the position of the cursor with respect the center of the home pad in the x-plane and the top of the home pad in the y-plane. Units are pixels. Note that the responses could be stopped at stage right up to the point of reaching the target (see target staircase), which illustrates the lack of a ballistic stage in responding (de Jong et al., 1990; McGarry & Franks, 1997)