The effect of visual competition on saccadic behavior in a stop signal task

Visual inhibition paradigms, such as the stop-signal task, often assume a unitary model of inhibition. While useful for gathering quantitative data on disorders like ADHD and schizophrenia, this oversimplifies inhibitory processes, which previous studies have shown to be distinct between and within go/no-go, Stroop, stop-signal and stop-change tasks (Boecker 2013 et al., Raud et al. 2019, Verbruggen et al. 2004). Our study further investigates the possibility of distinct yet interacting mechanisms of inhibition during a saccadic stop-signal task by examining the effects of visual competition between a stop-signal and target. If stop signal reaction time (SSRT) measures the influences of not only “high-level” inhibition but also low-level dynamics in the oculomotor system, a visual stop signal at fixation will cause greater interference when a saccade target appears at a small eccentricity compared to a larger eccentricity due to competition. This greater interference was predicted to aid the stop-process and produce shorter SSRT values. Failed stop trials were also expected to have smaller saccade amplitudes and slower velocities than successful go trials due to this stop signal interference. To test this, two target eccentricity conditions (2.5 and 7.5 degrees) were intermixed in a stop-signal task, with a central visual stop signal matched in luminance to the target. The results did not show the predicted differences in SSRTs between eccentricity conditions; however, significant differences in the characteristics of saccades in terms of reduced amplitude and slower velocity on go versus failed-stop trials were aligned with predictions. These findings suggest that saccades remain susceptible to competition's influence, even when visual competition does not significantly alter the winner of the race between stop and go processes. These findings have important implications for how the specific neural subsystems involved in generating and inhibiting saccades interact.