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A "Gap Effect" on Stop Signal Reaction Times in a Human Saccadic Countermanding Task

¹Canadian Institute of Health Research Group in Action and Perception, ²Graduate Program in Neuroscience, ³Departments of Physiology and Pharmacology and ⁴Psychology, University of Western Ontario, London, Ontario, Canada

Submitted 7 August 2008; accepted in final form 12 November 2008

The "gap effect" describes a phenomenon whereby saccadic reaction times are expedited by the removal of a visible fixation point prior to target presentation. Here we investigated whether processes controlling saccade cancellation are also subjected to a gap effect. Human subjects performed a countermanding experiment that required them to try to cancel an impending saccade in the presence of an imperative visual stop signal, across different fixation conditions. We found that saccadic cancellation latencies, estimated via derivation of the stop signal reaction time (SSRT), were 40 ms shorter on trials with a 200-ms gap between fixation point removal and target presentation compared with when the fixation point remained illuminated. Follow-up experiments confirmed that the reduction in SSRTs were primarily due to removal of a foveal fixation point (as opposed to a generalized warning effect) and persisted with an auditory stop signal that controlled for potential differences in stop signal saliency across different fixation conditions. Saccadic RTs exhibited a gap effect in all experiments with reductions in RTs being due to both removal of a foveal fixation point and a generalized warning effect. Overall, our results demonstrate that processes controlling saccade cancellation can be expedited by a 200-ms gap. The simultaneous priming of both saccade cancellation and generation is of particular interest considering the mutually antagonistic relationship between the saccade fixation and generation networks in the oculomotor system.