Gap Effect in Reflexive and Intentional Prosaccades

The analysis of saccadic eye movements has assumed an important role as a pathophysiological approach in neuropsychiatric diseases. However, before abnormal cognitive saccade behavior can be fully understood, some basic psychophysiological aspects have to be further investigated. Previous studies have demonstrated a shortening of saccade latency when the fixation stimulus was removed prior to the appearance of a peripheral visual target. This gap effect is smaller for antisaccades (AS; away from the target) than for reflexive saccades (RS; towards the target). Apart from the generation of an intentional eye movement, AS rely on different processes including reflex inhibition and spatial reversal of the saccade direction. We wondered whether intentional prosaccades (IpS) that do not rely on these additional processes also show a reduced gap effect. Therefore, we compared the gap effect in a saccade paradigm for RS and IpS in healthy subjects. We found a smaller gap effect in IpS than in RS and suggest that this reduction reflects endogenous strategic processes. Our findings further imply that reflex inhibition is not the unique cause for the smaller gap effect in AS than in RS and suggest that an endogenous component may be involved. We hypothesize that frontal control on the superior colliculus fixation cell activity may continue even after the fixation stimulus has disappeared, thus inhibiting a fixation offset and reducing the gap effect.