Human express saccade makers are impaired at suppressing visually evoked saccades.

Abstract

1. We report the oculomotor behavior of human subjects who produce unusually high numbers (> 30%) of express saccades (latency range 85-135 ms) in the overlap saccade task, where express saccades are usually absent or small in number (< 15%). We refer to these subjects as "express saccade makers" (ES makers). 2. We tested the hypothesis that ES makers have difficulties in maintaining fixation and in suppressing unwanted saccades to a suddenly appearing peripheral target by comparing the performances of 10 ES makers and 10 control subjects in gap and overlap antisaccade tasks and in a memory-guided saccade task. 3. The ES makers produced between 35% and 95% incorrect saccades toward the stimulus (prosaccades) in the antisaccade tasks, compared with control subjects, who produced < 20%. Their correct antisaccades appeared to be normal. 4. We further tested the ability of ES makers to maintain fixation and to avoid reflexive saccades to the onset of a target in the memory-guided saccade task. ES makers tended to glance to the briefly presented cue in many trials (4 of them in 50-80% of the trials) instead of delaying the saccade until fixation point offset. Most of the inappropriate saccades had latencies in the range of express saccades. 5. These results can be associated with the finding of fixation related neurons in different cortical and subcortical brain regions (e.g., inferior-parietal and frontal cortex, basal ganglia, superior colliculus). The unusual number of express saccades made by the ES makers in the standard overlap and gap tasks, and their unwanted short-latency reflexive saccades to the target in the memory-guided saccade task, are reminiscent of the performance in these tasks of monkeys whose collicular fixation neurons were chemically deactivated. The collicular fixation neurons are probably the final common pathway in the control of active fixation, and are in mutual inhibitory relationship with the saccade cells. 6. The decreased saccadic control observed in the ES makers suggests that saccade execution in humans is also gated by a fixation system. These ES makers may have reduced voluntarily control over saccade generation as a result of a defect or poor development of their fixation system.