Strategic control over saccadic eye movements: Studies of the fixation offset effect

dWe studied the strategic (presumably cortical) control of ocular fixation in experiments that measured the fixation offset effect (FOE) while manipul ating readiness to make reflexive or voluntary eye movements. The visual gr asp reflex, which generates reflexive saccades to peripheral visual signals , reflects an opponent process in the superior colliculus (SC) between fixa tion cells at the rostral pale, whose activity helps maintain ocular positi on and increases when a stimulus is present at fixation, and movement cells , which generate saccades and are inhibited by rostral fixation neurons. Vo luntary eye movements are controlled by movement and fixation cells in the frontal eye field (FEF). The FOE-a decrease in saccade latency when the fix ation stimulus is extinguished-has been shown to reflect activity in the co llicular eye movement circuitry and also to have an activity correlate in t he FEF. Our manipulation of preparatory set to make reflexive or voluntary eye movements showed that when reflexive saccades were frequent and volunta ry saccades were infrequent, the FOE was attenuated only for reflexive sacc ades. When voluntary saccades were frequent and reflexive saccades were inf requent, the FOE was attenuated only for voluntary saccades. We conclude th at cortical processes related to task strategy are able to decrease fixatio n neuron activity even in the presence of a fixation stimulus, resulting in a smaller FOE. The dissociation in the effects of a fixation stimulus on r eflexive and voluntary saccade latencies under the same strategic set sugge sts that the FOEs for these two types of eye movements may reflect a change in cellular activity in different neural structures, perhaps in the SC for reflexive saccades and in the FEF for voluntary saccades.