What neural pathways mediate express saccades?

Conditions that alter saccadic eye movement latencies and affect target choice to visual stimuli and to electrical stimulation of area V1 in the monkey

In this study, we examined procedures that alter saccadic latencies and target selection to visual stimuli and electrical stimulation of area V1 in the monkey. It has been shown that saccadic eye movement latencies to singly presented visual targets form a bimodal distribution when the fixation spot is turned off a number of milliseconds prior to the appearance of the target (the gap period); the first mode has been termed express saccades and the second regular saccades. When the termination of the fixation spot is coincident with the appearance of the target (0 ms gap), express saccades are rarely generated. We show here that a bimodal distribution of saccadic latencies can also be obtained when an array of visual stimuli is presented prior to the appearance of the visual target, provided the elements of the array overlap spatially with the visual target. The overall latency of the saccadic eye movements elicited by electrical stimulation of area V1 is significantly shortened both when a gap is introduced between the termination of the fixation spot and the stimulation and when an array is presented. However, under these conditions, the distribution of saccadic latencies is unimodal. When two visual targets are presented after the fixation spot, introducing a gap has no effect on which target is chosen. By contrast, when electrical stimulation is paired with a visual target, introducing a gap greatly increases the frequency with which the electrical stimulation site is chosen.

The effects of visual scene composition on the latency of saccadic eye movements of the rhesus monkey

This study examined how variations in the visual scene affect the generation of bimodal saccadic latency distributions, the first mode of which is called the population of "express saccades". The surface media used to make stimuli visible and the composition of the background were varied to determine the conditions under which express saccades can be generated in rhesus monkeys. The results show that express saccades to singly presented targets can readily be elicited when the stimuli are made visible by virtue of either luminance contrast, color contrast or motion cues. Express saccades are rarely obtained when stimuli are made visible by virtue of only stereoscopic depth or texture cues. Express saccades can, however, be elicited using random-dot stereograms or textures when luminance or chrominance information is added to the target. When single target stimuli are presented simultaneously with a set of non-target stimuli, express saccades are for the most part prevented unless either the non-target stimuli are near threshold or their numerosity is very high, in which case they form a texture-like array. However, when the non-target stimuli are continuously present in the display, express saccades reemerge. These findings suggest that express saccades are not unique to experimental situations in which only a single stimulus appears on an otherwise homogeneous surface; they can readily be generated as long as the target stimulus is made visible by virtue of luminance, chrominance, motion or a combination of more than one surface medium and as long as the target does not appear concurrently with a salient group of other non-target stimuli.

Express saccades elicited during visual scan in the monkey

Monkeys trained to saccade to visual targets can develop separate "express" and "regular" modes in their distribution of saccadic latencies. The purpose of this study was to determine whether this occurs under more natural viewing conditions, when targets are suddenly presented in a structured visual field during visual scan. It was found that scanning saccades stopped appearing 60 msec after a target's onset, and subsequent saccades, which were directed toward the suddenly appearing target, had a bimodal distribution of latencies. Express saccades were more likely to occur as the target was presented later in a fixation. Regular mode saccades were more likely to occur with longer target durations. Scanning saccades made to stimuli of the structured visual field always had unimodal inter-saccadic interval distributions. All these effects were apparent after only 2-3 days of training. These findings, taken together with recent physiological results, suggest that the visuomotor cells of the superior colliculus mediate latency bimodality.

The effects of lateral geniculate nucleus, area V4, and middle temporal (MT) lesions on visually guided eye movements

Visually guided saccadic eye movements to singly presented stationary targets form a bimodal distribution. After superior colliculus lesions, the so called "express saccades" that form the first mode of the distribution are no longer obtained. The aim of this study was to determine what role several other neural systems play in the generation of express and regular saccades, with the latter being those that form the second mode in the bimodal distribution. Lesions were made in the parvocellular and magnocellular portions of the lateral geniculate nucleus to disrupt either the midget system or the parasol system that originates in the retina and areas V4 and MT. The effects of the lesions were examined on the accuracy and latency of saccadic eye movements made to stationary and to moving visual targets. Following magnocellular and MT lesions deficits were observed in smooth pursuit and in the amplitude of saccades made to moving targets. However, none of the lesions produced significant changes in the bimodal distribution of saccadic latencies to stationary targets. The results suggest that express saccades and regular saccades are not selectively mediated by either the midget or the parasol systems or by areas V4 and MT. Neither are the frontal eye fields involved as had previously been shown. We suggest that the superior colliculus plays a central role in producing both express and regular saccades by virtue of highly convergent input from numerous cortical structures.