Inhibition of Saccadic Eye Movements by vergence stimuli

Abstract

Abstract: : Purpose:In children with a central suppression scotoma eye–movements in response to step disparity vergence stimuli include a combination of saccadic and vergence components. These responses may be due to the fact that some portions of the visual stimuli are seen binocularly (vergence stimuli) and others are seen monocularly (saccadic stimuli). To understand the observed responses, eye–movements to visual stimuli with both vergence and saccadic components were studied. Methods:Six subjects with normal binocular vision (16–33 years) viewed stereoscopic visual stimuli that were displayed on a computer screen through a pair of liquid crystal shutter glasses. Eye–movements were recorded using a binocular head mounted eye tracker. Disparity vergence stimuli were presented at five different retinal eccentricities simultaneously with a fixed central monocular saccadic stimulus. Pure vergence stimuli and pure monocular saccadic stimuli were used as control stimuli. Results: The rates of responses with a saccadic component increased significantly (p = 0.0338) with the eccentricity of vergence stimuli. In combined vergence saccadic responses, vergence appeared first at a latency of 142 ± 40 milliseconds. The saccadic component appeared later at a latency of 368 ± 107 milliseconds. The mean latency of saccades in combined vergence–saccades was significantly longer (p < 0.001) than the latency of saccades (277 ± 56 milliseconds) to pure monocular saccadic stimuli. No significant differences (p= 0.708) were demonstrated between the mean latency of the vergence component in combined vergence–saccadic responses and the mean latency of vergence to pure disparity vergence stimuli (136 ± 37 milliseconds). Eye–movement responses without a saccadic component had significantly higher (p < 0.001) initial vergence amplitudes (recorded 150 milliseconds following the initiation of vergence) compared to combined vergence–saccadic responses. Conclusions:When the saccadic and vergence control systems are stimulated simultaneously, the response of the saccadic system can be delayed or even inhibited by the vergence response. The probability of triggering a saccade following the initiation of vergence is affected by the amplitude of the vergence response and its ability to compensate for retinal errors induced by the visual stimulus. The reduced capacity of the vergence control system to inhibit the saccadic response may explain the inability to develop symmetric vergence eye–movements in children with a central suppression scotoma.

Keywords: vergence • strabismus • eye movements: conjugate