Correction of Saccade-Induced Midline Errors in Responses to Pure Disparity Vergence Stimuli.

John L Semmlow,Tara L Alvarez,Yung-Fu Chen,Bérangère Granger-Donnetti

doi:10.16910/jemr.2.5.1

Abstract

Purely symmetrical vergence stimuli aligned along the midline (cyclopean axis) require only a pure vergence response. Yet, in most responses saccades are observed and these saccades must either produce an error in the desired midline response or correct an error produced by asymmetry in the vergence response. A previous study (Semmlow, et al. 2008) has shown that the first saccade to appear in a response to a pure vergence stimulus usually increased the deviation from the midline, although all subjects (N = 12) had some responses where the initial saccade corrected a vergence induced midline error. This study focuses on those responses where the initial saccade produces an increased midline deviation and the resultant compensation that ultimately brings the eyes to the correct binocular position. This correction is accomplished by a higher level compensatory mechanism that uses offsetting asymmetrical vergence and/or corrective saccades. While responses consist of a mixture of the two compensatory mechanisms, the dominant mechanism is subject-dependent. Since fixation errors are quite small (minutes of arc), some feedback controlled physiological process involving smooth eye movements, and possibly saccades, must move the eyes to reduce binocular error to fixation disparity levels.

Highlights

Pure vergence stimuli can be constructed in the laboratory and are useful for the study of disparity vergence eye movements
The goal of this paper is to describe the mechanism used for returning the eyes to midline binocular fixation after an initial saccade has produced a midline error
There is still some residual error at the end of the second saccade and it is eventually corrected by a combination of asymmetrical vergence and other saccades, Figure 2B and 2C, respectively

Summary

Introduction

Pure vergence stimuli can be constructed in the laboratory and are useful for the study of disparity vergence eye movements. Such stimuli can be presented using a stereo pair of images moving in equal and opposite directions or by two targets placed at different depths along the midline (i.e., mid-sagittal or cyclopean axis). In the latter case, an accommodative (i.e., blurdriven) stimulus may drive the vergence response. Pure vergence responses would follow along the mid-sagittal plane with no version component to the response.

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Conclusion