Binocular co‐ordination of human horizontal saccadic eye movements.

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Abstract
The binocular co-ordination of human horizontal saccades was analysed for the first time systematically over the full oculomotor range with a precise and accurate scleral sensor coil technique. Effects of amplitude (1.25-80 deg), direction (adduction vs. abduction and centrifugal vs. centripetal) and eccentricity (symmetrical about primary or between primary and secondary positions) were systematically investigated in three subjects. To minimize extraneous effects of stimulus presentation on the programming of saccades, subjects were instructed to voluntarily change their gaze between two continuously visible targets. These were positioned on an iso-vergence locus, and thus contained no stimulus for disjunctive eye movements. Under these conditions the amplitudes of the primary saccades of the two eyes were remarkably accurate; undershooting of the target by about 0.5 deg (independent of amplitude in the range 10-70 deg) was typical. This finding contrasts with the undershooting by about 10% described in the literature as characteristic for other stimulus conditions. Saccadic peak velocities saturated at a mean asymptotic levels of 502 .+-. 32 (S.D.) deg/s for saccades of 40 deg and larger. The duration was linearly related to amplitude for saccades up to 50 deg; up to 50 deg; for saccades of larger sizes the duration increased progressively more steeply. Skewness values (accelaration time as a fraction of total saccadic duration) decreased from about 0.45 for saccades up to 10 deg to about 0.20 for saccades of 50 deg and larger. Binocular saccades showed an abduction-adduction asymmetry and were not well yoked dynamically. The saccades of the abducting eye consistently had a larger size, a higher peak velocity, a shorter duration and were more skewed than the concomitant adducting saccades of the fellow eye. As a result in the eyes diverged transiently by as much as 3 deg during horizontal saccades. Saccades also showed a marked centrifugal-centripetal asymmetry. Peak velocities of saccades towards the primay position were about 10% higher than peak velocities of corresponding centrifugal saccades. These directional asymmetries were the main source of variability in the pool of saccades. In comparisons, intra- and intersubject variability was minor in our sample. Post-saccadic drift consisted of a vergence and a version component. The vergence component of this drift was a continuation of the vergence movement occurring during saccades. The version component, generally smaller than the vergence component, was directed towards the target position. The result of post-saccadic drift was the fovea of each eye was guided towards the target. The net post-saccadic drift of the abducting eye was smaller than that of the adducting eye, a result appropriate to reduce fixation errors remaining at saccadic offset. A tight relationship between skewness and saccade duration was not found. For instance, velocity profiles of centrifugal saccades were more skewed than those of centripetal saccades whose duration was equal. The main parameters of saccades were not different for binocular and monocular viewing; the degree of yoking, however, was decreased when the targets were viewed monocularly. We conclude that binocular horizontal saccades between continously visible stationary targets are accurate and highly reproducible within and between subjects. However, saccadic parameters are systematically affected not only by size, but also by direction and initial position.