VOLUNTARY BINOCULAR GAZE-SHIFTS IN THE PLANE OF REGARD - DYNAMICS OF VERSION AND VERGENCE

We studied the dynamics of voluntary, horizontal, binocular gaze-shift s between pairs of continuously visible, real three-dimensional target s. Subjects were stabilized on a biteboard to allow full control of ta rget angles, which were made to differ only in distance (pure vergence ), only in direction (pure version; conjugate saccades) or in both dis tance and direction (disjunctive saccades). A wide range of changes in vergence (0-25 deg) and version (0-65 deg) was recorded to study the dynamics of disjunctive saccades, described until now for limited rang es, throughout the horizontal oculomotor range within manual working s pace, and to study the velocity-duration-amplitude relations (''main s equence'') of disjunctive vs conjugate saccades. Pure vergence was alm ost never observed; divergence, especially, was always associated with saccades. Likewise, horizontal saccades were never strictly conjugate , they always contained a transient divergence-convergence sequence. T he amplitude and velocity of these transient components varied systema tically with saccadic size. In combined version-vergence movements, ve rgence was, in general, accelerated and shortened as a function of inc reasing version. This effect was fairly uniform for divergence, which appeared to increase in velocity by about as much as the transient pea k divergent velocity of the version saccade. The intrasaccadic fractio n of divergence increased from about 50% to close to 100% as a functio n of increasing version. For convergence, saccades up to about 20 deg were also accelerating; in this case it appeared as if the transient p eak convergent velocity of the version saccade was added to the basic convergence velocity. For larger saccades this effect was partly count eracted by the penetration of an initial divergence associated with th e saccade. This initial divergence delayed and slowed down convergence . The intrasaccadic fraction of convergence varied between about 40% a nd 70%. In disjunctive saccades the individual eyes did not follow the main-sequence parameters of conjugate saccades of comparable sizes, e xcept for the eye that moved with the combination ''abduction and dive rgence''. For all other combinations of vergence and version, disjunct ive saccades had lower peak velocities and longer durations than conju gate saccades. As a consequence, disjunctive version was also slower t han conjugate version. Thus, while version accelerates vergence, verge nce slows down version: in the generalized case of three-dimensional g aze-shifts, peak velocities and durations are in between those of the limiting cases of pure version and pure vergence. We conclude that, wi thin manual working space, binocular gaze-shifts are effected by the h ighly integrated action of conjugate and disjunctive mechanisms, both of which are expressed preferentially in fast, saccadic movements.