SACCADE-VERGENCE TRAJECTORIES UNDER FREE-SPACE AND INSTRUMENT-SPACE ENVIRONMENTS

Purpose. The purpose of this study was to examine in detail the binocu lar fixation top-view trajectories of saccade-vergence responses to as ymmetrical targets, and to compare latency difference between saccade and vergence, under the free- and instrument-space viewing environment s. Methods. Binocular eye movements were recorded using the infrared r eflection technique in five visually-normal subjects. Responses were o btained for various asymmetrical target positions under both free- and instrument-space environments. Results. Four types of top-view trajec tories that represented normal variations in saccade and vergence cont rol were found: straight, overshoot, undershoot, and saccade-vergence. Also, it was found that under the instrument-space environment, there was a predominance of saccade-vergence trajectories and a scarcity of overshoot trajectories, whereas under the free-space environment, the re was a predominance of overshoot trajectories, and a scarcity of sac cade-vergence trajectories. Further, under the instrument-space enviro nment, latency was significantly longer for saccade than vergence (35. 9 +/- 15.7 msec; t = 5.1, degrees of freedom (df) = 4, P < 0.01), wher eas under the free-space environment, there was no latency difference (-10.5 +/- 14.8 msec; t = -1.6, df = 4, P > 0.05). Conclusions. The di fferences in response profiles under the two viewing environments coul d be accounted for by differences in timing of saccade and vergence on set. Moreover, in contrast to some recent investigations that suggest higher center control of individual trajectories, which was dependent on the naturalistic scene, these trajectories could be accounted for b y known neural and oculomotor mechanisms, with the higher centers usin g a priori information about spatial location of the target, to assist in the synchrony of saccade and vergence onset under the free-space e nvironment.