The influence of initial eye position on motor commands for the control of saccadic eye movements: Behavioral and neurophysiological observations
Ellen J. Barton · Scholarly Commons (University of Pennsylvania) · 2000
The intent of this research was to further the understanding of the influence of starting eye position on saccadic eye movements and on the concomitant neural discharge. The effect of initial eye position on remembered and visually-guided saccades was examined in rhesus monkeys. A pilot study was also conducted in humans. Unlike visually-guided saccades, remembered saccades were markedly influenced by starting position. For the monkeys, orbital position affected remembered saccades to horizontal or vertical targets to a striking degree. The amplitude or direction of the movement changed depending on whether the starting position shifted in the direction of the target or the orthogonal direction, respectively. The curvature of visually-guided saccades displayed a similar pattern of initial position influence on movement direction. The human data indicated a similar effect of starting position on the amplitude of remembered saccades as the monkey data. However, the humans did not exhibit the influence of starting position on movement direction that the monkeys showed. To examine the source of these errors, single-unit recordings were done in the region of the MVN in monkey. The cells linearly coded the deviation in direction of remembered saccades caused by shifts in starting position. A modest amount of neural data suggested that the MVN also encode the errors in amplitude. In the process of examining the MVN tonic discharge for visually-guided saccades, some unexpected observations occurred. The tonic activity for eye positions achieved by visually-guided saccades was affected not only by the final eye positions occurring along one cardinal axis, but also by final positions along the orthogonal axis, with oblique movements to a particular quadrant producing the highest discharge. Furthermore, the path the visually-guided saccades took to the final position modulated the discharge. Preliminary recording studies were done in the SC and indicated that the SC doesn't produce the errors of remembered saccades caused by shifts in the vertical starting position, but rather sends an accurate command. The effects of initial eye position were unanticipated and should be considered during neural studies of remembered saccades, and investigations of the compensation for the elastic restoring forces of the plant.