Compensation for smooth eye and head movements by gaze saccades during head-unrestrained tracking

Abstract

Gaze orientation is generally performed using combined eye and head movements. It has been shown in head fixed conditions that smooth eye movements occurring during the saccade latency period are compensated for if the brain has enough time to integrate eye velocity [2,3,4]. Here, we asked whether this was the same when the head was free to move. Subjects sat in front of a 1-m distant tangential screen. They were instructed to pursue a sinusoidal target (Frequency [0.6 to 1.2Hz]) moving along a straight line in 2D (Orientation [0 to 360°] and amplitude [20 to 25°] randomly chosen). 2.2-3.8s after target motion onset, a second target was briefly flashed at a random position on the screen. The position of both eyes was recorded by a video-based recording device (200 Hz), head position was recorded by active infrared marker tracking (200 Hz) and gaze orientation was reconstructed [5]. We analyzed how the orienting gaze shift towards the flash was programmed and how these saccades compensated for the smooth gaze displacement [SGD] during the latency period. Multiple regression analysis showed that gaze saccades were programmed using position error at flash time [PE] and an estimation of SGD. Both the smooth eye (90%) and head displacement (75%) were used in programming the saccade. In conclusion, we propose that the gaze control system uses a similar mechanism to program head restrained and head unrestrained saccades. Eye and head displacements during the saccade latency period were integrated to compensate for intervening eye and head movements. © 2009 Springer Berlin Heidelberg.