Compensation for smooth eye and head movements by gaze saccades during head-unrestrained tracking
Pierre Daye, Gunnar Blohm, Philippe Lefèvre

Gaze orientation is generally performed using a combined eye-head movement. The programming and coordination of both motor systems during 2D gaze shifts is not yet established. In head fixed conditions, smooth eye movements that occur during the saccade latency are compensated if the nervous system has enough time to integrate eye velocity (Blohm & al., 2003, 2005 and 2006). We have previously shown (Daye & al., SFN2007) that a similar compensatory mechanism exists during gaze tracking in 2D. In this study, we differentiate the integration of eye movements from head movements occurring during the saccade latency period.

Subjects sat in front of a 1-m distant tangential screen. They were instructed to pursue with their eyes and head a sinusoidal target (Frequency [0.6 to 1.2 Hz]) moving along a straight line in 2D (Orientation [0 to 360°] and amplitude [20 to 25°] randomly chosen). 1.2-1.8s 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 cameras (200 Hz) and gaze orientation was reconstructed (Ronsse & al., 2007).

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 up to 150 ms before saccade onset (R : 0.94, p-value<0.001). Both the smooth eye (90%) and head displacement (75%) were used in programming the saccade. Consistent with these findings, additional analysis of the remaining error after multiple intervening saccades to the target revealed a slow compensation process for SGD.

In conclusion, we propose that the gaze control system use similar mechanism to program head restrained and head unrestrained saccades. Eye and head displacements during gaze latency were integrated to compensate for intervening eye and head movements.