Independent control of head and gaze movements during head-free pursuit inhumans

1. Head and gaze movements are usually highly co-ordinated. Here we demonst rate that under certain circumstances they can be controlled independently and we investigate the role of anticipatory activity in this process. 2. In experiment 1, subjects tracked, with head and eyes, a sinusoidally mo ving target. Overall, head and gaze trajectories were tightly coupled. From moment to moment, however, the trajectories could be very different and he ad movements were significantly more variable than gaze movements. 3. Predictive head and gaze responses can be elicited by repeated presentat ion of an intermittently illuminated, constant velocity target. In experime nt 2 this protocol elicited a build-up of anticipatory head and gaze veloci ty, in opposing directions, when subjects made head movements in the opposi te direction to target movement whilst maintaining gaze on target. 4. In experiment 3, head and gaze movements were completely uncoupled. Subj ects followed, with head and gaze, respectively, two targets moving at diff erent, harmonically unrelated frequencies. This was possible when both targ ets were visual, and also when gaze followed a visual target at one frequen cy whilst the head was oscillated in time with an auditory tone modulated a t the second frequency. 5. We conclude that these results provide evidence of a visuomotor predicti ve mechanism that continuously samples visual feedback information and stor es it such that it can be accessed by either the eye or the head to generat e anticipatory movements. This overcomes time delays in visuomotor processi ng and facilitates time-sharing of motor activities, making possible the pe rformance of two tasks simultaneously.