Evidence for separate eye and head position command signals in unrestrained rats

Compensatory horizontal eye-head movements of unrestrained rats were recorded with search coils in a magnetic field in response to combined optokinetic plus vestibular sinusoidal oscillations (0.05-1 Hz). The velocity contribution of compensatory slow head movements for image stabilization was relatively small (about 30%). The beating field of ocular nystagmus shifted during each half cycle in quick phase direction. These changes in eye position were counterbalanced by concomitant changes in head position. As a result, the orientation of gaze position was kept straight ahead with respect to the body length axis. These results imply independent and task-specific recruitment orders for the ocular and neck motor system.