Eye and head movements during vestibular stimulation in the alert rabbit.

Rabbits passively oscillated in the horizontal plane with a free hand tended to stabilize their head in space (re: earth-fixed surroundings) by moving the head on the trunk (neck angular deviation, NAD) opposite the passively imposed body rotation. The gain (NAD/body rotation) of head stabilization varied from 0.0 to 0.95 (nearly perfect stability) and was most commonly above 0.5. Horizontal eye movement (HEM) was inversely proportional to head-in-space stability, i.e. the gaze (sum of HEM, NAD, and body rotation) was stable in space (regardless of the gain of head stabilization). When the head was fixed to the rotating platform, attempted head movements (head torque) mimicked eye movements in both the slow and fast phases of vestibular nystagmus; tonic eye position was also accompanied by conjugate shifts in tonic head torque. Thus, while eye and head movements may at times be linked, that the slow eye and head movements vary inversely during vestibular stimulation with a free head indicates that the linkage is not rigid. Absence of a textured stationary visual field consistently produced a response termed 'visual inattentiveness,' which was characterized by, among other things, a reduction of head and gaze stability in space. This behavioral response could also be reproduced in a subject allowed vision during prolonged vestibular stimulation in the absence of other environmental stimuli. It is suggested that rabbits optimize gaze stability (re: stationary surroundings), with the head contributing variably, as long as the animal is attending to its surroundings.