THE ROLE OF COMPENSATORY EYE AND HEAD MOVEMENTS IN THE RAT FOR IMAGE STABILIZATION AND GAZE ORIENTATION

Compensatory horizontal eye movements of head restrained rats were com pared with compensatory horizontal eye-head movements of partially res trained rats (head movements limited to the horizontal plane). Respons es were evoked by constant velocity optokinetic and vestibular stimuli (10-60-degrees/s) and recorded with search coils in a rotating magnet ic field. Velocity and position components of eye and head responses w ere analysed. The velocity gains of optokinetic and vestibular respons es of partially restrained and of head restrained rats were similarly high (between 0.8 and 1.0). Eye movements in partially restrained rats also contributed most (about 80%) to the velocity components of the r esponses. At stimulus velocities above 10-degrees/s, the ''beating fie ld'' of the evoked optokinetic and vestibular nystagmus was shifted tr ansiently in the direction of ocular quick phases. The amplitude of th is shift of the line of sight was about 3-10-degrees in head restraine d and about 20-30-degrees in partially head restrained rats. Most of t his large, transient gaze shift (about 80%) was accomplished by head m ovements. We interpret this gaze shift as an orienting response, and c onclude that the recruitment of the ocular and the neck motor systems can be independent and task specific: head movements are primarily use d to orient eye, ear and nose towards a sector of particular relevance , whereas eye movements provide the higher frequency dynamics for imag e stabilization and vergence movements.