Saccade-vestibulo-ocular reflex co-operation and eye-head uncoupling during orientation to flashed target.

1. Eye-head co-ordination in the horizontal plane was studied in four human subjects using two successive flashes in the same direction, either increasing in eccentricity (IE), or decreasing in eccentricity (DE). 2. Results showed that for both conditions, head movements preceded eye movements and were typically longer or followed by a slow gaze movement. This slow movement was due to a vestibulo-ocular reflex gain of less than one. Gaze accuracy was achieved by small head movement adjustments. 3. Gaze movement to an IE stimulus had a staircase pattern, and to a DE stimulus, a pulse-step pattern or one gaze saccade to the final flash eccentricity. 4. In some cases, however, in response to a DE stimulus, the eye and head movements were directed to different displacements (dissociation); i.e. the head movement started towards the first flash eccentricity with a concomitant eye saccade to the second flash eccentricity. When this occurred, gaze movement did not resemble a pulse-step pattern. 5. It is suggested that non-visually orienting gaze is driven mainly by head movement. Eye and head movements can be either tightly coupled or dissociated, depending on the stimulus pattern.