Model of coupled or dissociated eye-head coordination.

Tight coupling between eye and head movements have been observed in response to a single visual target offset or a single flash. On this basis, when the visual stimulus consists of two successive steps or flashes in the same (horizontal) direction, either increasing in eccentricity or decreasing in eccentricity, gaze should be due to concomitant eye and head angular displacement. The main findings were that in response to increasing eccentricity stimulus, the majority of the gaze movement had a staircase pattern, and to decreasing eccentricity stimulus, a pulse-step pattern. Some responses were one gaze movement to the final target (or flash) offset. In all these responses, eye and head were tightly coupled. In some cases, however, in response to two flashes or two successive steps decreasing in eccentricity, the initial eye and head movements were dissociated: eye and head were directed to different offsets. Current eye-head models assume a strong coupling between the head displacement and the eye saccade. We have modified the Laurutis and Robinson model to accommodate the new findings of saccade-VOR cooperation and eye-head dissociated responses. we suggest that when gaze falls short of the target offset, the retinal error signal is not only fed to the saccadic system but also partially suppresses the VOR gain control. This allows for "corrective VOR" movements after the gaze saccade has occurred.(ABSTRACT TRUNCATED AT 250 WORDS)