The superior colliculus and movements of the head and eyes in cats

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Abstract
The superior colliculus was studied in alert, restrained cats whose head and eye movements were monitored. Microstimulation within the rostral part of the colliculus, which represents the central 25.degree. of the visual field, evokes saccadic eye movements moving the area centralis to the region of visual space previously occupied by the receptive fields of the stimulated cells (foveation). These saccades are not generally accompanied by a movement of the head. At more caudal locations the visual receptive fields of collicular neurons lie at a greater eccentricity relative to the area centralis than the maximum possible deviation of the eyes from the central position in normal circumstances. At these sites electrical stimulation produces a combined movement of the head and eyes whose co-ordination is identical to natural gaze changes in response to novel stimuli. Prolonged stimulation results in the addition of further coordinated eyehead movements. The addition of a head movement does not increase the area of visual space that may be foveated in a single gaze change. Head movements are compensated by the vestibulo-ocular reflex. The visual receptive fields of cells at more caudal locations are not foveated by a single gaze change. A 3rd class of response to electrical stimulation is occasionally found in the caudal part of the colliculus. The head movement often begins before an accompanying eye movement and continues smoothly for the entire stimulation duration or until limited by the range of mobility. Electrical microstimulation never produced so-called goal-directed eye movement, in which the eyes move in a single saccade to a fixed orbital position regardless of their starting position. Cells (99) were recorded from the superior colliculus and classified into 4 types based on their responses, or lack of responses, during or preceding eye and head movements. Type 1 cells did not show changes in activity prior to gaze changes. Type 2 cells were inhibited prior to and during eye movements. Cells discharging before normal saccadic eye movements (type 3) were found only in the rostral part of the colliculus. Cells discharging before head movements (type 4) were found only in the caudal part. The production of gaze changes in the cat is discussed.