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

1. The superior colliculus has been studied in alert cats which were restrained and whose head and eye movements were monitored.2. Microstimulation within the rostral part of the colliculus, which represents the central 25 deg of the visual field, evokes saccadic eye movements that carry the area centralis to that region of visual space previously occupied by the receptive fields of the cells that were stimulated (;foveation'). These saccades are not generally accompanied by a movement of the head.3. At more caudal locations the visual receptive fields of collicular neurones 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 that of natural gaze changes in response to novel stimuli. Prolonged stimulation results in the addition of further co-ordinated eye-head movements.4. The addition of a movement of the head does not increase the area of visual space that may be foveated in a single gaze change. Movements of the head are compensated by the vestibulo-ocular reflex. The visual receptive fields of cells at more caudal locations cannot be foveated by a single gaze change.5. A third class of response to electrical stimulation is also 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.6. Electrical microstimulation was never found to produce 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.7. Ninety-nine cells were recorded from the superior colliculus and classified into four 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.8. These results are discussed with respect to the production of gaze changes in the cat.