The fixation area of the cat superior colliculus: effects of electrical stimulation and direct connection with brainstem omnipause neurons.

The superior colliculus has long been recognized as an important structure in the generation of saccadic displacements of the visual axis. Neurons with presaccadic activity encoding saccade vectors are topographically organized and form a "motor map." Recently, neurons with fixation-related activity have been recorded at the collicular rostral pole, at the area centralis representation or fixation area. Another collicular function which deals with the maintenance of fixation behavior by means of active inhibition of orientation commands was then suggested. We tested that hypothesis as it relates to the suppression of gaze saccades (gaze = eye in space = eye in head + head in space) in the head-free cat by increasing the activity of the fixation cells at the rostral pole with electrical microstimulation. Long stimulation trains applied before gaze saccades delayed their initiation. Short stimuli, delivered during the gaze saccades, transiently interrupted both eye and head components. These results provide further support for a role in fixation behavior for collicular fixation neurons. Brainstem omnipause neurons also exhibit fixation-related activity and have been shown to receive a direct excitatory input from the superior colliculus. To determine whether the collicular projection to omnipause neurons arises from the fixation area, the deep layers of the superior colliculus were electrically stimulated either at the rostral pole including the fixation area or in more caudal regions where stimulation evokes orienting responses. Forty-nine neurons were examined in three cats. 61% of the neurons were found to be orthodromically excited by single-pulse stimulation of the rostral pole, whereas only 29% responded to caudal stimulation. In addition, stimuli delivered to the rostral pole activated, on average, omnipause neurons at shorter latencies and with lower currents than those applied in caudal regions. These results suggest that excitatory inputs to omnipause neurons from the superior colliculus are principally provided by the fixation area, via which the superior colliculus could play a role in suppression of gaze shifts.