Effects of eye position on saccades evoked electrically from superior colliculus of alert cats.

Electrical stimulation was carried out in the intermediate and deep gray layers of the superior colliculus in alert cats. The heads of the animals were fixed, and their eye movements were recorded with the scleral search coil method. Stimulation in the anterior two-thirds of the colliculus with long-duration pulse trains produced multiple saccades, as in the primate (45, 51), but their directions and amplitudes were influenced significantly by the initial position of the eye. Stimulation in the posterior part of the colliculus evoked saccades that appeared to be "goal-directed," whereas stimulation at the extreme caudal edge of the colliculus yielded centering saccades. These observations confirm previous reports of Roucoux and Crommelinck (48) and Guitton et al. (24). Saccades evoked during bilateral simultaneous stimulation of the superior colliculi were also dependent on the initial position of the eye. At certain relative intensities of stimulation on the two sides, saccades failed to occur when the eye was within a particular part of the oculomotor range. When the eye was outside this region, the same stimuli triggered an eye movement that drove the eye toward the zone of saccade failure. These findings indicate that saccadic commands resulting from focal collicular stimulation in the cat can be modified by information about current eye position. It is not certain where in the brain this occurs or by what neural mechanisms, but a local feedback model of the saccadic control system (46) can account for the main observations. The functional significance of these findings depends in large measure on the degree to which focal collicular stimulation reproduces naturally occurring patterns of neural activity.