Evidence for gaze feedback to the cat superior colliculus: discharges reflect gaze trajectory perturbations.

Rapid coordinated eye-head movements, called saccadic gaze shifts, displace the line of sight from one location to another. A critical structure in the gaze control circuitry is the superior colliculus (SC) of the midbrain, which drives gaze saccades by relaying cortical commands to brainstem eye and head motor circuits. We proposed that the SC lies within a gaze feedback loop and generates an error signal specifying gaze position error (GPE), the distance between target and current gaze positions. We investigated this feedback hypothesis in cats by briefly stopping head motion during large ( approximately 50 degrees ) gaze saccades made in the dark. This maneuver interrupted intended gaze saccades and briefly immobilized gaze (a plateau). After brake release, a corrective gaze saccade brought the gaze on goal. In the caudal SC, the firing frequency of a cell gradually increased to a maximum that just preceded the optimal gaze saccade encoded by the position of the cell and then declined back to zero near gaze saccade end. In brake trials, the activity level just preceding a brake-induced plateau continued steadily during the plateau and waned to zero only near the end of the corrective saccade. The duration of neural activity was stretched to reflect the increased time to target acquisition, and firing frequency during a plateau was proportional to the GPE of the plateau. In comparison, in the rostral SC, the duration of saccade-related pauses in fixation cell activity increased as plateau duration increased. The data show that the cat's SC lies in a gaze feedback loop and that it encodes GPE.