Abstract
Rapid coordinated eye - head movements, called saccadic gaze shifts, displace the line ofsight from one location to another. A critical structure in the gaze control circuitry is themidbrain's superior colliculus (SC), which drives gaze saccades by relaying corticalcommands to brain stem eye and head motor circuits. We have proposed that the SC lieswithin a gaze feedback loop and generates an error signal specifying gaze position-error(GPE), the distance between target and current gaze positions. We investigated thisfeedback hypothesis, in cat, by briefly stopping head motion during large (around 50°) gazesaccades made in the dark. In the caudal SC, a cell's firing frequency gradually increased to a maximum that just preceded the optimal gaze saccade encoded by the cell's position. In "brake" trials the activity-level just preceding a brake-induced gaze 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 plateau's GPE. By 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. [Jpn J Physiol 54 Suppl:S180 (2004)]
