Fixation neurons in the superior colliculus encode distance between current and desired gaze positions

A visual scene is scrutinized during sequential periods of steady fixation, connected by saccades that shift the visual axis (gaze) to new positions. During such exploratory scan paths, gaze frequently strays from and then returns to salient features. How the brain keeps track of major end-gears and intermediate subgoals is not understood. We studied the discharge of fixation neurons of the brainstem's superior colliculus during multiple-step gaze shifts composed of a sequence of saccades made in the dark and separated by short periods of steady fixation. Cells were initially silent. As sequential gaze saccades approached the goal, firing began; its frequency increased progressively and peaked when gaze was on the remembered target location. We conclude that these fixation neurons encode the error between desired and actual gaze positions, irrespective of trajectory characteristics.