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-goals 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.
