Visual fixation as equilibrium: evidence from superior colliculus inactivation.

During visual fixation, the image of an object is maintained within the fovea. Previous studies have shown that such maintenance involves the deep superior colliculus (dSC). However, the mechanisms by which the dSC supports visual fixation remain controversial. According to one view, activity in the rostral dSC maintains gaze direction by preventing neurons in the caudal dSC from issuing saccade commands. An alternative hypothesis proposes that gaze direction is achieved through equilibrium of target position signals originating from the two dSCs. Here, we show in monkeys that artificially reducing activity in the rostral half of one dSC results in a biased estimate of target position during fixation, consistent with the second hypothesis, rather than an inability to maintain gaze fixation as predicted by the first hypothesis. After injection of muscimol at rostral sites in the dSC, fixation became more stable since microsaccade rate was reduced rather than increased. Moreover, the scatter of eye positions was offset relative to preinactivation baselines. The magnitude and the direction of the offsets depended on both the target size and the injected site in the collicular map. Other oculomotor parameters, such as the accuracy of saccades to peripheral targets and the amplitude and velocity of fixational saccades, were largely unaffected. These results suggest that the rostral half of the dSC supports visual fixation through a distributed representation of behaviorally relevant target position signals. The inactivation-induced fixation offset establishes the foveal visual stimulation that is required to restore the balance of activity between the two dSCs.