The superior colliculus and its control of fixation behavior via projections to brainstem omnipause neurons.

Evidence, obtained in the animal whose head is unrestrained, has shown that the superior colliculus (SC) controls, not specifically eye-in-head motion, but rather saccadic shifts of the visual axis (gaze) composed of coordinated eye and head movements. The SC has also been implicated in fixation control. In a current hypothesis, activity on the SC motor map reflects two conflictual behavioral states: 'Orient!' versus 'Don't orient!' The latter behavior is thought to be commanded from a 'fixation zone' in the rostral SC that includes the foveal representation of the retinotopic map. 'Fixation neurons' (SCFNs) in this zone project to brainstem 'omnipause neurons' (OPNs) that inhibit the gaze-saccade generating circuits. It has been proposed that activity in SCFNs during active fixation, drives OPNs which in turn inhibit the gaze saccade generator, thereby maintaining the visual axis stable on a target of interest. Cats with head unrestrained frequently orient in the dark, to a briefly visible target, using multiple-step gaze shifts with a variable number of gaze saccades interspersed with periods of steady fixation. We found that SCFNs are not always active during the inter-step fixation periods when the visual axis is immobile--whereas OPNs are tonically active--thereby implying that activity in FNs and OPNs can be decoupled. In cats with head unrestrained, SCFNs encode the error between desired and actual gaze positions, not necessarily that the visual axis is immobile. By comparison, OPNs are tonically active when the visual axis is immobile, but some also encode gaze position error. Thus, the discharge of an OPN may reflect combined inputs from SCFNs combined with a steady bias.