Fixation and orientation control by the tecto-reticulo-spinal system in the cat whose head is unrestrained

The role of the tecto-reticular and tecto-reticulo-spinal neurons (here called TR(S)Ns) in gaze control is described. TR(S)Ns, located in the deeper layers of the cat superior colliculus (SC), project onto the eye and head premotor circuitry. TR(S)Ns located in the caudal SC had sustained and phasic discharges related to the control of gaze movements. The sustained discharge occurred when the visual axis was positioned at some vector quantity away from a target of interest. Each cell has its preferred vector corresponding to the cell's location on the collicular retinotopic map. This tonic discharge acted as a preamble to the phasic discharge and served to pre-excite the relevant oculomotor circuitry. The phasic discharge preceded gaze shifts whose direction and magnitude matched the preferred vector. The intensity of this discharge was correlated to the acceleration and velocity of the movement. TR(S)Ns situated in the rostral SC were maximally active when the cat fixated a target of interest. These neurons decreased their discharge rate during gaze shifts. Thus, TR(S)Ns provide both fixation and orientation signals to the eye and head premotor circuitry. A scheme is proposed where TR(S)Ns lie within a gaze feedback loop that controls eye and head movements via inputs to long lead burst neurons and omnipause neurons.