Neuronal connectivity between accommodative and active fixation systems

Abstract

The superior colliculus (SC) has long been recognized as an important structure in the generation of saccadic eye movements. The SC of the cat and the monkey has recently been shown to be involved in the control of visual fixation. A subpopulation of collicular cells exhibits tonic discharge when the animal fixates on a target of interest. These cells are located in the rostral SC where the central visual field is represented. Active fixation is thought to be important for the ocular near response; accommodation, vergence and pupillo-constriction, and these systems are functionally linked. Therefore, it is possible that the rostral SC is also involved in the control of accommodation or vergence. The results of several recent studies have suggested that the rostral SC is also involved in the control of accommodation. The accommodation-related area in the rostral SC also corresponds to the area of representation of the central visual field. The accommodation-related area in the SC receives heavy projections from the accommodation area in the lateral suprasylvian (LS) area of the cat cortex. It is well known that the LS area is also involved in the control of vergence and pupillo-constriction. The rostral SC projects to the pretectal nuclei (PT) where accommodative responses are evoked by microstimulation, the raphe interpositus (RIP), in which omnipause neurons are located, and the dorsomedial portion of the nucleus reticularis tegmenti pontis. Many neurons in the intermediate layers of the rostral SC have divergent axon collaterals to the PT and the RIP. The rostral SC is likely a key structure involved in the near response and visual fixation.