Responses of Single Neurons in the Pretectum of Monkeys to Visual Stimuli in Three‐Dimensional Space

In their first paper on the cat’s pretectum Straschill and Hoffmannl described neurons with a strong preference for objects moving in three-dimensional space either towards or away from the animal. They concluded that change of retinal image size rather than displacement of retinal image or change in retinal disparity seemed to be the most effective stimulus for these units. When tested with continuously increasing or decreasing stationary projections of bright discs or shadows, units with preference for movements towards the animal were excited during increase of stimulus size, whereas units with preference for movements away from the animal were more effectively activated during decreasing stimulus size. Occlusion of one eye changed the strength but not the type of reaction. Continuous increase or decrease of intensity of a stationary stimulus did not produce a response, and preferred direction (towards or away from the animal) remained unchanged when bright objects were substituted for dark ones. The anatomical location of these units was in the deep pretectal nuclei. In recent years we have studied extensively the more superficial pretectal structures in cats and monkeys.24 One of these structures is the nucleus of the optic tract (NOT). The mammalian NOT and dorsal terminal nucleus of the accessory optic tract (DTN) consist of scattered groups of neurons located along the ventral margin of the brachium of the superior colliculus (BSC).4 Lesion and stimulation studies in vertebrates as diverse as amphibians,” reptiles, rats,” rabbits,’* cats,I3-l4 and monkeys‘s’6 have shown that the NOT and the related nuclei of the accessory optic tract are necessary structures for the optokinetic reflex or for visual modification of the vestibulo-ocular reflex.” The major known projection of N O T and DTN cells is to the dorsal cap of the inferior (IO), which relays visual information to the cerebellum as a climbing fiber input. NOT neurons projecting to the inferior olive are very sensitive to retinal slip, and their activity can be directly related to slow phase eye velocity during optokinetic nystagmus (OKN). In monkeys all these NOT neurons are binocular, which is a prerequisite for symmetrical horizontal OKN under monocular viewing conditions. OKN is called symmetrical when monocular stimulation in the temporonasal and the nasotemporal directions lead to about equal eye velocity. It may be suggested that symmetrical OKN in primates is important to stabilize the eyes with respect to the binocularly fused visual world, that is, objects on the horopter. Visual stimuli before and behind the horopter would then be less effective in eliciting OKN, and their influence on the activity of NOT cells should be decreased. We therefore tested the NOT cells’ sensitivity to retinal disparity in macaque monkeys.