Discharges of Visual Neurons in Eye Movements

Abstract

In animals with sharp vision, only a small portion of the retina is specialized for detailed vision. This portion, the fovea, has a high density of photoreceptors and in our eyes we can count as many as 7000 cones in a small area which covers only two degrees of the visual field. The fovea has outstanding spatial sensitivity. For the rest of the retina, however, the spatial sensitivity is extremely poor. Because of such a difference in capabilities we have to move our eyes in order to bring the image onto the fovea; therefore, the eye movement is especially well developed in animals having a discrete fovea. The function of eye movements is to acquire a visual target and then track it so that the image stays on the fovea. For that purpose the brain has to select a visual target and transform its spatial coordinates into a motor signal necessary for the production of accurate eye movements by the oculo motor system. The information about the location of the target could be derived from either branch of the visual system, namely, the midbrain pathway terminating primarily in the superior colliculus or the geniculo-cortical pathway terminating chiefly in the striate cortex. There is no doubt that the control of eye movements is strongly dependent on the feedback of sensory information from the retina. The primary pathway for visual information is by axons of retinal ganglion cells from the eyeball to the lateral geniculate nucleus and then through the optic radiations to the visual cortex. Some fibers of retinal ganglion cells also travel to the midbrain, terminating chiefly in the superior colliculus and the pretectal region. In this session on “Neuronal Substrates of Eye Movement,” I have been asked to discuss the properties of neurons in the afferent pathway of the eye movement control system. In order to study response properties of visual neurons during eye movements, I recorded single unit activity from the optic tract and the lateral geniculate nucleus in the chronic cat. The cat had permanently implanted, nonpolarizing EOG electrodes, with which both horizontal and vertical EOGs were measured. The cat was fully awake and making frequent eye movements. A stainless steel microelectrode was advanced by a mechanical drive until it reached the target structure; thereafter, it was driven by a hydraulic system. Action potentials of retinal ganglion cells were recorded from the optic tract, a few millimeters posterior to the optic chiasm. In the second series of experiments, spikes were recorded from relay cells of the lateral geniculate nucleus.