Visual Processing In The Primate Brain

National Eye Institute

Investigators

Linked publications & trials

Abstract

The major visual pathway into the brain, and the one most extensively studied, is that from the retina through the lateral geniculate nucleus of the thalamus to primary visual cortex. Another pathway from the retina to the superior colliculus (SC) then to the pulvinar nucleus of the thalamus also projects to visual cortex. We have now begun to address what information is carried by this second pathway to the cerebral cortex. The primate pulvinar is one of the largest nuclei in the primate brain and also the least understood. It receives massive inputs from the SC, and projects to multiple areas in visual cortex. These observations have led to the hypothesis that the pulvinar provides a secondary pathway for visual signals to reach cortex. Two major questions must be answered before this putative pathway can be understood. First, are there single cells in pulvinar that serve as relays from SC to cortex? Second, what signals does this pathway convey? We used classic stimulation techniques to identify neurons in the macaque pulvinar that receive input from SC and project to extrastriate motion area MT. We then characterized their function by recording extracellularly while the monkey performed visual and eye movement tasks. Our first and primary finding is that the pulvinar contains single-neuron relays from SC to MT. Relay neurons are clustered in a small zone posterior to the lateral geniculate nucleus, and are surrounded by a larger zone in which neurons connect to either SC or MT. Our second finding is that relay neurons carry visual signals, often resembling neurons in the superficial layers of SC. Ongoing experiments will examine the modification of these visual signals by extraretinal influences such as attention and saccadic eye movements.