Vision’s First Steps: Anatomy, Physiology, and Perception in the Retina, Lateral Geniculate Nucleus, and Early Visual Cortical Areas

Abstract

This chapter reviews the functional anatomical bases of visual perception in the retina, the lateral geniculate nucleus (LGN) in the visual thalamus, the primary visual cortex (area V1, also called the striate cortex, and Brodmann area 17), and the extrastriate visual cortical areas of the dorsal and ventral pathways. The sections dedicated to the retina and LGN review the basic anatomical and laminar organization of these two areas, as well as their retinotopic organization and receptive field structure. We also describe the anatomical and functional differ- ences among the magnocellular, parvocelullar and koniocellular pathways. The section dedicated to area V1 reviews the functional maps in this area (retino- topic map, ocular dominance map, orientation selectivity map), as well as their anatomical relationship to each other. Special attention is given to the modular columnar organization of area V1, and to the various receptive field classes in V1 neurons. The section dedicated to extrastriate cortical visual areas describes the "where" and "what" pathways in the dorsal and ventral visual streams, and their respective physiological functions. The temporal dynamics of neurons throughout the visual pathway are critical to understanding visibility and neural information processing. We discuss the role of lateral inhibition circuits in processing spatiotemporal edges, corners, and in the temporal dynamics of vision. We also discuss the effects of eye movements on visual physiology and percep- tion in early visual areas. Our visual and oculomotor systems must achieve a very delicate balance: insufficient eye movements lead to adaptation and visual fading, whereas excessive motion of the eyes produces blurring and unstable vision during fixation. These issues are very important for neural prosthetics, in which electrodes are stabilized on the substrate.