The central visual system.

Modern neuroscience, of which the study of the visual system is an integral part, has surprisingly recent beginnings. Its cornerstone, the neuron doctrine, was formulated late in the 19th century, to a large extent as a result of evidence collected by Santiago Ramon y Cajal using the fortuitously discovered Golgi method. Ironically, the idea that nerve cells were independent biological units was vehemently opposed by Camillio Golgi who was a proponent of the reticular theory. Theirs was a controversy bitter enough to compel Golgi to devote his Nobel address, a prize he shared with Cajal in 1906, to an all out attack on the neuron doctrine. The latter part of the 19th century also witnessed the emergence of ideas about the functional localization of brain function. Broca (1861) and Wernicke (1874) studied the effects of brain damage in humans’ localized speech areas, and the electrical stimulation studies of Fritsch and Hitzig (1870) identified the site of motor cortex. The localization of visual cortex, also made at this time, did not arise without controversy. Based on his lesion studies Ferrier (1876) believed that parts of the parietal lobe comprised primary visual cortex, precipitating an acrimonious debate with Hermann Munk (1881), who correctly identified the occipital lobe as the primary site. Other developments forming the underpinnings of modem neuroscience soon followed. The microelectrode, which became central in the study of the properties of single neurons and on which much of what we know about the functional organi~tion of the visual system is based, was first used on plants during the early part of this century (Bretag, 1983; Hoyle, 1983). Its refinement was gradual; not until the 1940s did Ling and Gerard devise the double pull method to yield tips small enough to study central nervous system neurons intracellularly. Using this electrode in the late 194Os, Eccles made his key discoveries about the nature of synaptic transmission (Eccles, 1957). The Hbrsley-Clark stereotaxic apparatus, which bec.ame such an important tool for the accurate localization of neural structures, was also developed shortly after the turn of the century. The electron microscope, which eventually provided most of the essential information about the fine structure of the brain, was assembled first by Max Knoll and Ernest Rushka in the early 193Os, but was not applied to the nervous system with real success until the 1950s. The first definitive experiments demonstrating chemical synaptic transmission were made by Loewi in 1921; the idea for his famous experiments on frog hearts came to him in a dream (McGreer et al., 1978). Today neuroscience is a wideranging discipline using a multitude of methods; it spans the study of the brain from mind to molecule. Studies of the visual system have played a major role in its growth. The discoveries made on visual system function have resulted in several Nobel prizes, including those to George Wald, H. K. Hartline, and Ragnar Granit in 1967, and to David Hubel, Torsten Wiesel and Roger Sperry in 1981. Most of our knowledge of the visual system has arisen during the past 25 years. In this chapter I will describe the central developments as they occurred, with emphasis on work that provided new understanding about vision and visuo-motor functions. I will begin with the geniculo-striate system, and will then discuss the higher visual stations, the tectal system, and the accessory optic system.