[Neurobiological principles of binocular space perception. Historical review on the development of the concept and...

Binocular vision and space perception are not readily explained on the basis of direct anatomical evidence on the visual system. Galen (2nd century A.D.) localized the mechanism underlying binocular vision into the optic chiasm. Nearly 2000 years later the famous physicist Newton (1704), undoubtedly stimulated by clinical reports on hemianopia and Descartes' (1686) hypothesis on the retinal image projections on the brain, came up with a new hypothesis on the optic chiasm which could explain how information from homonymous visual hemifields could converge from the two eyes to one and the same area in the brain. He predicted that the temporal retinal fibers of both eyes fail to cross. Irrefutable proof of the special arrangement of the optic chiasm was provided by Ramon y Cajal (1899) with the aid of the Golgi method. It took more than 50 years before binocularly oriented cells were discovered in the visual cortex by Baumgartner and colleagues (1958), and before neurons were found in that area by Hubel and Wiesel (1959) which responded to stimulation from corresponding retinal sites of the two eyes. These electrophysiological studies profited by the localization of the cortical visual center in the occipital lobe by Panizza (1856), the identification of the corpus geniculatum laterale as a relais station of the retino-cortical pathway by Monakow (1883, 1885) and the discovery of the projection of homonymous retinal halves upon alternating layers of this structure by Minkowski (1920). Wheatstone opened the road to the understanding of stereoscopic vision by inventing the stereoscope in 1838. He based his research largely upon the the investigation of Aguilonius (1613), Vieth (1818) and Müller (1926) on the horopter. The most elegant evidence on the significance of visual disparity for depth perception came recently from Julesz and his demonstration of "random-dot-stereograms" (1971). Binocular neurons which are sensitive to minimal stimulus disparities within Panum's area (1858) have only recently been observed in the striate cortex by electrophysiological recordings (Barlow et al., 1967; Poggio and Fischer, 1977). These cells were designated as "disparity or depth detectors". They displace the "cyclopean eye" of Helmholtz (1867) and Hering (1879) from its Homeric seat in the forehead to the cortical field whose striations were already noted by Gennari (1782) near the posterior end of the cranium.