Visual neuroscience before the neuron
Nicholas J. WADE
Visual neuroscience is considered to be a contemporary concern, based in large part on relating characteristics of neural functioning to visual experience. It presupposes a detailed knowledge of neural activity for which the neuron doctrine is a fundamental tenet. However, long before either the neuron doctrine had been advanced or the nerve cell had been described attempts were made to estimate the dimensions of nerve fibers from measures of visual resolution. In the seventeenth century, the microscopes of Hooke and Leeuwenhoek were unable to resolve structures as small as nerves adequately. However, it was not Hooke's microscope that led to an estimate of the dimensions of nerve fibers but his experiments on the limits of visual resolution. Hooke determined that a separation of one minute of arc was the minimum that could normally be seen. Descartes had earlier speculated that the retina consisted of the terminations of fibers of the optic nerve, and that their size defined the limits of what could be seen. Estimates of the diameters of nerve fibers were made on the basis of human visual acuity by Porterfield in 1738; he calculated the diameters of nerve fibers in the retina as one 7,200th part of an inch (0.0035 mm), based on the resolution of one minute as the minimum visible. In the same year, Jurin questioned the reliability of such estimates because of variations in visual resolution with different stimuli. The measurement of visual acuity was refined by Mayer in 1755, using dots, gratings, and grids as stimuli. In the 1830s, Treviranus fused the microscopic and acuity approaches to determining the dimensions of nerve fibers. His indirect estimates of the dimensions of retinal fibers were close to those derived from microscopic observation. However, the suggestion that the retina consisted of terminations of nerve fibers influenced Treviranus's detailed illustrations of its microscopic structure. Contrary to the situation that obtained after the microscopic structure of the retina had been established, a function of vision (acuity) was used to determine the dimensions of the structures (retinal elements) that were thought to mediate it.
Session II -- Visual Neuroscience
Montreal, Quebec, Canada