Clinical and Anatomical Evidence for Parallel Processing in the Human Visual System

Classes of retinal ganglion cells have been morphologically and physiologically described in several experimental animals. Whereas no such classification of retinal ganglion cell types has been made in man, there have been psychophysical and retinal electrophysiological human studies suggestive of a similar segregation of retinal ganglion cells into classes which subserve different functions. The neuro-ophthalmic clinician often has psychophysical evidence of the separation of visual functions. For example, it is well known that a patient with optic neuritis may have a pronounced afferent pupillary defect despite having normal visual acuity. Moreover, such a patient may measure best for Snellen visual acuities when the contrast is neither too high nor too low. Indeed, we offer evidence that brightness-sense is a sensitive measure of optic nerve disease which may be singularly impaired. Thus we see that several diseases of the optic nerve produce impairments of function which are out of proportion to other defects of ViSiOn. This mismatch between certain parameters of function by ophthalmic diseases has major clinical and scientific implications.