Central mechanisms of vision: parallel processing.

Despite repeated attempts by several laboratories to discover increasingly complex "feature detector" neurons in higher visual centers of mammals, there has been little convincing evidence for the existence of such neurons. What had been reported instead is that many neurons in higher centers show less rather than more specificity when compared with cells in areas 17, 18 and 19. Studies in mammalian retina have revealed multiple processing systems apparently operating in parallel at the level of the ganglion cells. Striate cortex receives at least two different kinds of visual input from the lateral geniculate, and sends at least two different parallel outputs to other brain regions. Within striate cortex there is some segregation of different functional cell types in separate layers. The accumulated evidence suggests the existence of parallel visual processing mechanisms beginning in mammalian retina and extending through striate cortex to higher cortical centers. The notion of separate processing systems for the detection of different features in the visual world recalls earlier work by Lettvin, Maturana, McCulloch and Pitts concerning visual processing in the frog retina and optic tectum.