ABSTRACT
One of the critical events in modern visual neuroscience was the discovery by Hubel and Wiesel (1959, 1968) of cells in the mammalian visual cortex that responded to specific patterns of light falling on the retina. These patterns included oriented contours of sharp changes in brightness, lines of both dark-on-light and light-on-dark polarity, and line endpoints. The visual system implements this detection process by a multistage process. At each stage neural signals, each associated with a specific limited region of the retina (a receptive field, as described in Chapter 2), are combined to produce a set of output signals. The actual mechanisms are complicated and vary in important ways over different portions of the eye and brain. Common to most of the process is that receptive fields consist of excitatory regions, in which retinal stimulation increases output, and inhibitory regions, in which retinal stimulation attenuates the effects of the excitatory regions. The effect is well approximated by linear signal processing that combines blurring and differentiation, though some aspects of neural processing are clearly nonlinear, and responses to temporal changes are also pervasive.
