Microcircuitry of the cat retina.

As a device for extracting information from a visual image, the vertebrate retina is unparalleled in its range, reliability, and compactness. Signaling in the retina is slower by six orders of magnitude than in an integrated digital circuit. The advantage of the biological structure must therefore derive from the variety of its fundamental elements and from the subtlety of their connections. Each of the five major classes of retinal neuron, whose synaptic contacts were first described systematically by Dowling & Boycott (1966), is now known to have multiple types, totaling in the cat about 60. Specific local circuits involving about one-third of these neurons have been recog­ nized in the electron microscope. Physiological responses have also been documented for about one-third of the types, and evidence regarding the neural transmitter, or at least the sign of the synapse, has accumulated also for about one-third. These discoveries have abundantly supported certain concepts of retinal function developed in the 1960s by Lettvin & Maturana. The function of the retina, they proposed, "is not to transmit information about the point­ to-point distribution of light and dark in the image, but to analyze this image at every point in terms of ... arbitrary contexts ... " (Maturana et at 1960). Each of'these "contexts," they suggested, corresponds to some operation on the local image performed by a ganglion cell of particular size and shape (Lettvin et al 1961). This idea, based on studies of the frog, seemed for a time inapplicable to the cat, which was thought to have a "simple" retina with only center-surround type ganglion cells. Subsequent studies to be reviewed here have firmly established for the cat the validity of this idea.