Parallel processing in the mammalian retina: the Proctor Lecture.

T projection from the eye to the mammalian brain is organized into parallel routes. In the optic pathway retinal axons separate into several tracts that terminate in different subcortical areas, such as the suprachiasmatic nucleus, the lateral geniculate complex, the pretectum, the superior colliculus, and the accessory optic nuclei. In this way different aspects of the light signal are transfered to these centers. Parallel routes also can be distinguished in the projection from the eye through the lateral geniculate nucleus to the visual cortex. Here, in primates, the parvocellular (P) and magnocellular (M) pathways are well established.' Recently a further pathway in parallel to these has been found in the interlaminar regions (K-layers) of the geniculate.' This may carry a blue cone signal among others. The specific tracts are made up by the axons of the 10 to 20 different types of retinal ganglion cells." Their dendritic fields cover the retina homogeneously without leaving gaps, so a spot of light on the retina—independent of retinal location— hits at least one member of each of the ganglion cell types. Thus, the ganglion cells represent 10 to 20 niters that encode, in parallel, different aspects of the image projected onto the retina and their axons send specific messages into the different visual centers of the brain. A brightness signal, for instance, is sent to the suprachiasmatic nucleus for regulating the diurnal rhythm (see Ref. 19), a signal about retinal image movements to the accessory optic nuclei, which stabilize the eyes' positions and so on. The input from the photoreceptors to the ganglion cells is through bipolar and amacrine cells' synapses onto the ganglion cells' dendrites in the inner plexiform layer GPL)The IPL is precisely organized: ganglion cells have dendrites at characteristic levels within the IPL. There is an overall subdivision of the IPL into ONand OFF-layers. Dendrites of OFF-ganglion cells stratify in the outer half of the IPL, those of ON-ganglion cells in the inner half. Within the ONand OFF-layers there are further subdivisions. Dendrites of ONand OFF-parasol cells in primates keep a very narrow level of stratification close to the center of the IPL. In contrast, dendrites of ONand OFFmidget ganglion cells stratify more diffusely and are found more toward the inner and outer regions of the IPL, respectively. The two dendritic planes of the small bistratified (blue) ganglion cells stratify even further toward the outer and inner edges of the IPL. This suggests that when a retinal