All information about the retinal image that is directly available to the brain is transmitted by the axons of retinal ganglion cells. In higher mammals, and particularly primates, the projection to the cortex via the lateral geniculate nucleus of the thalamus appears to be essential for conscious visual perception. Retinal axons also reach the hypothalamus, superior colliculus, pretectum, and various other nuclei of the brain stem and diencephalon that subserve a variety of functions, such as reflex orientation to visual stimuli, stabilization of gaze, and control of pupil diameter. This chapter focuses primarily on the retino-geniculate component of the projection to the cerebral cortex, but several of the principles dealt with here are relevant to brain-stem projections as well.

Parallel Processing of the Retinal Image and Classes of Ganglion Cells

Previous chapters have shown how the retinal circuitry establishes one channel to signal increments and another to signal decrements of illumination. These on and off channels encode complementary versions of the distribution of light on the retina and transmit them in parallel to the central nervous system. Also, as discussed earlier, other features of the retinal image, such as movement direction, can be signaled by specialized ganglion cells. Thus, the retinal image is not transmitted in raw form to the brain, but is analyzed in different ways by different ganglion cells, which then communicate their “Views” of the image along separate channels. Different animals segregate different aspects of the retinal image as part of this strategy of parallel processing.