The rods and cones detect the intensity, color, and pattern of light entering the eye through the lens. However, it is their interaction with a highly ordered array of neurons in the retina that begins the first steps of visual sensory processing (Fig. 36.19). The retina consists of five layers of cells that form a signal-processing network just in front of a pigmented epithelium. The rods and cones are at the back of the retina, so that light must pass through several layers before reaching these light-sensitive cells. Because they are photoreceptors, rod and cone cells hyperpolarize in response to light but do not fire action potentials. They synapse on to bipolar cells, which also do not fire action potentials, but rather adjust their release of neurotransmitter in response to the input from multiple rod and cone cells. Depending on their receptors, some bipolar cells are inhibited when rod and cone cells reduce their release of glutamate, whereas other bipolar cells are excited by the reduction in glutamate release.

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The bipolar cells, in turn, synapse on to ganglion cells located on the front of the retina. If activated, ganglion cells transmit action potentials by the optic nerve (a cranial nerve) to the visual cortex in the brain, the part of the brain that processes images. The optic nerve begins at the front of the retina and exits at the back, creating an area without light-sensitive cells: This is the blind spot. In octopus and squid, photoreceptors are in the front of the retina, not in the back as they are in vertebrates. As a result, octopus and squid don’t have a blind spot.

In addition to these three primary layers of neurons, two additional sets of nerve cells combine information laterally across the retina. Horizontal cells communicate between neighboring groups of photoreceptors and bipolar cells, enhancing contrast and adjusting photoreceptor sensitivity to light levels. Although their function is less well understood, we know that amacrine cells communicate between neighboring bipolar cells and ganglion cells, enhancing motion detection and likely reinforcing the adjustment of photoreceptor light sensitivity by horizontal cells.

Photoreceptor cells, bipolar cells, ganglion cells, horizontal cells, and amacrine cells form a neural network that processes visual information before it is sent to the brain for further processing and interpretation. Visual signals from approximately 131 million photoreceptors converge to approximately 1 million ganglion cell neurons that communicate with the brain. Properties of this network were first studied in the mammalian retina by the American neurophysiologists Stephen Kuffler, David Hubel, and Torsten Wiesel in the 1950s and 1960s (Fig. 36.20). Hubel and Wiesel shared the 1981 Nobel Prize in Physiology or Medicine for their later work on how visual information is processed in the brain.