How the Retina Works
Much of the construction of an image takes place in the retina itself through the use of specialized neural circuits
The parallel sets of visual channels for ON (detecting light areas on dark backgrounds) and OFF (detecting dark areas on light backgrounds) qualities of an image are fundamental to our seeing. Vertebrate vision depends on perceiving the contrast between images and their backgrounds. For example, we read black letters against a white background using the OFF channels that start in the retina. Parallel bipolar channels transmit inputs to ganglion cells. Early in development the architecture of the inner plexiform layer, full of synapses between bipolar and ganglion cells, shows that synaptic connections become segregated in distinct, parallel pathways. Connections occur between ON bipolar cells and ON ganglion cells and also between OFF bipolar cells and OFF ganglion cells in demarcated portions of the inner plexiform layer.
If the retina were simply to transmit opposite-contrast images directly from the photoreceptors to the brain, the resulting vision would probably be coarse-grained and blurry. Further processing in the retina defines precise edges to images and allows us to focus on fine details. The honing of the image starts at the first synaptic level in the retina, where horizontal cells receive input from cones. Each horizontal cell actually receives input from many cones, so its collection area or receptive field is large. Horizontal cells' receptive fields become even broader because their plasma membranes fuse with those of neighboring horizontal cells at gap junctions. The membrane potentials of a whole sheet of cells become the same; consequently, horizontal cells respond to light over a very large area. Meanwhile, a single bipolar cell receives input from a handful of cones and thus has a medium-size receptive field.
Whereas a single bipolar cell with its OFF or ON light response would carry a fairly blurry response to its ganglion cell, horizontal cells add an opponent signal that is spatially constrictive, giving the bipolar cell what is known as a center surround organization (Figure 9). The bipolar center signals either ON or OFF, and the horizontal cells add an OFF or ON surround signal, by one of two means. The horizontal cells can either signal the bipolar cell or feed information back to the cone photoreceptors themselves, which then feed forward information to the bipolar cells the cones contact. Feedback to the cones is now proposed to occur by means of an unusual electrical synapse consisting of half a gap junction; these hemi gap junctions are thought to change the ionic environment across the membrane of the cone photoreceptor. This complicated circuit from horizontal cell to cone to bipolar cells is still a subject of hot debate in the community of retina scientists.
Horizontal-cell function has occupied many vision scientists for decades, and much is now known about the role of these cells in the organization of visual messages. Horizontal cells respond to more than the photoreceptors that link to them. Feedback signals from the inner plexiform layer influence horizontal-cell activity as well. These feedback signals are transmitted via substances such as dopamine, nitric oxide and retinoic acid. The result is that horizontal cells modulate the photoreceptor signal under different lighting conditions—allowing signaling to become less sensitive in bright light and more sensitive in dim light—as well as shaping the receptive field of the bipolar cells, as we have seen. The horizontal cells can even make the bipolar cells' response color-coded, all apparently through feedback circuits to the cones.
The ganglion cells have a receptive field organized as concentric circles. The amacrine-cell circuitry in the inner plexiform layer conveys additional information to the ganglion cell, possibly sharpening the boundary between center and surround even further than the horizontal-cell input does. In human retinas, two basic types of ganglion cells—ON center and OFF center—form the major output of the retina to the visual centers in the brain (Figure 10, left). ON-center ganglion cells are activated when a spot of light falls in the center of their receptive field and are inactivated when light falls on the field's periphery. OFF-center ganglion cells react in the opposite way: Their activity increases when the periphery of their receptive field is lit and decreases when light falls on the center of the field. (The receptive fields of ganglion cells are modeled as the difference between Gaussian distributions, giving them a so-called Mexican-hat shape.)
In contrast to the rest of the retina, the human fovea contains midget ganglion cells, which have minute dendritic trees connected in a one-to-one ratio with midget bipolar cells (Figure 10, right). The channel from midget bipolar to midget ganglion cell carries information from a single cone, thus relaying a point-to-point image from the fovea to the brain. Each red or green cone in the central fovea connects to two midget ganglion cells, so at all times each cone can either transmit a dark-on-light (OFF) signal or a light-on-dark (ON) message. The message that goes to the brain carries both spatial and spectral information of the finest resolution.
Messages from blue cones are not processed in the same way as from red and green cones for some reason, possibly because the blue system is older in evolutionary terms. Blue cones are found in the retinas of most species. The typical mammalian retina also has green cones; primates have the additional red cones. Blue cones transmit information through a special blue cone bipolar cell to a different type of ganglion cell, which can carry both a blue ON and a yellow OFF response.
Electrical recordings show that several types of ganglion cells do not have concentric organization, especially in animals whose eyes lack a fovea. This includes most nonmammalian species and mammalian species that have retinas with visual streaks. Compared with species with foveas, the species with visual streaks do even more image processing in the retina itself before sending a message to the brain; their retinas can immediately synthesize information about image motion and direction of motion.