We now consider in more detail the basic retinal circuit and how it accounts for the intricate response properties of retinal ganglion cells.
Parallel Pathways Originate in Bipolar Cells
The photoreceptor forms synapses with bipolar cells and horizontal cells (see Figure 26–3A). In the dark the cell's synaptic terminal releases glutamate continuously. On illumination the photoreceptor hyperpolarizes, less Ca2+ enters the terminal, and the terminal releases less glutamate. Photoreceptors do not fire action potentials; like bipolar cells they release neurotransmitter in a graded fashion using a specialized structure, the ribbon synapse. In fact, most retinal processing is accomplished with graded membrane potentials: Action potentials occur only in certain amacrine cells and in ganglion cells.
The two principal varieties of bipolar cells, ON and OFF cells, respond to glutamate at the synapse through distinct mechanisms. The OFF cells use ionotropic receptors, namely glutamate-gated cation channels of the AMPA-kainate variety (AMPA = α-amino-3-hydroxy-5-methylisoxazole-4-propionate). The glutamate released in darkness depolarizes these cells. The ON cells use metabotropic receptors that are linked to a G protein whose action ultimately closes cation channels. Glutamate activation of these receptors thus hyperpolarizes the cells in the dark.
Bipolar ON and OFF cells differ in shape and especially in the levels within the inner plexiform layer where their axons terminate. The axons of ON cells end in the proximal (lower) half, those of OFF cells in the distal (upper) half (Figure 26–15). There they form specific synaptic connections with amacrine and ganglion cells whose dendritic trees ramify in specific levels of the inner plexiform layer. The ON bipolar cells excite ON ganglion cells, while OFF bipolar cells excite OFF ganglion cells (see Figure 26–3A). Thus the two principal subdivisions of the retinal output signal, the ON and OFF pathways, are already established at the level of bipolar cells.
Bipolar cells in the macaque retina.
The cells are arranged according to the depth of their terminal arbors in the inner plexiform layer. The horizontal line dividing the distal and proximal levels of this layer represents the border between the axonal terminals of OFF and ON types. Bipolar cells with axonal terminals in the upper (distal) half are presumed to be OFF cells, those in the lower (proximal) half ON cells. Cell types are diffuse bipolar cells (DB), ON and OFF midget bipolars (IMB, FMB), S-cone ON bipolar (BB), and rod bipolar (RB). (Reproduced, with permission, from Boycott and Wässle 1999.)
Bipolar cells can also be distinguished by the morphology of their dendrites (Figure 26–15). In the central region of the primate retina the midget bipolar cell receives input from a single cone and excites a P-type ganglion cell. This explains why the centers of P-cell receptive fields are so small. The diffuse bipolar cell receives input from many cones and excites an M-type ganglion cell. The receptive-field centers of M-cells are accordingly much larger. Thus stimulus representations in the ganglion cell population originate in dedicated bipolar cell pathways that are differentiated by their selective connections to photoreceptors and postsynaptic targets.
Spatial Filtering Is Accomplished by Lateral Inhibition
Signals in the parallel vertical pathways are modified by lateral interactions with horizontal and amacrine cells (see Figure 26–3A). Horizontal cells have broadly arborizing dendrites that spread laterally in the outer plexiform layer. The tips of these arbors contact photoreceptors at terminals shared with bipolar cells. Glutamate released by the photoreceptors excites the horizontal cell. In addition, horizontal cells are electrically coupled with each other through gap junctions.
A horizontal cell effectively measures the average level of excitation of the photoreceptor population over a broad region. This signal is fed back to the photoreceptor terminal through an inhibitory synapse. Thus the photoreceptor terminal is under two opposing influences: light falling on the receptor hyperpolarizes it, but light falling on the surrounding region depolarizes it through the sign-inverting synapses from horizontal cells. As a result, the bipolar cell, which shares the photoreceptor's glutamatergic terminals with the horizontal cells, has an antagonistic receptive field structure.
This spatial antagonism in the receptive field is enhanced by lateral inhibition from amacrine cells in the inner retina. Amacrine cells are axonless neurons with dendrites that ramify in the inner plexiform layer. Approximately 30 types of amacrine cells are known, some with small arbors only tens of micrometers across, and others with processes that extend all across the retina. Amacrine cells generally receive excitatory signals from bipolar cells at glutamatergic synapses. Some amacrine cells feed back directly to the presynaptic bipolar cell at a reciprocal inhibitory synapse. Some amacrine cells are electrically coupled to others of the same type, forming an electrical network much like that of the horizontal cells.
Through this inhibitory network a bipolar cell terminal can receive inhibition driven by other, distant bipolar cells, in a manner closely analogous to the lateral inhibition of photoreceptor terminals (see Figure 26–3A). Amacrine cells also inhibit retinal ganglion cells directly. These lateral inhibitory connections contribute substantially to the antagonistic receptive field component of retinal ganglion cells.
Temporal Filtering Occurs in Synapses and Feedback Circuits
For many ganglion cells a step change in light intensity produces a transient response, an initial peak in firing that declines to a smaller steady rate (see Figure 26–10). Part of this sensitivity originates in the negative-feedback circuits involving horizontal and amacrine cells.
For example, a sudden decrease in light intensity depolarizes the cone terminal, which excites the horizontal cell, which in turn repolarizes the cone terminal (see Figure 26–3A). Because this feedback loop involves a brief delay, the voltage response of the cone peaks abruptly and then settles to a smaller steady level. Similar processing occurs at the reciprocal synapses between bipolar and amacrine cells in the inner retina.
In both cases the delayed-inhibition circuit favors rapidly changing inputs over slowly changing inputs. The effects of this filtering, which can be observed in visual perception, are most pronounced for large stimuli that drive the horizontal cell and amacrine cell networks most effectively. For example, a large spot can be seen easily when it flickers at a rate of 10 Hz but not at a low rate (see Figure 26–14).
In addition to these circuit properties, certain cellular processes contribute to shaping the temporal response. For example, the AMPA-kainate type of glutamate receptor undergoes strong desensitization. A step increase in the concentration of glutamate at the dendrite of a bipolar or ganglion cell leads to an immediate opening of additional glutamate receptors. As these receptors desensitize, the postsynaptic conductance decreases again. The effect is to render a step response more transient.
Retinal circuits seem to go to great lengths to speed up their responses and emphasize temporal changes. One likely reason is that the very first neuron in the retinal circuit, the photoreceptor, is exceptionally slow (see Figure 26–7C). Following a flash of light a cone takes about 40 ms to reach the peak response, an intolerable delay for proper visual function. Through the various filtering mechanisms in retinal circuitry, subsequent neurons respond sensitively during the rising phase of the cone's response. Indeed, some ganglion cells have a response peak only 20 ms after the flash. Temporal processing in the retina clearly helps to reduce visual reaction times, a life-extending trait in highway traffic as on the savannas of our ancestors.
Color Vision Begins in Cone-Selective Circuits
Throughout recorded history philosophers and scientists have been fascinated by the perception of color. This interest was fueled by the relevance of color to art, later by its relation to the physical properties of light, and finally by commercial interests in television and photography. The 19th century witnessed a profusion of theories to explain color perception, of which two have survived modern scrutiny. They are based on careful psychophysics that placed strong constraints on the underlying neural mechanisms.
Early experiments on color matching showed that the percept of any given light could be matched by mixing together appropriate amounts of three primary lights. Thomas Young and Hermann von Helmholtz accordingly postulated the trichromatic theory of color perception based on absorption of light by three mechanisms, each with a different sensitivity spectrum. We now know that these correspond to the three cone types (see Figure 26–6), whose measured absorption spectra fully explain the color-matching results both in normal individuals and those with genetic anomalies in the pigment genes.
In an effort to explain our perception of different hues, Ewald Hering proposed the opponent-process theory, later formalized by Leo Hurvich and Dorothea Jameson. According to this theory, color vision involves three processes that respond in opposite ways to light of different colors: (y–b) would be stimulated by yellow and inhibited by blue light; (r–g) stimulated by red and inhibited by green; and (w–bk) stimulated by white and inhibited by black. We can now recognize some of these processes in the post-receptor circuitry of the retina.
In the central 10° of the human retina a single midget bipolar cell that receives input from a single cone excites each P-type ganglion cell. An L-ON ganglion cell, for example, has a receptive field center consisting of a single L cone and an antagonistic surround involving a mixture of L and M cones. When stimulated with a large spot that extends over both the center and the surround, this neuron is depolarized by red light and hyperpolarized by green light. Similar antagonism holds for the three other P-cells: L-OFF, M-ON, and M-OFF. These P-cells send their signals to the parvocellular layers of the lateral geniculate nucleus.
Although S cones are relatively rare, a dedicated type of S-ON bipolar cell collects their signals selectively and transmits them to ganglion cells of the small bistratified type. Because this ganglion cell also receives excitation from L-OFF and M-OFF bipolar cells, it is depolarized by blue light and hyperpolarized by yellow light. Another ganglion cell type shows the opposite signature: S-OFF and (L + M)-ON. These signals are transmitted to the koniocellular layers of the lateral geniculate nucleus.
The M cells are excited by diffuse bipolar cells, which in turn collect inputs from many cones regardless of pigment type. These ganglion cells therefore have large receptive fields with broad spectral sensitivity. Their axons project to the magnocellular layers of the lateral geniculate nucleus.
In this way chromatic signals are combined and formatted by the retina for transmission to the thalamus and cortex. In the primary visual cortex these signals are recombined in different ways, leading to a great variety of receptive field layouts. Note that only about 10% of cortical neurons are preferentially driven by color contrast rather than luminance contrast. This likely reflects the fact that color vision—despite its great esthetic appeal—makes only a small contribution to our overall fitness. As an illustration of this, recall that colorblind individuals, who in a sense have lost half of their color space, can grow up without noticing that defect.
Congenital Color Blindness Takes Several Forms
Few people are truly colorblind in the sense of being wholly unable to distinguish a change in color from a change in the intensity of light, but many individuals have impaired color vision and experience difficulties in making distinctions that for most of us are trivial, for example between red and green. Most such abnormalities of color vision are congenital and have been characterized in detail; some other abnormalities result from injury or disease of the visual pathway.
The study of inherited variation in color vision has contributed in important ways to our understanding of the mechanisms of normal color vision. The first major insight, well understood in the 19th century, is that some people have only two classes of receptors instead of the three in normal trichromatic vision. These dichromats find it difficult or impossible to distinguish some surfaces whose colors appear distinct to trichromats. The dichromat's problem is that every surface reflectance function is represented by a two-value description rather than a three-value one, and this reduced description causes dichromats to confuse many more surfaces than do trichromats. Simple tests for color-blindness exploit this fact. Figure 26–16 shows an example from the Ishihara test, in which the numerals defined by colored dots are seen by normal trichromats but not by most dichromats.
A test for some forms of color blindness.
The numerals embedded in this color pattern can be distinguished by people with trichromatic vision but not by certain dichromats, including the Editor of this section of the book, who are weak in red–green discrimination. (Reproduced, with permission, from Ishihara 1993.)
When a person with normal color vision fails to distinguish two physically different surface reflectance functions, a dichromat will also fail to distinguish them. This failure means that each class of cone gives rise to the same signal when absorbing light reflected by either surface, so the fact that the dichromat is confused by the same surfaces that confuse a trichromat shows that the cones in the dichromat have normal pigments.
Although there are three forms of dichromacy, corresponding to the loss of each of the three types of cones, two kinds of dichromacy are much more common than the third. The common forms correspond to the loss of the L cones or M cones and are called protanopia and deuteranopia, respectively. Protanopia and deuteranopia almost always occur in males, each with a frequency of about 1%. The conditions are transmitted by women who are not themselves affected, and so implicate genes on the X chromosome. A third form of dichromacy, tritanopia, corresponds to the loss or dysfunction of the S cone. It affects only about 1 in 10,000 people, afflicts women and men with equal frequency and has a gene on chromosome 7.
Because the L and M cones exist in large numbers, one might think that the loss of one or the other type would impair vision more broadly than just weakening color vision. In fact, this does not happen because the total number of L and M cones in the dichromat retina is not altered. All cells destined to become L or M cones are probably converted to L cones in deuteranopes and to M cones in protanopes.
In addition to the relatively severe forms of colorblindness represented by dichromacy, there are milder forms, again affecting mostly males, that result in an impaired capacity to distinguish different reflectance functions that are readily distinguished by normal trichromats. People with these milder impairments are referred to as anomalous trichromats, for their cones provide three-value descriptions of the light reflected by surfaces. In contrast to dichromats, however, they do not see as identical the physically different spectral functions distinguished by a normal trichromat.
These anomalous trichromats have cones whose spectral sensitivities differ from those of cones in normal trichromats. Anomalous trichromacy occurs in different forms, corresponding to the replacement of one of the normal cone pigments by an altered protein with a different spectral sensitivity. Two common forms, protanomaly and deuteranomaly, together affect about 7% of males and represent respectively the replacement of the L or M cones by a pigment with some intermediate spectral sensitivity.
The occurrence of sex-linked inherited defects of color vision points to the X chromosome as the locus of genes that encode the visual pigments of L and M cones. These genes, and the amino acid sequences of the pigments they encode, have now been identified, largely through the work of Jeremy Nathans and his colleagues. Their discovery reveals some interesting complexities in the molecular organization underlying color vision. Molecular cloning of the genes for the L and M pigments shows the genes to be very similar and arranged head-to-tail on the X chromosome (Figure 26–17A). The pigments also have very similar structures, differing in only 4% of their amino acids. People with normal color vision possess a single copy of the gene for the L pigment and from one to three—occasionally as many as five—nearly identical copies of the gene for the M pigment.
L- and M-pigment genes on the X chromosome.
A. Arrangement of L- and M-pigment genes in color-normal males. The base of each arrow corresponds to the 5′end of the gene, and the tip corresponds to the 3′end. Males with normal color vision can have one, two, or three copies of the gene for the M pigment on each X chromosome. (Adapted, with permission, from Nathans, Thomas, and Hogness 1986.)
B. Because they lie next to each other on the chromosome, the L- and M-pigment genes can undergo recombinations that lead to the generation of a hybrid gene (3 and 4) or the loss of a gene (1), the patterns observed in colorblind men. Spurious recombination can also cause gene duplication (2), a pattern observed in some people with normal color vision. (Adapted, with permission, from Stryer 1988.)
The proximity and similarity of these genes is thought to predispose them to varied forms of recombination, leading either to the loss of a gene or to the formation of hybrid genes that account for the common forms of red-green defect (Figure 26–17B). Examination of these genes in dichromats reveals a loss of the L-pigment gene in protanopes and a loss of one or more M-pigment genes in deuteranopes. Anomalous trichromats have L-M or M-L hybrid genes that code for visual pigments with shifted spectral sensitivity, the extent of the shift depending on the point of recombination. In tritanopes, the loss of S-cone function arises from mutations in the S-pigment gene.
Rod and Cone Circuits Merge in the Inner Retina
For vision under low-light conditions the mammalian retina has an ON bipolar cell that is exclusively connected to rods (see Figure 26–3B). By collecting inputs from up to 50 rods, this rod bipolar cell can pool the effects of dispersed single-photon absorptions in a small patch of retina. This neuron is excited by light and there is no corresponding OFF bipolar cell dedicated to rods.
Unlike all other bipolar cells, the rod bipolar cell does not contact ganglion cells directly but instead excites a dedicated neuron called the AII amacrine cell. This amacrine cell receives inputs from several rod bipolar cells and conveys its output to cone bipolar cells. It sends excitatory signals to ON bipolar cells through gap junctions as well as glycinergic inhibitory signals to OFF bipolar cells. These cone bipolar cells in turn excite ON and OFF ganglion cells as described above. Thus the rod signal is fed into the cone system after a detour, involving the rod bipolar and AII amacrine cells, that produces the appropriate signal polarities for the ON and OFF pathways. The purpose of these added interneurons may be to allow greater pooling of rod signals than of cone signals.
Rod signals also enter the cone system through two other pathways. Rods can drive neighboring cones directly through electrical junctions, and they make connections with an OFF bipolar cell that services primarily cones. Once the rod signal has reached the cone bipolars through these pathways, it can take advantage of the same intricate circuitry of the inner retina. One gets the impression that the rod system of the mammalian retina is an evolutionary afterthought added to the cone circuits.