The opponent process theory is based on the observation that certain color pairs activate opposing neural processes. Opposed color pairs include green and red, yellow and blue, and black and white. When a gray area is surrounded by a green ring, the gray area appears reddish. Additionally, there are no greenish red or bluish yellow colors. These observations are supported by the finding that neurons stimulated by green wavelengths are inhibited by red wavelengths. Similarly, neurons excited by blue wavelengths may be inhibited by yellow wavelengths. These types of neurons are found in the retina and higher levels of the visual pathway, enhancing the ability to perceive the contrast between opposing colors.

A diagram of the retina's basic circuitry is shown in Figure 8.7. Noteworthy features of this circuitry include the input provided by light striking the photoreceptors, and the output carried by the axons of retinal ganglion cells to the brain. Within the retina, information is processed by interneurons. The most direct pathway through the retina is from a photoreceptor to a bipolar cell and then to a ganglion cell. There are also indirect pathways that involve photoreceptors, bipolar cells, amacrine cells, and ganglion cells, as well as horizontal cells that facilitate lateral interactions between adjacent pathways.

Observations on color blindness support the trichromacy theory. In color blindness, which is a genetic defect (sex-linked recessive), one or more cone mechanisms are lost. Individuals with normal color vision are trichromats because they possess three cone mechanisms. Those who lack one cone mechanism are called dichromats. When the long-wavelength cone mechanism is absent, the condition is called protanopia. Absence of the medium-wavelength system causes deuteranopia, and absence of the short-wavelength system causes tritanopia. Monochromats lack two or more cone mechanisms.

Rod and cone pathways exhibit several important functional differences in their phototransduction mechanisms and retinal circuitry. Rods have more visual pigment and a better signal amplification system compared to cones. There are also significantly more rods than cones. Consequently, rods function better in dim light (scotopic vision), and loss of rod function leads to night blindness. Moreover, all rods contain the same visual pigment, rendering them incapable of signaling color differences. Additionally, many rods converge onto individual bipolar cells, resulting in large receptive fields and low spatial resolution. Finally, in bright light, most rhodopsin is bleached, rendering rods non-functional under photopic conditions.

Cones have a higher light threshold and therefore are not activated in dim light after dark adaptation. However, they perform optimally in daylight conditions. Cones provide high-resolution vision as only a few cones converge onto individual bipolar cells in cone pathways. In the fovea, where cones make one-to-one connections to bipolar cells, no convergence occurs. As a result of reduced convergence, cone pathways have small receptive fields and can resolve stimuli originating from sources very close to each other. Cones also exhibit good temporal resolution when responding to sequential stimuli. Finally, cones have three different types of visual pigments.
The opponent process theory is based on observations that certain pairs of colors seem to activate opposing neural processes. Green and red are opposed, as are yellow and blue, as well as black and white. For example, if a gray area is surrounded by a green ring, the gray area appears to acquire a reddish color. Furthermore, a greenish red or a bluish yellow color does not exist. These observations are supported by findings that neurons activated by green wavelengths are inhibited by red wavelengths. Similarly, neurons excited by blue wavelengths may be inhibited by yellow wavelengths. Neurons with these characteristics are present both in the retina and at higher levels of the visual pathway and seem to serve to increase the ability to see the contrast between opposing colors.
A diagram of the basic circuitry of the retina is shown in
Fig. 8.7 . Several features of this circuitry are noteworthy. Input to the retina is provided by light striking the photoreceptors. The output is carried by axons of the retinal ganglion cells to the brain. Information is processed within the retina by the interneurons. The most direct pathway through the retina is from a photoreceptor to a bipolar cell and then to a ganglion cell (see
Fig. 8.7).
More indirect pathways that provide for intraretinal signal processing involve photoreceptors, bipolar cells, amacrine cells, and ganglion cells, as well as horizontal cells to provide lateral interactions between adjacent pathways.
Observationsoncolorblindnessareconsistentwiththetrichromacytheory.Incolorblindness,ageneticdefect(sex-linkedrecessive),oneormoreconemechanismsarelost.Peoplewithnormalcolorvisionaretrichromatsbecausetheyhavethreeconemechanisms.Individualswholackoneoftheconemechanismsarecalleddichromats.Whenthelong-wavelengthconemechanismisabsent,theresultingconditioniscalledprotanopia;absenceofthemedium-wavelengthsystemcausesdeuteranopia;andabsenceoftheshort-wavelengthsystemcausestritanopia.Monochromatslacktwoormoreconemechanisms.
Rod and cone pathways have several important functional differences in their phototransduction mechanisms and their retinal circuitry. As described previously, rods have more visual pigment and a better signal amplification system than cones do, and there are many more rods than cones. Thus rods function better in dim light (scotopic vision), and loss of rod function results in night blindness. In addition, all rods contain the same visual pigment, so they cannot signal color differences. Furthermore, many rods converge onto individual bipolar cells and the results are very large receptive fields and low spatial resolution. Finally, in bright light, most rhodopsin is bleached, so that rods no longer function under photopic conditions.
Cones have a higher threshold to light and thus are not activated in dim light after dark adaptation. However, they operate very well in daylight. They provide high-resolution vision because only a few cones converge onto individual bipolar cells in cone pathways. Moreover, no convergence occurs in the fovea, where the cones make one-to-one connections to bipolar cells. As a result of the reduced convergence, cone pathways have very small receptive fields and can resolve stimuli that originate from sources very close to each other. Cones also respond to sequential stimuli with good temporal resolution. Finally, cones have three
CHAPTER 8 The Special Senses •Fig. 8.7 Basic Retinal Circuitry. Thearrow attheleftindicatesthedirectionoflightthroughtheretina.Photoreceptors(R)synapseonthedendritesofbipolarcells(B)andhorizontalcells(H)intheouterplexiformlayer.Thehorizontalcellsmakereciprocalsynapticconnectionswithphotoreceptorcellsandareelectricallycoupledtootherhorizontalcells.Bipolarcellsreachsynapseonthedendritesofganglioncells(G)andontheprocessesofamacrinecells(A)intheinnerplexiformlayer.Amacrinecellsconnectwithganglioncellsandotheramacrinecells.