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.