As described previously, absorption of a photon causes 11-cis retinal to be converted to all-trans retinal, which then splits from the opsin (bleaching). The visual pigments in rods and cones are bleached at a similar rate; however, regeneration of the visual pigment occurs much more rapidly in cones than in rods. This difference is, at least in part, due to the cones’ ability to utilize a second pathway for regeneration (see previous section). This more rapid regeneration of visual pigment prevents cones from becoming unresponsive in bright light conditions. In contrast, the slowness of the regeneration of rhodopsin molecules means that at light levels not much above those found in evening hours, essentially all of the rhodopsin molecules are bleached. Thus in bright-light conditions, only the cone system is functioning, and the retina is said to be light-adapted.
When entering a darkened movie theater, a person can observe evidence of the existing light adaptation (decreased light sensitivity in association with the reduced amount of rhodopsin) in the inability to see the empty seats (or much else). The gradual return of the ability to see the seats while the person remains in the theater reflects the slow regeneration of rhodopsin and recovery of function of the rod system, a process known as dark adaptation.
This process refers to the gradual increase in light sensitivity of the retina when in low-light conditions. Rods adapt to darkness slowly as their rhodopsin levels are restored, and, indeed, it may take more than 30 minutes for the retina to become fully dark-adapted. In contrast, cones adapt rapidly to darkness, but their adapted threshold is relatively high, and so they do not function when the ambient light level is low. Thus within 10 minutes in a dark room, rod vision is more sensitive than cone vision and becomes the main system for seeing.
In sum, in the dark-adapted state, primarily rod vision is operative, and thus visual acuity is low and colors are not distinguished (this is called scotopic vision). However, when light levels are higher (e.g., when the movie is projected) and cone function resumes (this is called photopic vision), visual acuity and color vision are restored. There is an intermediate range of light levels at which rod and cones are both functional (mesopic vision).
The visual pigments in the cone outer segments contain different opsins. As a result of these differences, the three types of cones absorb light best at different wavelengths. Although the cone pigments have maximum efficiency closer to violet, green, and yellow wavelengths, they are referred to as blue, green, and red pigments, respectively (see
Fig. 8.5). The differences in the cone absorption spectra underlie humans’ ability to see colors, as opposed to only shades of gray.
According to the trichromacy theory, the differences in absorption efficiency of the cone visual pigments are presumed to account for color vision because a suitable mixture of three colors can produce any other color. However, a neural mechanism must also exist for the analysis of color brightness because the amount of light absorbed by a visual pigment, as well as the subsequent response of the cell, depends on both the wavelength and the intensity of the light (see
Fig. 8.5 ). Two or three of the cone pigments may absorb a particular wavelength of light, but the amount absorbed by each differs according to its efficiency at that wavelength. If the intensity of the light is increased (or decreased), all will absorb more (or less), but the ratio of absorption among them will remain constant. Consequently, there must be a neural mechanism to compare the absorption of light of different wavelengths by the different types of cones for the visual system to distinguish different colors. At least two different kinds of cones are required for color vision. The presence of three kinds decreases the ambiguity in distinguishing colors when all three absorb light, and it ensures that at least two types of cones will absorb most wavelengths of visible light.