Automatic Regulation of Retinal Sensitivity-Light and Dark Adaptation
Light and Dark Adaptation. If a person has been in brightlight for hours, large portions of the photochemicals in both the rods and the cones will have been reduced to retinal and opsins. Furthermore, much of the retinal of both the rods and the cones will have been converted into vitamin A. Because of these two effects, the con-centrations of the photosensitive chemicals remaining in the rods and cones are considerably reduced, and the sensitivity of the eye to light is correspondingly reduced. This is called light adaptation.
Conversely, if a person remains in darkness for a long time, the retinal and opsins in the rods and cones are converted back into the light-sensitive pigments. Furthermore, vitamin A is converted back into retinal to give still more light-sensitive pigments, the final limit being determined by the amount of opsins in the rods and cones to combine with the retinal. This is called dark adaptation.
Figure 50–8 shows the course of dark adaptation when a person is exposed to total darkness after having been exposed to bright light for several hours.
Note that the sensitivity of the retina is very low on first entering the darkness, but within 1 minute, the sensitivity has already increased 10-fold—that is, the retina can respond to light of one tenth the previously required intensity. At the end of 20 minutes, the sensi-tivity has increased about 6000-fold, and at the end of 40 minutes, about 25,000-fold.
The resulting curve of Figure 50–8 is called the darkadaptation curve. Note, however, the inflection in thecurve.The early portion of the curve is caused by adap-tation of the cones, because all the chemical events of vision, including adaptation, occur about four times as rapidly in cones as in rods. However, the cones do not achieve anywhere near the same degree of sensitivity change in darkness as the rods do. Therefore, despite rapid adaptation, the cones cease adapting after only a few minutes, while the slowly adapting rods continue to adapt for many minutes and even hours, their sen-sitivity increasing tremendously. In addition, still more sensitivity of the rods is caused by neuronal signal con-vergence of 100 or more rods onto a single ganglion cell in the retina; these rods summate to increase their sensitivity.
Other Mechanisms of Light and Dark Adaptation. In additionto adaptation caused by changes in concentrations of rhodopsin or color photochemicals, the eye has two other mechanisms for light and dark adaptation. The first of these is a change in pupillary size. This can cause adaptation of approxi-mately 30-fold within a fraction of a second, because of changes in the amount of light allowed through the pupillary opening.
The other mechanism is neural adaptation, involving the neurons in the successive stages of the visual chain in the retina itself and in the brain. That is, when light intensity first increases, the signals transmitted by the bipolar cells, horizontal cells, amacrine cells, and gan-glion cells are all intense. However, most of these signals decrease rapidly at different stages of transmission in the neural circuit. Although the degree of adaptation is only a fewfold rather than the many thousandfold that occurs during adaptation of the photochemical system, neural adaptation occurs in a fraction of a second, in contrast to the many minutes to hours required for full adaptation by the photochemicals.
Value of Light and Dark Adaptation in Vision. Between thelimits of maximal dark adaptation and maximal light adaptation, the eye can change its sensitivity to light as much as 500,000 to 1 million times, the sensitivity automatically adjusting to changes in illumination.
Because registration of images by the retina requires detection of both dark and light spots in the image, it is essential that the sensitivity of the retina always be adjusted so that the receptors respond to the lighter areas but not to the darker areas. An example of maladjustment of retinal adaptation occurs when a person leaves a movie theater and enters the bright sunlight. Then, even the dark spots in the images seem exceedingly bright, and as a consequence, the entire visual image is bleached, having little contrast among its different parts. This is poor vision, and it remains poor until the retina has adapted sufficiently so that the darker areas of the image no longer stimulate the receptors excessively.
Conversely, when a person first enters darkness, the sensitivity of the retina is usually so slight that even the light spots in the image cannot excite the retina. After dark adaptation, the light spots begin to regis-ter. As an example of the extremes of light and dark adaptation, the intensity of sunlight is about 10 billion times that of starlight, yet the eye can function both in bright sunlight after light adaptation and in starlight after dark adaptation.