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Discuss the purpose, methods, and results of the dark…
Discuss the purpose, methods, and results of the dark adaptation experiment discussed in Chapter 2 of your text. What do the results of these studies inform us about the functions of the rods and cones?
experiment provided below:
Adapting to the Dark
When we discussed measuring perception in Chapter 1, we noted that when a person goes from a lighted environment to a dark place, it may be difficult to see at first, but that after some time in the dark, the person becomes able to make out lights and objects that were invisible before (Figure 1.16). This process of increasing sensitivity in the dark, called dark adaptation, is measured by determining a dark adaptation curve. In this section we will show how the rod and cone receptors control an important aspect of vision: the ability of the visual system to adjust to dim levels of illumination. We will describe how the dark adaptation curve is measured, and how the increase in sensitivity that occurs in the dark has been linked to properties of the rod and cone visual pigments.
Measuring the Dark Adaptation Curve
The study of dark adaptation begins with measuring the dark adaptation curve, which is the function relating sensitivity to light to time in the dark, beginning when the lights are extinguished.
MethodMeasuring the Dark Adaptation Curve
The first step in measuring a dark adaption curve is to have the subject look at a small fixation point while paying attention to a flashing test light that is off to the side (Figure 2.12). Because the subject is looking directly at the fixation point, its image falls on the fovea, so the image of the test light falls on the peripheral retina, which contains both rods and cones. While still in the light, the subject turns a knob that adjusts the intensity of the flashing light until it can just barely be seen. This threshold for seeing the light, the minimum amount of energy necessary to just barely see the light, is then converted to sensitivity. Because , this means that a high threshold corresponds to low sensitivity. The sensitivity measured in the light is called the light-adapted sensitivity, because it is measured while the eyes are adapted to the light. Because the room (or adapting) lights are on, the intensity of the flashing test light has to be high to be seen. At the beginning of the experiment, then, the threshold is high and the sensitivity is low.
Figure 2.12
Viewing conditions for a dark adaptation experiment. In this example, the image of the fixation point falls on the fovea, and the image of the test light falls on the peripheral retina.
Once the light-adapted sensitivity to the flashing test light is determined, the adapting light is extinguished so the subject is in the dark. The subject continues adjusting the intensity of the flashing light so he or she can just barely see it, tracking the increase in sensitivity that occurs in the dark. As the subject becomes more sensitive to the light, he or she must decrease the light’s intensity to keep it just barely visible. The result, shown as the red curve in Figure 2.13, is a dark adaptation curve.
Figure 2.13
Three dark adaptation curves. The red line is the two-stage dark adaptation curve, with an initial cone branch and a later rod branch, which occurs when the test light is in the peripheral retina, as shown in Figure 2.12. The green line is the cone adaptation curve, which occurs when the test light falls on the fovea. The purple curve is the rod adaptation curve measured in a rod monochromat. Note that the downward movement of these curves represents an increase in sensitivity. The curves actually begin at the points indicating “light-adapted sensitivity,” but there is a slight delay between the time the lights are turned off and when measurement of the curves begins.
The dark adaptation curve shows that as adaptation proceeds, the subject becomes more sensitive to the light. Note that higher sensitivity is at the bottom of this graph, so movement of the dark adaptation curve downward means that the subject’s sensitivity is increasing. The red dark adaptation curve indicates that the subject’s sensitivity increases in two phases. It increases rapidly for the first 3 to 4 minutes after the light is extinguished and then levels off. At about 7 to 10 minutes, it begins increasing again and continues to do so until the subject has been in the dark for about 20 or 30 minutes (Figure 2.13). The sensitivity at the end of dark adaptation, labeled dark-adapted sensitivity, is about 100,000 times greater than the light-adapted sensitivity measured before dark adaptation began.
Dark adaptation was involved in a 2007 episode of the Mythbusters program on the Discovery Channel, which was devoted to investigating myths about pirates. One of the myths was that pirates wore eye patches to preserve night vision in one eye so that when they went from the bright light outside to the darkness below decks, removing the patch would enable them to see. To determine whether this would work, the Mythbusters carried out some tasks in a dark room just after both of their eyes had been in the light and did some different tasks with an eye that had previously been covered with a patch for 30 minutes. It isn’t surprising that they completed the tasks much more rapidly when using the eye that had been patched. Anyone who has taken a course on sensation and perception could have told the Mythbusters that the eye patch would work because keeping an eye in the dark triggers the process of dark adaptation, which causes the eye to increase its sensitivity in the dark.
Whether pirates actually used patches to help them see below decks remains an unproven hypothesis. One argument against the idea that pirates wore eye patches to keep their sensitivity high is that patching one eye causes a decrease in depth perception, which might be a serious disadvantage when the pirate is working on deck.
Although the Mythbusters showed that dark adapting one eye made it easier to see with that eye in the dark, we have a more specific goal. We are interested in showing that the first part of the dark adaptation curve is caused by the cones and the second part is caused by the rods. We will by running two additional dark adaptation experiments, one measuring adaptation of the cones and another measuring adaptation of the rods.
