Our visual system operates over an enormous range of light levels. This range extends from night time when one can barely see anything to high noon with a cloudless sky and the ground covered with clean white snow. Or, if you prefer a summer example, high noon on a beach with clean white sand. In the latter two cases one usually requires sunglasses to see comfortably. The night time example is characterized by our lack of color vision and low acuity. Someone once said, "at night all cats are gray." If you look at a newspaper under these night time conditions you probably can read only the headlines. There are two ways to think about this range: 1. Once having been exposed to a certain light level what are the factors that allow us to become "use" (adapt) to lower light levels. Think about a time when you were outside if very bright sunlight and then went indoors. For a while it was very difficult to see, but in time the ease with which once was able to see returned. 2. As indicated in the opening sentence our visual system responds over an enormous range of light intensities. We will discuss the visual mechanisms that make this possible. Let us start with dark adaptation.
Adapting To Darkness
There are several factors, including two classes of receptors (rods and cones), the amount of photopigment in the outer segments of the receptors and neural gain control factors that influence dark adaptation. I will start out by primarily looking at the scotopic function because it is simpler than the photopic system with its three classes of cone receptors.
We have two classes of receptors in our retina: rods and cones. One of the features of these receptors, not previously discussed, is their connections to the optic nerve. Cones tend to individually connect to individual optic nerve fibers. Multiple rods, on the other hand, converge onto single optic nerve fibers. It was discovered many years ago that it took only one quantum of light energy to activate a rod, but it took several such hits for a threshold visual response. It would seem therefore that rods would have an advantage over cones because rods can pool the signals by the convergence of multiple receptors onto a single optic nerve fiber.
The outer segments of visual receptors contain pigments which absorb light and start the electrophysiological chain of events that result in our seeing. When the photopigments absorb light they bleach after which they can not absorb more light until the photopigment regenerates. With the aid of an apparatus called the retinal densitometer it was possible to follow the course of rhodopsin regeneration in a human eye. If all of the rhodopsin is in the unbleached state they sensitivity of the visual system (actually of the rods) would be very great. On the other hand, when 100% of the rhodopsin is bleached scotopic sensitivity would be nil. It is, therefore, very tempting to ascribe the full range of scotopic sensitivity to the amount of rhodopsin available to absorb light. Unfortunately, things aren't quite so simple. If they were then one should obtain exactly the same shaped dark adaptation function regardless of the size, duration, wavelength or other parameters. If the amount of unbleached photopigment was the only important variable in this function then one should only see a vertical shift, a change in sensitivity. The shape of the function should not change. But it does. Consequently, there must be other factors in addition to the amount of unbleached photopigment that contribute to dark adaptation. So, let us now see what they might be. The size of the stimulus would be important. Multiple rods converge on to single optic nerves. Consequently, a large scotopic stimulus should be more easily seen than a smaller one.