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.
The 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.
Photopigments
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.