Some basic facts of vision
So far, attention has been directed to what are essentially the preliminaries to vision. It is now time to examine some of the elementary facts of vision and to relate them to the structure of the retina and, later, to chemically identifiable events.
Measurement of the threshold
An important means of measuring a sensation is to determine the threshold stimulus—i.e., the minimum energy required to evoke the sensation. In the case of vision, this would be the minimum number of quanta of light entering the eye in unit time. If it is found that the threshold has altered because of a change of some sort, then this change can be said to have altered the subject’s sensitivity to light, and a numerical value can be assigned to the sensitivity by use of the reciprocal of the threshold energy.
Practically, a subject may be placed in the dark in front of a white screen, and the screen may be illuminated by flashes of light. For any given intensity of illumination of the screen, it is not difficult to calculate the flow of light energy entering the eye. One may begin with a low intensity of flash and increase this successively until the subject reports that he or she can see the flash. In fact, at this threshold level, the individual will not see every flash presented, even though the intensity of the light is kept constant. For this reason, a certain frequency of seeing—e.g., four times out of six—must be selected as the arbitrary point at which to fix the threshold.
When measurements of this sort are carried out, it is found that the threshold falls progressively as the subject is maintained in the dark room. This is not due to dilation of the pupil, because the same phenomenon occurs if the subject is made to look through an artificial pupil of fixed diameter. The eye, after about 30 minutes in the dark, may become about 10,000 times more sensitive to light. Vision under these conditions is, moreover, characteristically different from what it is under ordinary daylight conditions. Thus, in order to obtain best vision, the eye must look away from the screen so that the image of the screen does not fall on the fovea. If the screen is continuously illuminated at around this threshold level, it will be found to disappear if its image is brought onto the fovea, and it will become immediately visible on looking away. The same phenomenon may be demonstrated on a moonless night if the gaze is fixed on a dim star: it disappears on fixation and reappears on looking away. This feature of vision under these near-threshold, or scotopic, conditions suggests that the cones are effectively blind to weak light stimuli, since they are the only photoreceptors in the fovea. This is the basis of the duplicity theory of vision, which postulates that when the light stimulus is weak and the eye has been dark-adapted, it is the rods that are utilized because, under these conditions, their threshold is much lower than that of the cones. When the subject first enters the dark, the rods are the less sensitive type of photoreceptor, and the threshold stimulus is the light energy required to stimulate the cones. During the first five or more minutes, the threshold of the cones decreases; i.e., they become more sensitive. The rods then increase their sensitivity to the point that they are the more sensitive, and it is they that now determine the sensitivity of the whole eye, the threshold stimuli obtained after 10 minutes in the dark, for example, being too weak to activate the cones.
Scotopic sensitivity curve
When different wavelengths of light are employed for measuring the threshold, it is found, for example, that the eye is much more sensitive to blue-green light than to orange. The interesting feature of this kind of study is that the subject reports only that the light is light; he or she distinguishes no colour. If the intensity of a given wavelength of light is increased step by step above the threshold, a point comes when the subject states that it is coloured. The difference between the threshold for light appreciation and this, the chromatic threshold, is called the photochromatic interval. This suggests that the rods give only achromatic, or colourless, vision, and that it is the cones that permit wavelength discrimination. The photochromatic interval for long wavelengths (red light) is about zero, which means that the intensity required to reach the sensation of light is the same as that to reach the sensation of colour. This is because the rods are so insensitive to red light. If the dark-adaptation curve is plotted for a red stimulus, it is found that it follows the cone path, like that for foveal vision at all wavelengths.
Loss of dark adaptation
If, when the subject has become completely dark-adapted, one eye is held shut and the other exposed to a bright light for a little while, it is found that, whereas the dark-adapted eye retains its high sensitivity, that of the light-exposed eye has decreased greatly; it requires another period of dark adaptation for the two eyes to become equally sensitive.
These simple experiments pose several problems, the answers to which throw a great deal of light on the whole mechanism of vision. Why, for example, does it require time for both rods and cones to reach their maximum sensitivity in the dark? Again, why is visual acuity so low under scotopic conditions compared with that in daylight, although sensitivity to light is so high? Finally, why do the rods not serve to discriminate different wavelengths?
Bleaching of rhodopsin
It may be assumed that a photoreceptor is sensitive to light because it contains a substance that absorbs light and converts this vibrational type of energy into some other form that is eventually transmuted into electrical changes, which may be transmitted from the photoreceptor to the bipolar cell with which it is immediately connected. When the retina of a dark-adapted animal is removed and submitted to extraction procedures, a pigment, originally called visual purple but now called rhodopsin, may be obtained. If the eye is exposed to a bright light for some time before extraction, little or no rhodopsin is obtained. When retinas from animals that had been progressively dark-adapted were studied, a gradual increase in the amount of rhodopsin that could be extracted was observed. Thus, rhodopsin, on absorption of light energy, is changed to some other compound, but new rhodopsin is formed, or rhodopsin is regenerated, during dark adaptation. The obvious inference is that rhodopsin is the visual pigment of the rods and that, when it is exposed to relatively intense light, it becomes useless for vision. When the eye is allowed to remain in the dark, the rhodopsin regenerates and thus becomes available for vision. There is now conclusive proof that rhodopsin is, indeed, the visual pigment for the rods. It is obtained from retinas that have only rods and no cones—e.g., the retinas of the rat or guinea pig—and it is not obtained from the pure cone retina of the chicken.
When the absorption spectrum is measured, it is found that its maximum absorption occurs at the point of maximum sensitivity of the dark-adapted eye. Similar measurements may be carried out on animals, but the threshold sensitivity must be determined by some objective means—e.g., the response of the pupil, or, better still, the electrical changes occurring in the retina in response to light stimuli. Thus, the electroretinogram (ERG) is the record of changes in potential between an electrode placed on the surface of the cornea and an electrode placed on another part of the body, caused by illumination of the eye.
The high sensitivity of the rods by comparison with the cones may be a reflection of the greater concentration in them of pigment that would permit them to catch light more efficiently, or it may depend on other factors—e.g., the efficiency of transformation of the light energy into electrical energy. The pigments responsible for cone vision are not easily extracted or identified, and the problem will be considered in the material on colour vision. An important factor, so far as sensitivity is concerned, is the actual organization of the photoreceptors and neurons in the retina.
Synaptic organization of the retina
The basic structure of the retina has been indicated earlier. As in other parts of the nervous system, the messages initiated in one element are transmitted, or relayed, to others. The regions of transmission from one cell to another are areas of intimate contact known as synapses. An impulse conveyed from one cell to another travels from the first cell body along a projection called an axon, to a synapse, where the impulse is received by a projection, called a dendrite, of the second cell. The impulse is then conveyed to the second cell body, to be transmitted further, along the second cell’s axon.
It will be recalled that the functioning cells of the retina are the photoreceptor cells, the rods and cones; the ganglion cells, the axons of which form the optic nerve; and cells that act in a variety of ways as intermediaries between the photoreceptors and the ganglion cells. These intermediaries are named bipolar cells, horizontal cells, and amacrine cells.
Plexiform layers
As was indicated earlier, the synapses occur in definite layers, the outer and inner plexiform layers. In the outer plexiform layer the bipolar cells make their contacts, by way of their dendrites, with the rods and cones, specifically the spherules of the rods and the pedicles of the cones. In this layer, too, the projections from horizontal cells make contacts with rods, cones, and bipolar cells, giving rise to a horizontal transmission and thereby allowing activity in one part of the retina to influence the behaviour of a neighbouring part. In the inner plexiform layer, the axons of the bipolar cells make connection with the dendrites of ganglion cells, once again at special synaptic regions. (The dendrites of a nerve cell carry impulses to the nerve cell; its axon, away from the cell.) Here, too, a horizontal interconnection between bipolar cells is brought about, in this case by way of the axons and dendrites of amacrine cells.
The bipolar cells are of two main types: namely, those that apparently make connection with only one photoreceptor—a cone—and those that connect to several photoreceptors. The type of bipolar cell that connects to a single cone is called the midget bipolar. The other type of bipolar cell is called diffuse. Varieties of the latter include the rod bipolar, the dendritic projections of which spread over an area wide enough to allow contacts with as many as 50 rods, and the flat cone bipolar, which collects messages from up to seven cones.
Ganglion cells are of two main types: namely, the midget ganglion cell, which apparently makes a unique connection with a midget bipolar cell, which in turn is directly connected to a single cone; and a diffuse type, which collects messages from groups of bipolar cells.
Convergence of the messages
The presence of diffuse bipolar and ganglion cells collecting messages from groups of photoreceptors and bipolar cells and, what may be even more important, the presence of lateral connections of groups of photoreceptors and bipolar cells through the horizontal and amacrine cells means that messages from photoreceptors over a rather large area of the retina may converge on a single ganglion cell. This convergence means that the effects of light falling on the receptive field may be cumulative, so that a weak light stimulus spread over about 1,000 rods is just as effective as a stronger stimulus spread over 100 or fewer. In other words, a large receptive field will have a lower threshold than a small one. This is, in fact, the basis for the high sensitivity of the area immediately outside the fovea, where there is a high density of rods that converge on single bipolar cells. Thus, if it is postulated that the cones do not converge to anything like the same extent as the rods, the greater sensitivity of the latter may be explained, and the anatomical evidence favours this postulate.
It has been indicated above that the regeneration of visual pigment is a cause of the increased sensitivity of the rods that occurs during dark adaptation. This, apparently, is only part of the story. An important additional factor is the change in functional organization of the retina during adaptation. When the eye is light-adapted, functional convergence is small, and sensitivity of rods and cones is low; as dark adaptation proceeds, convergence of rods increases. The anatomical connections do not change, but the power of the bipolar cells and ganglion cells to collect impulses is increased, perhaps by the removal of an inhibition that prevents this during high illumination of the retina.
Absolute threshold and minimum stimulus for vision
As was indicated earlier, the threshold is best indicated in terms of frequency of seeing since, because of fluctuations in the threshold, there is no definite luminance of a test screen at which it is always seen by the observer, and there is no luminance just below this at which it is never seen. Experiments, in which 60 percent was arbitrarily taken as the frequency of seeing and in which the image of a patch of light covered an area of retina containing about 20,000,000 rods, led to the calculation that the mean threshold stimulus represents 2,500 quanta of light that is actually absorbed per square centimetre of retina. This calculation leads to two important conclusions: namely, that at the threshold only one rod out of thousands comes into operation, and that during the application of a short stimulus the chances are that no rod receives more than a single quantum.
A quantum, defined as the product of Planck’s constant (6.63 × 10−27 erg-second) times the frequency of light, is the minimum amount of light energy that can be employed. A rod excited by a single quantum cannot excite a bipolar cell without the simultaneous assistance of one or more other rods. Experiments carried out in the 1940s indicated that a stimulus of about 11 quanta is required; thus, it may require 11 excited rods, each receiving one quantum of light, to produce the sensation of light.
Quantum fluctuations
With such small amounts of energy as those involved in the threshold stimulus, the uncertainty principle becomes important; according to this, there is no certainty that a given flash will have the expected number of quanta in it, but only a probability. Thus, one may speak of a certain average number of quanta and the actual number in any given flash, and one may compute on statistical grounds the shape of curve that is obtained by plotting frequency with which a flash contains, say, four quanta or more against the average number in the flash. One may also plot the frequency with which a flash is seen against the average number of quanta in the flash, and this frequency-of-seeing curve turns out to be similar to the frequency-of-containing-quanta curve when the number of quanta chosen is five to seven, depending on the observer. This congruence strongly suggests that the fluctuations in response to a flash of the same average intensity are caused by fluctuations in the energy content of the stimulus, and not by fluctuations in the sensitivity of the retina.
Spatial summation
In spatial summation, two stimuli falling on nearby areas of the retina add their effects; though either alone may be inadequate to evoke the sensation of light, they may do so when presented simultaneously. Thus, the threshold luminance of a test patch required to be just visible depends, within limits, on its size, a larger patch requiring a lower luminance and vice versa. Within a small range of limiting area—namely, that subtending about 10 to 15 minutes of arc—the relationship called Ricco’s law holds: threshold intensity multiplied by the area equals a constant. This means that over this area, which embraces several hundred rods, light falling on the individual rods summates, or accumulates, its effects completely, so that 100 quanta falling on a single rod are as effective as one quantum falling simultaneously on 100 rods. The basis for this summation is clearly the convergence of photoreceptors on ganglion cells, the chemical effects of the quanta of light falling on individual rods being converted into electrical changes that converge on a single bipolar cell through its branching dendritic processes. Again, the electrical effects induced in the bipolar cells may summate at the dendritic processes of a ganglion cell, so that the receptive field of a ganglion cell may embrace many thousands of rods.
