The human eye is capable of perceiving vision at a wide range of intensities from dim moonlight to bright sunlight. Be it a moonless night sky, a newsprint read, a candle flame, fluorescent tubes, tungsten lights or the bright sun, the visual system copes with the dynamic range of light intensities of a millionth of a million to one with at least twelve orders of magnitude roughly from 10-6 cd/m2 through to 106 cd/m2. However, a typical nerve cell responds to thousand to one range of intensity of light only.
The human visual system needs distinguished mechanisms to enable vision to work at the wide range of intensities. The four main strategies that the visual system uses are variations in pupil size, the duplex retina, and the dark adaptation and light adaptation mechanisms. These strategies much be accompanied by the scotopic and photopic light conditions to understand how the visual system adapts.
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A mechanism, common in mammals, is the dilation and constriction of the pupil by the muscular tissue of the iris. This gives a variation in the size of the pupil. Pupil constriction takes place in bright light or high light levels to limit the intensities reaching the retina and in turn to reduce the retinal blur circle due to spherical and optical aberrations. In dim light or at low light levels, when sensitivity is most important, the pupil dilates and thus expands to admit more light.
During high levels of light intensities falling on the retina, the parasympathetic pathways of the autonomic nervous system sends signals for miosis, constriction of the pupil, via the contraction of the circular muscles of the iris diaphragm. The third cranial nerve, oculomotor nerve, synapses in the ciliary ganglion cells to activate muscarinic receptors. On the other hand, at low levels of light intensities falling on the retina, the sympathetic pathways send signals for mydriasis, dilation of the pupil, via the contraction of the radial muscles of the iris. The oculomotor nerve synapses in the ciliary ganglion to activate the adrenergic receptors.
However, the pupil, in its complete state of constriction in the presence of very high light intensities has a diameter of about 2mm where as in the presence of extremely dim light conditions, it has a diameter of a maximum of 7.5mm. It is known that the pupil with its smallest diameter has an aperture which is only about fourteen to sixteen times smaller than when the pupil with its largest diameter. Therefore, the light levels that enter the eye through its fully constricted and fully dilated state differ only by a small factor and thus accounts for a small part of the wide range of intensities.
The vertebrate eye adapts to the larger range of light intensities in its retinal structure. The human eye is both, diurnal and nocturnal and is said to have a duplex retina due to the two types of photoreceptors present in the retina. The retinal photoreceptors are rods and cones. When electromagnetic radiation, particularly light wavelengths from the visible region enter the eye and stimulate the photoreceptors, the rods and cones function differently. Rods are present in the periphery of the retina and are most sensitive to dim conditions, and when these rod photoreceptors are active, vision is said to be scotopic. Rods are more sensitive to light than cones and have highly reduced thresholds. Cones are present mainly in the fovea, centre and near periphery of the retina and are most sensitive to bright conditions. These photoreceptors are responsible for the resolution of very fine detail and colour discrimination. Our vision is said to photopic when the cone photoreceptors are active. However, when both, rods and cones, are active, even though not in their peak efficiency, but still contribute to vision, it is called mesopic.
The retina's duplex nature is accounted for the differences between scotopic and photopic vision. The dual retina, also to an extent, accounts for the dark and light adaptation mechanisms and the transduction process. Photoreceptors contain photo-pigments that absorb the light that falls on our retina. Rods contain the photo-pigment rhodopsin. These photoreceptors are present in the periphery with a low threshold and therefore relatively slow adapting. On the other hand, cones contains three different photo-pigments known as Erythrolabe R, Chlorolabe G and Cyanlabe B. The photo-pigments in the cones are composed of opsins that are similar to rhodopsin. Cones are concentrated in the fovea with a high threshold and are relatively fast adapting. Each quantum of light that is absorbed by the photo-pigments triggers a series of complex photochemical reactions that involve biochemical, physical, and neural reactions that signals the receptors due to a change in voltage. These responses are sent to the brain for further analysis.
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Rods are highly sensitive to light and are known to respond to as much as a single quantum of light. High sensitivity of rods is achieved when the eye is completely dark adapted. During dark adaptation, it is observed that it takes several minutes before the subject can see contours when brought from a well illuminated environment to a dark room.
The dark adaptation mechanism can be explained in two stages separated by the rod-cone break. The threshold intensity decreases, and thus improves by 102 in the first seven to ten minutes followed by a much gradual decrease in threshold intensity. The high sensitivity is responsible for the 1010 range. During illuminated light conditions, the light sensitive photo pigments in the photoreceptors are partially bleached.
The rhodopsin pigment in rods breaks down very fast in bright light and is therefore said to be bleached, while the pigments in cones are less sensitive to light. These pigments slowly regenerate as the light levels decreases. This yields a cis- to trans- change in configuration of chromophore. Photo-transduction causes the breakage of the bond between the protein, opsin and the chromophore, retinal. This reaction activates a phosphodiesterase that acts as an enzyme in the hydrolysis of cGMP to form GMP. The regenerated pigments can then absorb and respond to light quanta with their high sensitivity to generate a photoreceptor response.
Where cones attain maximum sensitivity in 7 to 10 minutes, rods need approximately 30 - 45 minutes or more for maximum sensitivity after bright light is exposed. This is because, even though the rhodopsin molecule has regenerated, the threshold remains high due to the bleaching of pigment created by the bright light.
(4) Light adaptation.
When we move from a dark room into bright sunlight, vision is initially very bright and blinding, but quickly adapts. Having been in the dark, the eye is initially too sensitive, but sensitivity quickly reduces to a level appropriate for the current light level.
â€¢ Desensitization. The amount of desensitization can be evaluated by measuring Increment
Thresholds on different background intensities (see diagram). If the threshold #I increases in
proportion to the background intensity I, then we say that Weber's Law holds (#I = kI). When
Weber's Law holds, it is a certain level of contrast (#I/I) that determines visibility. This is not a
universal law like gravitation, but is often found over a wide range of intensities.
â€¢ Absolute threshold. There is a lower limit on light detection (the 'absolute threshold') that seems to be determined by internal noise in the receptors. This internal noise is (rather poetically) called the "dark light" (I0), and can be estimated from the corner point of the threshold function (see graph). Thus a more general form of Weber's Law is: #I = k(I+I0).
â€¢ Neural adaptation & 'gain control'. When Weber's law holds, it seems that the background light is acting in the retina to reduce the 'gain' or sensitivity of retinal elements, including cones and bipolar cells. Thus the neurons (nerve cells) of the retina operate over a limited range of intensities, but are able to shift this range to higher light levels in bright conditions. These shifts (light adaptation) are fast (0.1 to 1 sec) and localized. Thus the retina is continually adjusting (adapting) its sensitvity to different light levels in different parts of the image. A consequence of this local adaptation is the appearance of "negative afterimages", demonstrated in the lecture.
Another light-adaptive mechanism is neural adaptation, which is generated by retinal neurons at successive stages of the visual chain in the retina. A change in "neural gain" occurs in seconds and can improve night vision by a factor of 10 or more. Neural adaptation is rather like having low-speed and high-speed film simultaneously available in your camera. Furthermore, a large share of the inherently greater sensitivity of rod dark adaptation is a result of retinal summation. As many as 100 rods, or more, converge onto a single nerve fiber in the retina, which greatly increases sensitivity. Thus, if the rods are slightly stimulated, the summation of several low-level stimuli might be enough to initiate a light signal to the brain. Unlike the photoreceptor chemical changes, these mechanisms occur instantaneously.
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CENTRAL BLIND SPOT AT NIGHT
That portion of the retina responsible for the keenest visual acuity (VA) is the fovea, which corresponds to the center of the visual field. The foveola, or center of the fovea, possesses a high degree of cones, but is completely devoid of rods. Thus, if the ambient light is below cone threshold, when any small object is fixated centrally, it cannot be seen because, at light levels below dim starlight, a blind spot exists in the central 1 degree of the visual field. This central blind spot corresponds to the foveola, which is rod-free; it cannot function in diminished illumination.
Rods are present outside the central 1-degree foveolar area. The rods increase gradually with eccentricity from the foveola, and finally reach a maximum concentration at a point some 17 degrees from the fovea. Since the rods have a lower threshold than the cones, they are much more sensitive to light. A person attempting to see in scotopic illumination, light dimmer than moonlight, has to depend entirely on rods. To best detect small targets with the rods under such circumstances, the individual must look approximately 15-20 degrees to one side, above, or below an object to place the object of regard on the part of the retina that possesses the highest density of rods. Individuals can be taught to fixate to one side of an object to avoid the central blind spot and to scan, utilizing the most sensitive part of the retina to improve target detection at night. Therefore, proper education and training are helpful in maximizing visual function at night.
Rods and cones are not equally sensitive to visible wavelengths of light. Unlike the cones, rods are more sensitive to blue light and are not sensitive to wavelengths greater than about 640nm, the red portion of the visible spectrum.
The Purkinje shift is the relatively greater brightness of blue or green light, compared with yellow or red light, upon shifting from photopic to scotopic adaptation. For example, in a darkened room, if one looks at two dim lights of equal illumination (one red and one green) that are positioned closely together, the red light will look brighter than the green light when the eyes are fixating centrally. If one looks to the side of the dim lights about 15-20 degrees, the green light will appear brighter than the red. Central fixation involves the cones and photopic vision while fixating eccentrically involves rods and scotopic vision. The cones are more sensitive to yellow and red, but the rods are more sensitive to light of the blue and green wavelengths. The most sensitive wavelength for cones is 555nm (yellow-green). That is why the "optic yellow" tennis and golf balls are, in fact, easier to see under photopic conditions. The most sensitive wavelength for rods is 505nm (blue-green). Thus, blue-green lights will generally look brighter at night than red lights. The sensitivity of the eye changes from the red end of the visible spectrum toward the blue end when shifting from the photopic to scotopic vision.
An appropriate demonstration of the difference between photopic and scotopic sensitivity is to slowly decrease the intensity of a colored light until the cone threshold is reached. This is the point at which the color will disappear, but not the sensation of light. When this procedure is performed with any color except red, the color will disappear at the cone threshold, but the light will still be perceived by the rods as dim gray. If the intensity is further decreased until the rod threshold is reached, the light will disappear entirely. With red light, the color and sensation of light disappear at the same time.
The difference between the level of illumination at which the color of a light disappears (cone threshold) and that at which the light itself disappears (the rod threshold) is known as the photochromatic interval. There is a photochromatic interval for every color of the spectrum, except for the longer red wavelengths.