Photopsia (the ability to experience light)

PHOTOPSIA, the ability to experience light sensations. The usual irritant that causes us to have light sensations is light acting on the eye. Photopsia is distinguished from color perception, since in addition to light sensations that have no color tone, we can also, through the eye, receive sensations of various colors. All the multitude of light sensations is exhausted by the colors white, black, and gray of all possible brightnesses. Light sensations can arise in us as a result of the irritation of both rods and cones of the retina. However, while irritation of cones can cause both light sensations and color sensations, rods are capable of giving only light sensations. Therefore, photopsia is primarily a function of the rods of the retina. The basis of the light sensation caused by the irritation of the rods of the retina lies in the photochemical decomposition of visual purple or rhodopsin, located in the outer segments of the rods. The photochemical decomposition of visual purple, its fading, occurs as a result of the absorption of light by the visual purple. Rays of different wavelengths are absorbed by visual purple differently. The rays of green with a wavelength of about 510 nm are absorbed most; on both sides of the spectrum from this area, the absorption of rays by visual purple decreases. Accordingly, in the case of vision with only the rods of the retina, which occurs at very low light intensities, as well as in persons with complete color blindness, the area of green rays appears to be the brightest in the spectrum. Red rays with a wavelength greater than 625 nm do not act on visual purple at all, and therefore in conditions of "rod" or twilight vision, they are not felt by us at all. The situation is different in the case of vision with the cones of the retina. The sensitivity of the latter is maximum with respect to greenish-yellow rays and generally extends further into the long-wavelength side of the spectrum (up to 760 nm, and sometimes further). In this difference in the sensitivity curves of rods and cones with respect to rays of different wavelengths lies the reason that when passing from "colored," "cone" vision to "colorless," "rod" vision, in which only photopsia remains for us, the ratio of the brightnesses of colored objects changes sharply. Red-orange colors begin to seem almost black, while green-blue ones become relatively much lighter, gray (the so-called "Purkinje phenomenon"). The light sensitivity of the eye E is measured as the value inverse to the minimum brightness of a light irritant I that is first noticeable to us. Thus, E = 1/I. The light sensitivity of the eye is a variable value depending on many conditions. For example, in the center of the retina it is somewhat less than in its peripheral places; the maximum sensitivity falls in the peripheral zone between 10° and 20°. It also increases with the increase in the area of irritation: at the same brightness, we notice larger luminous objects more easily than small ones. The relationship between the angular size of an object and its minimum (threshold) brightness, at which our eye can see it, is expressed by various mathematical formulas depending on the absolute value of the object's size. For irritations with an angular size from 0.1' to 10', the so-called Ricco's law is justified, according to which the threshold brightness and the area of irritation are in a simple inverse proportion to each other: the larger the area of irritation, the smaller its brightness can be, and vice versa. Photopsia depends on the duration of the action of the irritant on the eye. As the irritation continues, the brightness of the sensation is first weak, then increases, reaches a certain maximum, after which it gradually declines, remaining at the end at more or less a constant, stationary level. When light acts on the eye, the sensitivity of the eye decreases. In the absence of a light irritant (in the dark), on the contrary, it increases. Such changes in the level of light sensitivity of the eye constitute what is called the adaptation of the eye to certain light conditions. Thanks to adaptation (see), the light sensitivity of the eye can change within very wide limits (in tens and hundreds of thousands of times!). Based on the kinetics of the photochemical reactions assumed in visual purple, it was possible to give a mathematical expression both for the course of the restoration of light sensitivity in the dark and for the reverse course of its decrease with a continuing light irritant. According to the theory developed by Lazarev: E = E0 / (1 + At + Bt^2), where E is the current sensitivity of the eye, E0 is its maximum sensitivity, t is the time elapsed after the eye has been immersed in the dark, A, B, y, a3 and a'3 are certain constant values. The decrease of light sensitivity when the eye is illuminated, according to the theory of the same scientist, is sufficiently satisfactorily expressed by the formula: E = E0 / (1 + Ae^(-t/tau)), where E denotes the light sensitivity of the eye at a given moment in time, E0 is its maximum sensitivity, A, B and beta are certain constant values and t is the time during which the light irritant acts on the eye. The light sensitivity of the eye depends not only on peripheral processes occurring in the visual purple of the rods of the retina, but also on the state of the brain centers. For example, experiments show that light sensitivity changes with age. At the age of 20–25 years it reaches its maximum, being lower both in earlier and later years. There are indications that the light sensitivity of the visual centers can be increased by preliminary brief illumination of the eye. Under conditions of oxygen starvation, for example during ascents to great heights, on the contrary, a noticeable decrease in the level of light sensitivity is observed. It has also been established that binocular light sensitivity is greater than monocular light sensitivity. A simultaneous auditory irritant is capable of increasing light sensitivity. Since light sensitivity depends, therefore, on very many factors, it is quite difficult to indicate exactly general values of it corresponding to "norm." In general, after an hour's stay in the dark, normal eyes are capable of seeing brightnesses of the order of millionths of a lux perpendicular to a white surface, that is, brightnesses of the order of 10^-10 sb/cm^2 (stilb). The discrimination of brightnesses by our eye obeys Weber-Fechner's law (see); under the most favorable conditions we can distinguish brightnesses differing from each other by only 1% and even less. At low brightnesses, however, the differential sensitivity of the eye turns out to be significantly lower. As Helmholtz thinks, the cause of such a lowering of our sensitivity to the discrimination of brightnesses in the case of insignificance of the latter is the constant presence in us of some weak light sensation, the so-called "intrinsic light of the retina." "Intrinsic light of the retina," according to Helmholtz, is the result of constantly existing physiological irritants acting on the endings of the optic nerve in the retina. Mixing with the light sensation caused by objective light, the "intrinsic light of the retina" hinders our discrimination of weak brightnesses. Photopsia is usually investigated by means of special tables and instruments (Ziegler's, Bjerrum's and Treitel's tables, and Ferster's, Nagel's and other systems of adaptometers, as well as the Masson-Helmholtz disc). In the tables mentioned above, we have rows of letters (in Ziegler's and Bjerrum's) and squares (in Treitel's), depicted so that the contrast of the signs with the background on which they are drawn changes from row to row, reaching a very insignificant one (dark gray on black or light gray on white). According to which row of the table the subject is able to distinguish, his photopsia is judged. This determines the so-called differential sensitivity of the eye. It can also be measured by the Masson-Helmholtz disc, which represents an ordinary disc of gray or white paper with thin intermittent narrow strips of lighter or darker paper pasted on it by radius. If such a disc is put on the shaft of a motor or a manual "color mixer" and brought into rotary motion, then we see concentric rings on it, the visibility of which decreases from the center to the periphery. The sensitivity of the examined eye to the discrimination of brightnesses can be calculated on the basis of which peripheral ring the eye still notices. When using one or the other method for determining the differential sensitivity of the eye, it is necessary to take care that the adaptation conditions be the same for all subjects, otherwise the results of the tests will not be comparable to each other. By means of adaptometers, not the differential, but the absolute light sensitivity of the eye is determined, finding that minimum brightness which the eye is generally capable of seeing after a certain time spent in the dark. The most common of these instruments, the Nagel adaptometer, allows varying the brightness of the light presented to the subject by 80 million times. Anomalies of photopsia most often appear in the form of hemeralopia (see).

A more rare disorder of S. is abnormally high sensitivity to light stimuli, so-called nyctalopia. The sufferer sees worse during the day than in twilight. Nyctalopia often has as its cause excessive dilation of the pupils, which do not react normally to light. A natural consequence of nyctalopia is greater or lesser photophobia.