From Big Medical Encyclopedia

PHOTOPERCEPTION — complex process of transformation of physical energy of light in biological. The nervous excitement arising at the level of peripheral department of the visual analyzer extends on visual nerve pathways to the visual centers of a cerebral cortex (see. Visual centers, ways ), where it is perceived as a visual signal. The page represents the earliest function of the visual analyzer (see. Sight , Visual analyzer ) also it is connected with perception of the achromatic, gray colors located between white and black. The page gives the chance to be guided in the conditions of the lowered illumination, i.e. in twilight and night time.

Initially S.'s process forms at the level of a photoreceptor (phototouch) layer retinas (see) it is also carried out by its bacillary device. The molecules of a rhodopsin which are in outside segments of sticks rhodopsin (see) absorb quanta light (see) and, decaying, provide the consecutive course of photochemical, electric, ionic and enzymatic processes. For actuating of all mechanism C. there is enough absorption by one molecule of rhodopsin of one light quantum. Rhodopsin under the influence of the beams absorbed by it, hl. obr. beams with wavelength apprx. 500 nanometers, i.e. beams of a green part of a range, changes color (fades). Therefore at bacillary sight of the brightest in a visible part of a range the area of green beams while the area red is perceived dark (Purkinye's phenomenon) seems.

Judgment of sensitivity of the visual analyzer to light is made on the basis of determination of levels of thresholds of Page. The weakest photoirritation causing a feeling of light is called the minimum threshold or absolute light threshold (J). Size, the return to a light threshold, characterizes light sensitivity (E):

Е = 1/J.

Light sensitivity changes under the influence of a number of factors. From them illumination most is important. Process of adaptation of the visual analyzer to various levels of illumination is called visual adaptation (see. visual adaptation ). So, at daylight light sensitivity quickly decreases (light adaptation), and in the dark slowly increases (dark adaptation), reaching the most high level in 50 — 60 min. Therefore absolute light sensitivity is defined during dark adaptation.

A light object as a direct irritant can exert impact on light sensitivity. Between brightness of a threshold irritant and the area of its influence (the angular size of an object) there is a certain dependence. So, with increase in the area of irritation light sensitivity increases. Light sensitivity depends also on distribution of sticks on the area of a retina (the maximum of their density is necessary between 10 and 20 ° from its center). Only for objects of the small angular sizes (to 10') the number of the sticks having per acre retinas does not influence light sensitivity since in this case becomes effective Rikko's law, according to Krom the work of threshold brightness on the area of influence of an irritant is a constant and about 2,1•10 corresponds - 9 lx on a pupil. In process of further increase in the area of irritation threshold brightness decreases, and the light threshold increases in nonlinear dependence; at the area equal 50 ° or it is more, threshold brightness does not change at increase in the area of irritation any more. Therefore definition of absolute light sensitivity consists in measurement of threshold brightness of an object with the angular size equal 50 ° or it is more, edges and is accepted to an absolute light threshold.

Light sensitivity depends also on duration of photoirritation, edges to a certain extent can compensate the insufficient brightness of an object. Duration of exposure of a light object at its smallest brightness, at a cut arises feeling of light of consider optimum. It is equal about 0,005 sec.

Unlike absolute light sensitivity contrast, or distinctive, light sensitivity allows to define a difference of brightness of two or several objects. Distinguishing of brightness submits to Weber's law — Fekhnera, expressing the quantitative relation between irritation and feeling (see). An eye of the person distinguishes change of brightness in relation to initial within 0,5%. Further decrease in distinctive light sensitivity at smaller brightness of objects is explained by spontaneous activity of the visual system causing residual categories of excitement of a retina even in the absence of light, so-called own light of a retina.

Distinctive light sensitivity increases at increase in contrast on border of a light object with a surrounding background. This process of interaction of the receptors of visual system perceiving first of all boundary sites of an object (as areas with constant illumination are a little informative), is known as lateral braking.

Light sensitivity depends as on the processes happening in rhodopsins of a retina and on a condition of the central departments of the visual analyzer. Change of illumination of one eye can change light sensitivity of another, sensitization to light in the course of dark adaptation is followed at the same time by increase in time of signal transmission in a brain because of increase of time of integration of excitement, etc.

S. investigate by means of various methods. Absolute light sensitivity is determined by Nagel's adaptometers, Belostotsky — Hoffman, etc. Use Kravkov's device to the accelerated research of twilight sight and dark adaptation — Vishnevsky. Abroad Hartinger's adaptometers, niktomata gained distribution (see. visual adaptation ).

The page can be broken at diseases of an organ of sight, at nek-ry general diseases, and also intoxications. Decrease in light sensitivity, so-called meets more often. hemeralopia (see), high sensitivity to light irritants is more rare (moonblink), at a cut of the patient sees in the afternoon worse, than at night.

See also Sight .

Bibliography: B. G. O box of visual dark adaptation, JI., 1958, bibliogr.; The multivolume guide to eye diseases, under the editorship of V. N. Arkhangelsky, t. 1, book 2, page 105, M., 1962; Gregory R. L. Eye and brain, The psychology of seeing, N. Y. — Toronto, 1966, bibliogr.

G. M. Laryukhina.