Purpose:
Determine the visual dark adaptation process, learn and master the changing rules of the visual dark adaptation process. Experimental variables: Independent variable: Intensity of light dependent variable: Dark acclimation accumulative time Subject: Visual or orthostatic vision Normal subjects Experimental materials: Dark adaptation instrument. Construction: power switch, light stimulation key (used to show light stimulation), dark adaptation reaction key (used to respond to the test during dark adaptation), visual mark key (used to change the visual target during dark adaptation), was Trial reaction key (subjects see the "see" reaction after seeing the target), dark adaptation to the shift key (change the intensity of the light in the dark adaptation window, 0-6 light intensity gradually weakens), time recording screen (record was The dark acclimation cumulative time when the test report sees the target
concept
When we walk from a bright place into a dark place, our eyes will not see anything at all. It will take a while before we will gradually adapt and gradually see things in the dark. This process is about 30. ~40 minutes, during which the adaptation process of the retina's sensitivity is gradually increased is dark adaptation, that is, the ability of the retina to adapt to the darkness
From the light to the dark, the sensitivity of the human eye to light gradually increased, reaching the maximum in about 30 minutes, said dark adaptation. Dark adaptation is a reflection of the basic function of the visual cell - sensitization. In the absence of nutrients and retinal lesions, dark adaptations often occur. Dark adaptation measurement is one of the important items for eye function tests
Cause
In dark places, cones in the human eye are inactive, and only rod photoreceptors are at work. In rod photoreceptor cells, there is a substance called rhodopsin, which is sensitive to weak light and can be gradually synthesized in the dark. According to eye expert statistics, 60% of rhodopsin can be generated within 5 minutes in the dark. It takes about 30 minutes to generate all. Therefore, the longer the time spent in a dark place, the higher the sensitivity to low light. However, some people have obstacles in the function of rod-shaped photoreceptor cells. In dark areas, rod-shaped pigment cells cannot work normally. No matter how long he stays in the dark, it cannot improve the sensitivity to low light. We take this phenomenon. Called night blindness. Some night blindness is caused by factors such as vitamin A deficiency, and some are caused by diseases such as primary retinitis pigmentosa. Rod photoreceptors do not have the ability to identify the color of an object, so looking at things in the dark is a color.
Dark reaction
Dark adaptation is a phenomenon in which the retina has increased visual acuity when it is adapted to a dark place or a low light intensity state, and is a corresponding word for light adaptation. The light that can't be seen from the light suddenly entering the darkness can gradually be seen with the extension of the stay in the dark. This is a daily phenomenon.
The time course of dark adaptation can be accurately determined by psychophysical methods for humans. After giving sufficient adaptation in advance and turning the room into a darkroom and measuring the time course of changes in the test light threshold, a dark adaptation curve can be obtained.
Collapse the first 2 to 3 minutes
The threshold drops sharply and then slows down.
After 5 to 10 minutes of folding
It began to decline sharply, causing the curve to twist (Kohlrausch's kink).
After that, the threshold value can decrease for about 30 minutes, and then it can slow down again and reach the extreme value after about 1 hour.
From the beginning to the appearance of the Colelau Shizhe is called the first phase or first-order adaptation, and is later called the second phase or secondary adaptation.
Since the first phase is mainly based on cone cell adaptation and the second phase is based on rod cell adaptation, only the first phase can be seen in the fovea of ​​the cone cell alone. Since the sensitivity of dark-adapted cone cells only increased by tens of times, the threshold was approximately 0.02-0.15 lux, but the sensitivity of rod cells varied from several thousand to tens of thousands of times, and the threshold value was as low as 0.569×10^-5 lux. Therefore, dark cells in the peripheral area of ​​the retina with many rod cells have good ability of dark adaptation and high sensitivity.
For this reason, a weak light in the dark (central vision) cannot be seen, resulting in a central scotoma or physiological night blindness. If you look away, you can see through the peripheral vision. Because the maximum wavelengths of the visual acuity curves of cone and rod cells are different, the dark adaptation curve varies with the wavelength of the test light. If red light is used, the second phase will not be visible due to the low sensitivity of the rod cells; if short-wave light with high sensitivity of the rod cells is used, the second phase appears early and the measured threshold is also extremely low.
These results measured in humans are very consistent with the experimental results obtained in other vertebrate animals using electroretinograms and peak discharges of the optic nerve fibers as indicators, suggesting that dark adaptation is a phenomenon produced by the retina. As for its mechanism, there has long been a view that the threshold value is reduced due to the resynthesis of the rod cell visual pigment, rhodopsin (photochemical theory), but according to the results of subsequent studies, the increase in the amount of rhodopsin and the decrease in the threshold are It is not exactly the relationship, and if the local retinal irradiation, the threshold of the non-irradiated area is also greatly increased, these facts emphasize the existence of neuronal dark adaptation mechanisms, so that the adaptation of the retina is considered to be the mechanism and The combination of photochemical dark adaptation may be appropriate.