What is Dark Adaptation of Light and How Does it Affect Vision?

[sameeralord]

Hello everyone,

What is dark adaptation of light? Is it the adaptation of rods and cones, when you are exposed to bright light for some time, and then move to a dark area. I know some chanes that occur.

1.the retinal and opsins in the rods and cones are converted back into the light-sensitive pigments
2. Pupils dilate- Why is that? I know light of more intensity enters, but if light sensitive pigments are not generated, how could this be beneficial.
3. The other mechanism is neural adaptation, involving the neurons in the successive stages of the visual chain in the retina itself and in the brain. What exactly is this neural adaptation.

Also what is the textbook saying in this paragraph.

Value of Light and Dark Adaptation in Vision. Between the limits of maximal dark adaptation and maximal light adaptation, the eye can change its sensitivity to light as much as 500,000 to 1 million times, the sensitivity automatically adjusting to changes in illumination.
Because registration of images by the retina requires detection of both dark and light spots in the image, it is essential that the sensitivity of the retina always be adjusted so that the receptors respond to the lighter areas but not to the darker areas. An example of maladjustment of retinal adaptation occurs when a person leaves a movie theater and enters the bright sunlight. Then, even the dark spots in the images seem exceedingly bright, and as a consequence, the entire visual image is bleached, having little contrast among its different parts. This is poor vision, and it remains poor until the retina has adapted sufficiently so that the darker areas of the image no longer stimulate the receptors excessively.
Conversely, when a person first enters darkness, the sensitivity of the retina is usually so slight that even the light spots in the image cannot excite the retina. After dark adaptation, the light spots begin to register. As an example of the extremes of light and dark adaptation, the intensity of sunlight is about 10 billion times that of starlight, yet the eye can function both in bright sunlight after light adaptation and in starlight after dark adaptation.

Thanks

 

[Lievo]

What is dark adaptation of light?

The darker the visual scene, the larger gain of the input/ouput function.

Is it the adaptation of rods and cones, when you are exposed to bright light for some time, and then move to a dark area.

Just moving to a dark area is enough.

1.the retinal and opsins in the rods and cones are converted back into the light-sensitive pigments

Converted is maybe a bit strong: the sensitivity is adapted.

2. Pupils dilate- Why is that?

That's one of the mechanism that increases the gain: the darker the visual scene, the larger the pupils, the more light on the retina, the larger gain.

3. The other mechanism is neural adaptation, involving the neurons in the successive stages of the visual chain in the retina itself and in the brain. What exactly is this neural adaptation.

It's again about the gain. Easy to say as a first order approximation, but the details can be quite complicated, and some likely unknown.

Also what is the textbook saying in this paragraph.

What's unclear?

 

[zhermes]

What is dark adaptation of light? Is it the adaptation of rods and cones, when you are exposed to bright light for some time, and then move to a dark area. I know some chanes that occur.

Adding to what has already been mentioned: the adaptation is mostly within each rod/cone: the number of neurotransmitter gates, the amount of neurotransmitters, and the number of enabled/disabled gates are all modulated to control sensitivity. Additionally, in regards to 'neural adaptation,' I don't remember any higher order mechanisms of adaptation---there certainly could be some I don't know of though.

And as Lievo said, you should be more specific about what you don't understand in the quotation.

 

[sameeralord]

Thanks both for replies However I'm still not clear about dark adapatation. Ok let's say I was exposed to bright light for some time, and then move to dark area, why can't I see things straightway, why does some adapatation need to take place. I know in real life adaptation occur, just asking why. Is the reason because in bright light, almost all light sensitive pigments are broken down, so you have to wait till they regenerate. But if you are in dark for some time, these light sensitive pigments regernerate, so if you then go to bright area why does adapatation have to occur, you already have all the light sensitive pigments.

Because registration of images by the retina requires detection of both dark and light spots in the image, it is essential that the sensitivity of the retina always be adjusted so that the receptors respond to the lighter areas but not to the darker areas. An example of maladjustment of retinal adaptation occurs when a person leaves a movie theater and enters the bright sunlight.

I don't understand dark and light spots in retina.