Adaptation (eye)
The eye takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and becomes 10,000 to 1,000,000 times more sensitive than at full daylight.
In response to varying ambient light levels, rods and cones of eye function both in isolation and in tandem to adjust the visual system.
[8] Many animals such as cats possess high-resolution night vision, allowing them to discriminate objects with high frequencies in low illumination settings.
[13] Therefore, the time required for dark adaptation and pigment regeneration is largely determined by the local concentration of 11-cis retinal and the rate at which it is delivered to the opsin in the bleached rods.
[14] The decrease in calcium ion influx after channel closing causes phosphorylation of metarhodopsin II and speeds up the cis-retinal to trans-retinal inactivation.
[18] Three factors affect how quickly the rod mechanism becomes dominant: Under scotopic conditions, intracellular cGMP concentration is high in photoreceptors.
Eventually, these deposits become clinically visible drusen that affect photoreceptor health, causing inflammation and a predisposition to choroidal neovascularization (CNV).
As a side effect of this process, the photoreceptors exhibit impaired dark adaptation because they require these nutrients for replenishment of photopigments and clearance of opsin to regain scotopic sensitivity after light exposure.
As such, research has shown that, by measuring dark adaptation, doctors can detect subclinical AMD at least three years earlier than it is clinically evident.
[29] In order for dark adaptation to be significantly accelerated an individual should ideally begin this practice 30 minutes prior to entering a low luminescence setting.
The insensitivity to red light will prevent the rod cells from further becoming bleached and allow for the rhodopsin photopigment to recharge back to its active conformation.
Although many aspects of the human visual system remain uncertain, the theory of the evolution of rod and cone photopigments is agreed upon by most scientists.
[31] Following the evolution of mammals from their reptilian ancestors approximately 275 million years ago there was a nocturnal phase in which complex colour vision was lost.
[32] It can be extrapolated that the high ratio of rods to cones present in modern human eyes was retained even after the shift from nocturnal back to diurnal.
It is believed that the emergence of trichromacy in primates occurred approximately 55 million years ago when the surface temperature of the planet began to rise.
A third cone photopigment was necessary to cover the entire visual spectrum enabling primates to better discriminate between fruits and detect those of the highest nutritional value.
The photopigment rhodopsin found in human rod cells is composed of retinal, a form of vitamin A, bound to an opsin protein.
[36] Vitamin A-based opsin proteins have been used for sensing light in organisms for most of evolutionary history beginning approximately 3 billion years ago.
[38] A second reason why retinal evolved to be vital for human vision is because it undergoes a large conformational change when exposed to light.
[38] This conformational change is believed to make it easier for the photoreceptor protein to distinguish between its silent and activated state thus better controlling visual phototransduction.
[45] In humans, anthocyanins are effective for a variety of health conditions including neurological damage, atherosclerosis, diabetes, as well as visual impairment.
[46] Anthocyanins frequently interact with other phytochemicals to potentiate biological effects; therefore, contributions from individual biomolecules remains difficult to decipher.
[44] As a result of anthocyanins providing bright colouration to flowers, the plants containing these phytochemicals are naturally successful in attracting pollinators such as birds and bees.
The aviators consumed this anthocyanin-rich food due to its many visual benefits, included accelerated dark adaptation, which would be valuable for night bombing missions.
[44] The daily intake of anthocyanins is estimated to be approximately 200 milligrams in the average adult; however, this value can reach several grams per day if an individual is consuming flavonoid supplements.
By having a diet rich in anthocyanins an individual is able to generate rhodopsin in shorter periods of time because of the increased affinity of opsin to retinal.
In a double-blind, placebo-controlled study conducted by Nakaishi et al. a powdered anthocyanin concentrate derived from black currants was provided to a number of participants.
Observing the data as a whole Nakaishi et al. concluded that anthocyanins effectively reduced the dark adaptation period in a dose-dependent manner.
[50] From these results Kalt et al. concluded that blueberry anthocyanins provide no significant difference to the dark adaptation component of human vision.
[55] In developed countries night blindness has historically been uncommon due to adequate food availability; however, the incidence is expected to increase as obesity becomes more common.
