Adaptation (eye)

The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart. However, in any given moment of time, the eye can only sense a contrast ratio of 1,000. What enables the wider reach is that the eye adapts its definition of what is black. 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 this process, the eye's perception of color changes as well (this is called the Purkinje effect). However, it takes approximately five minutes for the eye to adapt from darkness to bright sunlight. This is due to cones obtaining more sensitivity when first entering the dark for the first five minutes but the rods taking over after five or more minutes. Cone cells are able to regain maximum retinal sensitivity in 9–10 minutes of darkness whereas rods require 30–45 minutes to do so. Dark adaptation is far quicker and deeper in young people than the elderly. Cones vs. rods The human eye contains three types of photoreceptors, rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs). Rods and cones are responsible for vision and connected to the visual cortex. ipRGCs are more connected to body clock functions and other parts of the brain but not the visual cortex. Rods and cones can be easily distinguished by their structure. Cone photoreceptors are conical in shape and contain cone opsins as their visual pigments. There exist three types of cone photoreceptors, each being maximally sensitive to a specific wavelength of light depending on the structure of their opsin photopigment. Cone photoreceptors are concentrated in a depression in the center of the retina known as the fovea centralis and decrease in number towards the periphery of the retina. Conversely, rod photoreceptors are present at high density throughout most of the retina with a sharp decline in the fovea. Perception in high luminescence settings is dominated by cones despite the fact that they are greatly outnumbered by rods (approximately 4.5 million to 91 million). == Ambient light response ==

A minor mechanism of adaptation is the pupillary light reflex, adjusting the amount of light that reaches the retina very quickly by about a factor of ten. Since it contributes only a tiny fraction of the overall adaptation to light it is not further considered here. In response to varying ambient light levels, rods and cones of eye function both in isolation and in tandem to adjust the visual system. Changes in the sensitivity of rods and cones in the eye are the major contributors to dark adaptation. Above a certain luminance level (about 0.03 cd/m), the cone mechanism is involved in mediating vision; photopic vision. Below this level, the rod mechanism comes into play providing scotopic (night) vision. The range where two mechanisms are working together is called the mesopic range, as there is not an abrupt transition between the two mechanism. This adaptation forms the basis of the Duplicity Theory. == Advantages of night vision ==

'' Many animals such as cats possess high-resolution night vision, allowing them to discriminate objects with high frequencies in low illumination settings. The tapetum lucidum is a reflective structure that is responsible for this superior night vision as it mirrors light back through the retina exposing the photoreceptor cells to an increased amount of light. Most animals which possess a tapetum lucidum are nocturnal most likely because upon reflection of light back through the retina the initial images become blurred. Despite the fact that the resolution of human day vision is far superior to that of night vision, human night vision provides many advantages. Like many predatory animals, humans can use their night vision to prey upon and ambush other animals without their awareness. Furthermore, in the event of an emergency situation occurring at night, humans can increase their chances of survival if they are able to perceive their surroundings and get to safety. Both of these benefits can be used to explain why humans did not completely lose the ability to see in the dark from their nocturnal ancestors. == Dark adaptation ==

Rhodopsin, a biological pigment in the photoreceptors of the retina, immediately photobleaches in response to light. Visual phototransduction starts with the isomerizing of the pigment chromophore from 11-cis to all-trans retinal. Then this pigment dissociates into free opsin and all-trans retinal. Dark adaptation of both rods and cones requires the regeneration of the visual pigment from opsin and 11-cis retinal. The decrease in calcium ion influx after channel closing causes phosphorylation of metarhodopsin II and speeds up the cis-retinal to trans-retinal inactivation. Rods are more sensitive to light and so take longer to fully adapt to the change in light. Rods, whose photopigments regenerate more slowly, do not reach their maximum sensitivity for about two hours. Cones take approximately 9–10 minutes to adapt to the dark. The sensitivity of the rod pathway improves considerably within 5–10 minutes in the dark. Color testing has been used to determine the time at which rod mechanism takes over; when the rod mechanism takes over colored spots appear colorless as only cone pathways encode color. Three factors affect how quickly the rod mechanism becomes dominant: • Intensity and duration of the pre-adapting light: By increasing the levels of pre-adapting luminances, the duration of cone mechanism dominance extends, while the rod mechanism switch over is more delayed. In addition the absolute threshold takes longer to reach. The opposite is true for decreasing the levels of pre-adapting luminances. • Size and location on the retina: The location of the test spot affects the dark adaptation curve because of the distribution of the rods and cones in the retina. • Wavelength of the threshold light: Varying the wavelengths of stimuli also affect the dark adaptation curve. Long wavelengths—such as extreme red—create the absence of a distinct rod/cone break, as the rod and cone cells have similar sensitivities to light of long wavelengths. Conversely, at short wavelengths the rod/cone break is more prominent, because the rod cells are much more sensitive than cones once the rods have dark adapted. Intracellular signalling Under scotopic conditions, intracellular cGMP concentration is high in photoreceptors. cGMP binds to and opens cGMP gated Na channels to allow sodium and calcium influx. Sodium influx contributes to depolarization while calcium influx increases local calcium concentrations near the receptor. Calcium binds to a modulatory protein, which is proposed to be GUCA1B, removing this protein's stimulatory effect on guanylyl cyclase. Using Dark Adaptation Measurement to Diagnose Disease Numerous clinical studies have shown that dark adaptation function is dramatically impaired from the earliest stages of Age-related Macular Degeneration (AMD), Retinitis Pigmentosa (RP), and other retinal diseases, with increasing impairment as the diseases progress. AMD is a chronic, progressive disease that causes a part of the retina, called the macula, to slowly deteriorate over time. It is the leading cause of vision loss among people age 50 and older. It is characterized by a breakdown of the RPE/Bruch's membrane complex in the retina, leading to an accumulation of cholesterol deposits in the macula. Eventually, these deposits become clinically visible drusen that affect photoreceptor health, causing inflammation and a predisposition to choroidal neovascularization (CNV). During the AMD disease course, the RPE/Bruch's function continues to deteriorate, hampering nutrient and oxygen transport to the rod and cone photoreceptors. 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. Measurement of a patient's dark adaptation function is essentially a bioassay of the health of their Bruch's membrane. 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. ==Accelerating dark adaptation==

Several different methods, with varying levels of evidence, have been purported or demonstrated to increase the rate at which vision can adapt in the dark. Red lights and lenses As a result of rod cells having a peak sensitivity at a wavelength of 500 nanometers they cannot perceive all colours on the visual spectrum. Because rod cells are insensitive to long wavelengths, the use of red lights and red lens glasses has become a common practice for accelerating dark adaptation. 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. This practice will allow an individual to maintain their photopic (day) vision whilst preparing for scotopic vision. 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. 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. 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. • Submarines: Oftentimes submarines are "rigged for red", meaning that the boat is going to be surfacing or coming to periscope depth at night. During such times illumination within certain compartments is switched to red light to allow the eyes of the lookouts and officers to adjust to the darkness prior to looking outside of the boat. Additionally, compartments on a submarine may be illuminated with red light in order to simulate night conditions for the crew. Vitamin A Vitamin A is necessary for proper functioning of the human eye. The photopigment rhodopsin found in human rod cells is composed of retinal, a form of vitamin A, bound to an opsin protein. Upon the absorption of light rhodopsin was decomposed into retinal and opsin through bleaching. The average adult male and female should consume 900 and 700 micrograms of vitamin A per day, respectively. Sources of vitamin A Vitamin A is present in both animal and plant sources as retinoids and carotenoids, respectively. This feature has been passed from unicellular to multicellular organisms including Homo sapiens. The dark adaptation function was measured prior to supplementation, 1 day post-treatment, and 75 days post-treatment. It was observed that after merely one day of vitamin A supplementation the recovery kinetics of dark adaptation were significantly accelerated after photoreceptor bleaching. Within 8 days of oral supplementation of vitamin A both patients had their visual function restored to normal. Likewise, Vitamin A was shown to accelerate (to a lesser extent) dark adaptation in AMD. Anthocyanins Anthocyanins make up the majority of the 4000 known flavonoid phytochemicals. This group of approximately 600 bioactive antioxidants carries the strongest physiological effects of any plant compound. These chemicals are also the most visible of the flavonoid phytochemicals because they provide bright blue, red, or purple pigmentation to many plant species. In addition, the antioxidant, anti-inflammatory, and vasoprotective properties of anthocyanins allow them to demonstrate diverse health effects. Food sources Brightly coloured fruits and vegetables are rich in anthocyanins. This makes sense intuitively because anthocyanins offer pigmentation to plants. Blackberries are the most anthocyanin-rich foods, containing 89-211 milligrams per 100 grams. Anthocyanins accomplish this by binding directly to opsin upon the degradation of rhodopsin to its individual constituents by light. Participants received one of three doses of anthocyanins to measure if the result occurred in a dose-dependent manner. The period of dark adaptation was measured prior to and two hours following supplementation in all participants. Results from this experiment indicate that anthocyanins significantly accelerated dark adaptation at merely one dose level compared to the placebo. In neither study did the blueberry anthocyanin intake affect the length of dark adaptation. From these results Kalt et al. concluded that blueberry anthocyanins provide no significant difference to the dark adaptation component of human vision. == Light adaptation ==

With light adaptation, the eye has to quickly adapt to the background illumination to be able to distinguish objects in this background. The process for light adaptation occurs over a period of five minutes. The photochemical reaction is: : Rhodopsin ⇌ retinal + opsin === Increment threshold === Using increment threshold experiments, light adaptation can be measured clinically. In an increment threshold experiment, a test stimulus is presented on a background of a certain luminance, the stimulus is increased until the detection threshold is reached against the background. A monophasic or biphasic threshold versus intensity TVI curve is obtained through this method for both cones and rods. When the threshold curve for a single system (i.e., just cones or just rods) is taken in isolation it can be seen to possess four sections: ;1. Dark light: The threshold in this portion of the TVI curve is determined by the dark/light level. Sensitivity is limited by neural noise. The background field is relatively low and does not significantly affect threshold. ;2. Square root law: This part of the curve is limited by quantal fluctuation in the background. The visual system is usually compared with a theoretical construct called the ideal light detector. To detect the stimulus, the stimulus must sufficiently exceed the fluctuations of the background (noise). ;3. Weber's law: Threshold increases with background luminance proportional to the square root of the background. ;4. Saturation: At saturation, the rod system becomes unable to detect the stimulus. This section of the curve occurs for the cone mechanism under high background levels. == Insufficiency ==

Insufficiency of adaptation most commonly presents as insufficient adaptation to dark environment, called night blindness or nyctalopia. Night blindness is especially prominent in developing countries due to malnutrition and therefore a lack of vitamin A in the diet. 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. Increased obesity rates correspond to an increased number of bariatric surgeries, causing malabsorption of vitamin A in the human body. == See also ==