Adaptation (eye)
In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively responsible for night vision, as cone cells are only able to function at higher illumination levels. Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen. In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude. This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. Light adaptation, in contrast, works very quickly, within seconds.
Efficiency
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. 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. 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. The various cone cells are maximally sensitive to either short wavelengths, medium wavelengths, or long wavelengths. Rod photoreceptors only contain one type of photopigment, rhodopsin, which has a peak sensitivity at a wavelength of approximately 500 nanometers which corresponds to blue-green light.The distribution of photoreceptor cells across the surface of the retina has important consequences for vision. 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.
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, the cone mechanism is involved in mediating vision; photopic vision. Below this level, the rod mechanism comes into play providing scotopic 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. Humans, like their primate relatives, do not possess a tapetum lucidum and therefore were predisposed to be a diurnal species.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
, 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. 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. The decrease in calcium ion influx after channel closing causes phosphorylation of metarhodopsin II and speeds up the cis-retinal to trans-retinal inactivation. The phosphorylation of activated rhodopsin is mediated by recoverin. The regeneration of the photopigments occurs during dark adaptation albeit at markedly different rates. 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.Sensitivity to light is modulated by changes in intracellular calcium ions and cyclic guanosine monophosphate.
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
Inhibition
Inhibition by neurons also affects activation in synapses. Together with the bleaching of a rod or cone pigment, merging of signals on ganglion cells are inhibited, reducing convergence.Alpha adaptation, i.e., rapid sensitivity fluctuations, is powered by nerve control. The merging of signals by virtue of the diffuse ganglion cells, as well as horizontal and amacrine cells, allow a cumulative effect. Thus that area of stimulation is inversely proportional to intensity of light, a strong stimulus of 100 rods equivalent to a weak stimulus of 1,000 rods.
In sufficiently bright light, convergence is low, but during dark adaptation, convergence of rod signals boost. This is not due to structural changes, but by a possible shutdown of inhibition that stops convergence of messages in bright light. If only one eye is open, the closed eye must adapt separately upon reopening to match the already adapted eye.