View source for Adaptation (eye)

{{Short description|Response of the eye to light and dark}}

In [[visual perception|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 cell]]s are exclusively responsible for night vision, as [[cone cell]]s are only able to function at higher illumination levels.<ref name="Miller">{{cite journal |url=https://apps.dtic.mil/sti/citations/ADA257059 |archive-url=https://web.archive.org/web/20220104192142/https://apps.dtic.mil/sti/citations/ADA257059 |url-status=live |archive-date=January 4, 2022 |title=Night Vision Manual for the Flight Surgeon|vauthors=Miller RE, Tredici TJ |date=1 August 1992 |website=US Department of Defense, Defense Technical Information Center |access-date=4 January 2022}}</ref> Night vision is of lower quality than day vision because it is limited in [[Optical resolution|resolution]] and colors cannot be discerned; only shades of gray are seen.<ref name="Miller"/> In order for humans to transition from day to night vision they must undergo a '''dark adaptation''' period of up to two hours<ref>Rebecca Holmes, "Seeing single photons". Physics World, December 2016. http://research.physics.illinois.edu/QI/Photonics/pdf/PWDec16Holmes.pdf</ref> in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude.<ref name="Miller"/> This adaptation period is different between rod and cone cells and results from the regeneration of [[photopigments]] to increase retinal sensitivity.<ref name="Miller"/> Light adaptation, in contrast, works very quickly, within seconds.

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== Efficiency ==
{{human-centric}}
The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine [[order of magnitude|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 (vision)|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.<ref name="eye, human">"Sensory Reception: Human Vision: Structure and Function of the Human Eye" Encyclopædia Britannica, vol. 27, 1987</ref> [[Cone cells]] are able to regain maximum retinal sensitivity in 9–10 minutes of darkness whereas rods require 30–45 minutes to do so.<ref name="Sensory">"Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 179 Encyclopædia Britannica, 1987</ref>

Dark adaptation is far quicker and deeper in young people than the elderly.<ref name="effect of age">{{cite journal | pmid = 10748929 | volume=39 | issue=23 | title=Aging and dark adaptation |vauthors=Jackson GR, Owsley C, McGwin G Jr | journal=Vision Res | pages=3975–82 | doi=10.1016/s0042-6989(99)00092-9| year=1999 | doi-access=free }}</ref>

=== Cones vs. rods ===
{{See also|Photoreceptor cell}}
{{human-centric}}
[[File:Cone-absorbance-en.svg|thumb|upright=2|Normalised absorption spectra of the three human photopsins and of human rhodopsin (dashed).]]
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 [[opsin]]s 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.<ref name="Kolb">{{Cite book
| chapter = Photoreceptors
| first= Helga | last= Kolb |title= Webvision: The Organization of the Retina and Visual System
|url=https://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/photoreceptors/|access-date=2023-02-08|publisher= University of Utah
| date= 2013
| editor1= Helga Kolb
| editor2= Ralph Nelson
| editor3=Eduardo Fernandez
| editor4= Bryan Jones }}</ref> The various cone cells are maximally sensitive to either short wavelengths (blue light), medium wavelengths (green light), or long wavelengths (red light). 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.<ref name="Kolb"/>
The distribution of photoreceptor cells across the surface of the retina has important consequences for vision.<ref name="Purves">Purves, D., Augustine, G. J., & Fitzpatrick, D. (2001). Neuroscience. (2nd ed.). Sinauer Associates.</ref> 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.<ref name="Purves"/> 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).<ref name="Purves"/>

== Ambient light response ==
[[File:Pupillary light reflex.jpg|thumb|300x300px|The pupillary light reflex is a quick but minor mechanism of adaptation ]]
[[File:Dark Adaptation.png|thumbnail|right|Visual Response to Darkness. Cones work at high light levels (during the day but also during driving at night in the headlamp spotlight). Rods take over at twilight and night. The y-axis has logarithmic scaling.]]

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&nbsp;cd/m{{sup|2}}), the cone mechanism is involved in mediating vision; [[photopic vision]]. Below this level, the rod mechanism comes into play providing [[scotopic vision|scotopic (night) vision]]. The range where two mechanisms are working together is called the [[Mesopic vision|mesopic range]], as there is not an abrupt transition between the two mechanism. This adaptation forms the basis of the '''Duplicity Theory'''.<ref name="Light_Dark">{{cite web|url=http://webvision.med.utah.edu/book/part-viii-gabac-receptors/light-and-dark-adaptation/|title=Light and Dark Adaptation by Michael Kalloniatis and Charles Luu – Webvision|website=webvision.med.utah.edu}}</ref>

== Advantages of night vision ==
{{See also|Night vision}}
[[File:TapetumLucidum.JPG|thumb|Reflection of camera flash from ''[[tapetum lucidum]]'']]

Many animals such as [[cat]]s 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 cell|photoreceptor]] cells to an increased amount of light.<ref name="Ollivier">{{cite journal | last1 = Ollivier | first1 = F. J. | last2 = Samuelson | first2 = D. A. | last3 = Brooks | first3 = D. E. | last4 = Lewis | first4 = P. A. | last5 = Kallberg | first5 = M. E. | last6 = Komaromy | first6 = A. M. | year = 2004 | title = Comparative morphology of the tapetum lucidum (among selected species) | journal = Veterinary Ophthalmology | volume = 7 | issue = 1| pages = 11–22 | pmid = 14738502 | doi=10.1111/j.1463-5224.2004.00318.x}}</ref> 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.<ref name="Ollivier"/> Humans, like their primate relatives, do not possess a ''tapetum lucidum'' and therefore were predisposed to be a diurnal species.<ref name="Schwab">{{cite journal | last1 = Schwab | first1 = I. R. | last2 = Yuen | first2 = C. K. | last3 = Buyukmihci | first3 = N. C. | last4 = Blankenship | first4 = T. N. | last5 = Fitzgerald | first5 = P. G. | year = 2002 | title = Evolution of the tapetum | journal = Transactions of the American Ophthalmological Society | volume = 100 | pages = 187–200 | pmid = 12545693 | pmc=1358962}}</ref>

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.<ref name="Hall">{{cite journal | last1 = Hall | first1 = M. I. | last2 = Kamilar | first2 = J. M. | last3 = Kirk | first3 = E. C. | year = 2012 | title = Eye shape and the nocturnal bottleneck of mammals | journal = Proceedings of the Royal Society B | volume = 279| issue = 1749| pages = 4962–4968| doi = 10.1098/rspb.2012.2258 | pmid = 23097513 | pmc = 3497252}}</ref>

== Dark adaptation ==
[[File:USS Shiloh operations 150306-N-LX437-237.jpg|thumb|Extreme red light used on a ship's bridge at night to aid dark adaptation of the crew's eyes]]
{{See also|Purkinje effect}}
[[Rhodopsin]], a biological pigment in the photoreceptors of the retina, immediately [[Photobleaching|photobleaches]] in response to light.<ref name="Stuart_1996">{{cite book |vauthors=Stuart JA, Brige RR | editor = Lee AG | title = Rhodopsin and G-Protein Linked Receptors, Part A (Vol 2, 1996) (2 Vol Set) | publisher = JAI Press | location = Greenwich, Conn | year = 1996 | pages = 33–140 | isbn = 978-1-55938-659-3 | chapter = Characterization of the primary photochemical events in bacteriorhodopsin and rhodopsin }}</ref> [[Visual phototransduction]] starts with the isomerizing of the pigment [[chromophore]] from 11-cis to all-trans [[retinal]].<ref name="auto">{{cite journal|pmid=2707787|year=1989|last1=Bhatia|first1=K|title=Immunogenetic studies of two recently contacted populations from Papua New Guinea|journal=Human Biology|volume=61|issue=1|pages=45–64|last2=Jenkins|first2=C|last3=Prasad|first3=M|last4=Koki|first4=G|last5=Lombange|first5=J}}</ref> 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.<ref name="auto"/> 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.<ref>{{cite journal|pmid=15177205|year=2004|last1=Lamb|first1=T. D.|title=Dark adaptation and the retinoid cycle of vision|journal=Progress in Retinal and Eye Research|volume=23|issue=3|pages=307–80|last2=Pugh Jr|first2=E. N.|doi=10.1016/j.preteyeres.2004.03.001|s2cid=34362318}}</ref> The decrease in calcium ion influx after channel closing causes phosphorylation of metarhodopsin II and speeds up the cis-retinal to trans-retinal inactivation.<ref name="auto"/> The phosphorylation of activated rhodopsin is mediated by [[recoverin]].<ref name="auto"/> The regeneration of the photopigments occurs during dark adaptation albeit at markedly different rates.<ref name="American">{{Cite web|title=Clinical Guidelines|url=https://www.aoa.org/practice/clinical-guidelines?sso=y|access-date=2023-02-08|website=www.aoa.org|language=en}}</ref> [[Rod cell|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.<ref name="eye, human" /><ref>{{cite book|last=Passer and Smith|title=Psychology: The Science of Mind and Behavior|year=2008|edition=4th|page=[https://archive.org/details/psychologyscienc00pass/page/135 135]|isbn=978-0-07-256334-4|url=https://archive.org/details/psychologyscienc00pass/page/135}}</ref> [[Cone cell|Cones]] take approximately 9–10 minutes to adapt to the dark.<ref name="eye, human" />
Sensitivity to light is modulated by changes in intracellular [[calcium]] [[ion]]s and [[cyclic guanosine monophosphate]].<ref name="Hurley">{{cite journal | last=Hurley | first=JB | title=Shedding Light on Adaptation | journal=Journal of General Physiology | volume=119 | issue=2 | pages=125–128 |date=February 2002 | pmid=11815663 | doi=10.1085/jgp.119.2.125 | pmc=2233798 }}</ref>

The sensitivity of the [[Rod cell|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 cell|cone pathways]] encode color.<ref>Aubert H. Physiologie der Netzhaut. Breslau: E. Morgenstern; 1865.</ref>

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.<ref name="saturation">Bartlett NR. Dark and light adaptation. In: Graham CH, editor. Vision and visual perception. New
York: John Wiley and Sons, Inc.; 1965.</ref>
*Size and location on the retina: The location of the test spot affects the dark adaptation curve because of the distribution of the [[Photoreceptor cell|rods and cones]] in the retina.<ref>{{cite journal | author = Hallett PE | year = 1969 | title = The variations in visual threshold measurement | journal = J Physiol | volume = 202 | issue = 403–419| pages = 403–19 | pmid = 5784294 | pmc=1351489| doi = 10.1113/jphysiol.1969.sp008818 }}</ref>
*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.<ref name="Perkins">{{Cite web|title=Human eye {{pipe}} Definition, Anatomy, Diagram, Function, & Facts {{pipe}} Britannica|url=https://www.britannica.com/science/human-eye|access-date=2023-02-08|website=www.britannica.com|language=en}}</ref>

===Intracellular signalling===
Under [[Scotopic vision|scotopic]] conditions, intracellular [[Cyclic guanosine monophosphate|cGMP]] concentration is high in photoreceptors. cGMP binds to and opens [[Cyclic nucleotide–gated ion channel#Photoreceptors|cGMP gated Na{{sup|+}} channels]] to allow sodium and calcium influx.<ref name="auto1">{{cite journal |last1=Pugh |first1=E.N. Jr. |last2=Lamb |first2=T.D. |year=1990 |title=Cyclic GMP and calcium: The internal messengers of excitation and adaptation in vertebrate photoreceptors |journal=Vision Research |volume=30 |issue=12 |pages=1923–1948 |pmid=1962979 |doi=10.1016/0042-6989(90)90013-b|s2cid=22506803 |doi-access=free }}</ref> 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]],<ref>{{cite journal |last1=Gorczyca |first1=W.A. |last2=Gray-Keller |first2= M.P. |year=1994 |title=Purification and physiological evaluation of a guanylate cyclase activating protein from retinal rods |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=91 |issue=9 |pages=4014–4018 |doi=10.1073/pnas.91.9.4014|pmid=7909609 |pmc=43713 |bibcode=1994PNAS...91.4014G |doi-access=free }}</ref> removing this protein's stimulatory effect on [[guanylate cyclase|guanylyl cyclase]].<ref name="auto1" /> This reduces cGMP production by guanylyl cyclase to lower cGMP concentration during prolonged darkness. Elevated calcium concentration also increases the activity of [[phosphodiesterase]]<ref name="auto1" /> which hydrolyses cGMP to further reduce its concentration. This reduces opening of the cGMP gated Na{{sup|+}} channels to hyperpolarise the cell, once again making it sensitive to small increases in brightness. Without dark adaptation, the photoreceptor would remain depolarized under scotopic conditions and so also remain unresponsive to small changes in brightness.

=== Inhibition ===
Inhibition by neurons also affects activation in synapses. Together with the bleaching of a [[Photoreceptor cell|rod or cone pigment]], merging of signals on [[retinal ganglion cell|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.<ref name="eye, human" />

=== Measuring Dark Adaptation ===
Ophthalmologists sometimes measure patients' dark adaptation using an instrument known as a dark adaptometer.

There are multiple commercial dark adaptometers (AdaptDx, MetroVision MonCvONE, Roland Consult DARK-Adaptometer). Besides those free-viewing devices, a fundus-tracked workflow has been recently introduced by modifying a [[microperimetry]] device that enables testing of patients with unstable fixation.<ref>{{cite journal |vauthors=Oertli JM, Pfau K, Scholl HP, Jeffrey BG, Pfau M |title=Establishing Fully-Automated Fundus-Controlled Dark Adaptometry: A Validation and Retest-Reliability Study |journal=Transl Vis Sci Technol |volume=12 |issue=12 |pages=18 |date=December 2023 |pmid=38112496 |doi=10.1167/tvst.12.12.18|doi-access=free |pmc=10732091 }}</ref>

==== Using Dark Adaptation Measurement to Diagnose Disease ====
Numerous clinical studies have shown that dark adaptation function is dramatically impaired from the earliest stages of [[macular degeneration|Age-related Macular Degeneration]] (AMD), [[Retinitis Pigmentosa]] (RP), and other retinal diseases, with increasing impairment as the diseases progress.<ref>{{Cite journal|last1=Owsley|first1=C.|last2=Jackson|first2=G. R.|last3=White|first3=M.|last4=Feist|first4=R.|last5=Edwards|first5=D.|date=2001-07-01|title=Delays in rod-mediated dark adaptation in early age-related maculopathy|journal=Ophthalmology|volume=108|issue=7|pages=1196–1202|issn=0161-6420|pmid=11425675|doi=10.1016/s0161-6420(01)00580-2}}</ref><ref>{{Cite book|title=Structure, function, and pathology of Bruch's membrane. In: Ryan SJ, et al, eds. Retina, Vol 1, Part 2: Basic Science and Translation to Therapy. 5th ed|last=Curcio|first=CA|publisher=Elsevier|year=2013}}</ref> 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.<ref>{{Cite web|url=https://nei.nih.gov/health/maculardegen/armd_facts|title=Facts About Age-Related Macular Degeneration|last=NEI|website=NEI|access-date=2017-02-04|archive-date=2015-12-22|archive-url=https://web.archive.org/web/20151222151236/https://nei.nih.gov/health/maculardegen/armd_facts}}</ref> 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.<ref>{{Cite journal|last1=Owsley|first1=Cynthia|last2=McGwin|first2=Gerald|last3=Clark|first3=Mark E.|last4=Jackson|first4=Gregory R.|last5=Callahan|first5=Michael A.|last6=Kline|first6=Lanning B.|last7=Witherspoon|first7=C. Douglas|last8=Curcio|first8=Christine A.|date=2016-02-01|title=Delayed Rod-Mediated Dark Adaptation Is a Functional Biomarker for Incident Early Age-Related Macular Degeneration|journal=Ophthalmology|volume=123|issue=2|pages=344–351|doi=10.1016/j.ophtha.2015.09.041|issn=1549-4713| pmc=4724453 |pmid=26522707}}</ref>

==Accelerating dark adaptation==
[[File:Персеїди-2021 - екскурсія Головної астрономічної обсерваторії НАН України (5).jpg|thumb|Astronomer preserves night vision]]


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.<ref name="Abbott">[http://users.ipfw.edu/abbott/120/adaptation.html Link] {{Webarchive|url=https://web.archive.org/web/20180621160205/http://users.ipfw.edu/abbott/120/adaptation.html |date=2018-06-21 }}, Abbott, B. (2012). Sensation and perception.</ref> 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.<ref name="Watson">Watson, S., & Gorski, K. A. (2011). Invasive cardiology: A manual for cath lab personnel. (3rd ed., pp. 61-62). Sudbury, MA: Jones & Bartlett Learning.</ref> 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.<ref name="Abbott"/> Once an individual enters a dark setting most of their rod cells will already be accommodated to the dark and be able to transmit visual signals to the brain without an accommodation period.<ref name="Watson"/>

The concept of red lenses for dark adaptation is based upon experimentation by [[Antoine Béclère]] and his early work with radiology. In 1916, the scientist [[Wilhelm Trendelenburg]] invented the first pair of [[red adaptation goggles]] for radiologists to adapt their eyes to view screens during fluoroscopic procedures.

==== Evolutionary context ====
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. It is believed that the earliest visual pigments were those of cone photoreceptors, with rod opsin proteins evolving later.<ref name="Bowmaker">{{cite journal | last1 = Link | year = 1998| title = Evolution of colour vision in vertebrates| doi = 10.1038/eye.1998.143| pmid = 9775215| journal = Eye | volume = 12 | issue = 3| pages = 541–547 | doi-access = free}}</ref> 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.<ref name="Bowmaker"/> Being that these pro-mammals were nocturnal they increased their sensitivity in low luminescence settings and reduced their photopic system from tetrachromatic to dichromatic.<ref name="Bowmaker"/> The shift to a nocturnal lifestyle would demand more rod photoreceptors to absorb the blue light emitted by the moon during the night.<ref name="Roberts">{{Cite web|title=Circadian Rhythm and Human Health|url=http://photobiology.info/Roberts-CR.html|access-date=2023-02-08|website=photobiology.info}}</ref> 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.<ref name="Bowmaker"/> The primates were diurnal rather than nocturnal in nature and therefore required a more precise photopic visual system. 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.<ref name="Bowmaker"/>

====Applications ====
*Aviators commonly wear red lensed glasses or goggles prior to taking off in the dark to ensure that they are able to see outside of the aircraft. Furthermore, throughout flight the cockpit is illuminated with dim red lights. This lighting is to ensure that the pilot is able to read instruments and maps while maintaining scotopic vision for looking outside.<ref name="Federal">[https://www.faa.gov/air_traffic/publications/atpubs/aim/aim0801.html Link] {{webarchive |url=https://web.archive.org/web/20150326041755/https://www.faa.gov/air_traffic/publications/atpubs/aim/aim0801.html |date=March 26, 2015 }}, Federal Aviation Administration. (2015). Medical facts for pilots.</ref>
*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.<ref name="Summitt">Summitt, D. (2004). Tales of a cold war submariner. (1st ed., p. 138)</ref>

=== Vitamin A ===
{{See also|Vitamin A}}
[[File:11-cis-Retinal2.svg|thumb|upright=1.3|11-cis-Retinal2]]
[[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.<ref name="Wolf">{{cite journal | last1 = Wolf| first1 = G.| year = 2001| title = The discovery of the visual function of vitamin A | journal = The Journal of Nutrition | volume = 131 | issue = 6| pages = 1647–1650 | doi = 10.1093/jn/131.6.1647| pmid = 11385047| doi-access = free}}</ref> Upon the absorption of light rhodopsin was decomposed into retinal and opsin through bleaching.<ref name="Wolf"/> Retinal could then have one of two fates: it could recombine with opsin to reform rhodopsin or it could be converted into free retinol.<ref name="Wolf"/> The American scientist George Wald was the first to recognize that the visual system expends vitamin A and is dependent upon diet for its replacement.<ref name="Wolf"/>

Vitamin A serves many functions in the human body outside of healthy vision. It is vital in maintaining a healthy immune system as well as promoting normal growth and development.<ref name="Dieticians">{{Cite web|title=Vitamins and Minerals - Unlock Food|url=https://www.unlockfood.ca/en/Articles/Vitamins-and-Minerals|access-date=2023-02-08|website=www.unlockfood.ca}}</ref> The average adult male and female should consume 900 and 700 micrograms of vitamin A per day, respectively.<ref name="Dieticians"/> Consumption above 3000 micrograms per day can lead to acute or chronic hypervitaminosis A.<ref name="DRI VitA">{{cite book|author=Institute of Medicine |chapter=Vitamin A |chapter-url=https://www.nap.edu/read/10026/chapter/6 |title=Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc |publisher=Food and Nutrition Board of the Institute of Medicine| pages=82–161 |year=2001|isbn=0-309-07290-5}}</ref>

==== Sources of vitamin A ====
Vitamin A is present in both animal and plant sources as retinoids and carotenoids, respectively.<ref name="Dieticians"/> Retinoids can be used immediately by the body upon absorption into the cardiovascular system; however, plant-based carotenoids must be converted to retinol prior to utilization by the body.<ref name="Dieticians"/> The highest animal-based sources of vitamin A are liver, dairy products, and fish.<ref name="Dieticians"/> Fruits and vegetables containing high amounts of carotenoids are dark green, yellow, orange, and red in colour.<ref name="Dieticians"/>

==== Evolutionary context ====
Vitamin A-based opsin proteins have been used for sensing light in organisms for most of evolutionary history beginning approximately 3 billion years ago.<ref name="Zhong">{{cite journal | last1 = Zhong | first1 = M. | last2 = Kawaguchi | first2 = R. | last3 = Kassai | first3 = M. | last4 = Sun | first4 = H. | year = 2012 | title = Retina, retinol, retinal and the natural history of vitamin a as a light sensor | journal = Nutrients | volume = 4 | issue = 12| pages = 2069–2096 | doi = 10.3390/nu4122069 | pmid = 23363998 | pmc = 3546623 | doi-access = free }}</ref> This feature has been passed from unicellular to multicellular organisms including Homo sapiens.<ref name="Zhong"/> This vitamin was most likely selected by evolution for sensing light because retinal causes a shift in photoreceptor absorbance to the visible light range.<ref name="Zhong"/> This shift in absorbance is especially important for life on Earth because it generally matches the peak irradiance of sunlight on its surface.<ref name="Zhong"/> A second reason why retinal evolved to be vital for human vision is because it undergoes a large conformational change when exposed to light.<ref name="Zhong"/> 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.<ref name="Zhong"/>

==== Experimental evidence ====
Various studies have been conducted testing the effective of vitamin A supplementation on dark adaptation. In a study by Cideciyan et al. the length of dark adaptation was measured in a patient with systemic vitamin A deficiency (VAD) before and after vitamin A supplementation.<ref name="Cid">{{cite journal | title = Rod plateaux during dark adaptation in sorsby's fundus dystrophy and vitamin a deficiency | journal = Investigative Ophthalmology & Visual Science | volume = 38 | issue = 9| pages = 1786–1794 }}</ref> 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.<ref name="Cid"/> Dark adaptation was further accelerated following 75 days of treatment.<ref name="Cid"/>

A subsequent study by Kemp et al. studied dark adaptation in subjects with [[primary biliary cirrhosis]] and [[Crohn's disease]], both of whom had vitamin A deficiency.<ref name="Kemp">{{cite journal| doi=10.1016/S0014-4835(88)80076-9 | volume=46 | issue=2 | title=Visual function and rhodopsin levels in humans with vitamin A deficiency | year=1988 | journal=Experimental Eye Research | pages=185–197 | last1 = Kemp | first1 = Colin M. | last2 = Jacobson | first2 = Samuel G. | last3 = Faulkner | first3 = David J. | last4 = Walt | first4 = Robert W.| pmid=3350064 }}</ref> Within 8 days of oral supplementation of vitamin A both patients had their visual function restored to normal.<ref name="Kemp"/> Furthermore, adaptation kinetics significantly improved in both subjects following supplementation.<ref name="Kemp"/>

Later independent studies in Sorsby fundus dystrophy confirmed the effect of Vitamin A on dark adaptation.<ref>{{cite journal |vauthors=Raming K, Gliem M, Charbel Issa P, Birtel J, Herrmann P, Holz FG, Pfau M, Hess K |title=Visual Dysfunction and Structural Correlates in Sorsby Fundus Dystrophy |journal=Am J Ophthalmol |volume=234 |issue= |pages=274–284 |date=February 2022 |pmid=34352251 |doi=10.1016/j.ajo.2021.07.032 |url=}}</ref> Likewise, Vitamin A was shown to accelerate (to a lesser extent) dark adaptation in AMD.<ref>{{cite journal |vauthors=Owsley C, McGwin G, Jackson GR, Heimburger DC, Piyathilake CJ, Klein R, White MF, Kallies K |title=Effect of short-term, high-dose retinol on dark adaptation in aging and early age-related maculopathy |journal=Invest Ophthalmol Vis Sci |volume=47 |issue=4 |pages=1310–8 |date=April 2006 |pmid=16565362 |doi=10.1167/iovs.05-1292 |url=|doi-access=free }}</ref><ref>{{cite journal |vauthors=Pfau K, Jeffrey BG, Cukras CA |title=Low-Dose Supplementation with Retinol Improves Retinal Function in Eyes with Age-Related Macular Degeneration but Without Reticular Pseudodrusen |journal=Retina |volume=43 |issue=9 |pages=1462–1471 |date=September 2023 |pmid=37315571 |doi=10.1097/IAE.0000000000003840 |url=}}</ref>

=== Anthocyanins ===
{{See also|Anthocyanin}}
[[Anthocyanins]] make up the majority of the 4000 known [[Flavonoid|flavonoid phytochemicals]].<ref name="Lila">{{cite journal | year = 2004| journal = Journal of Biomedicine and Biotechnology | volume = 2004 | issue = 5| pages = 306–313 | pmc=1082894 | pmid=15577194 | doi=10.1155/S111072430440401X | title=Anthocyanins and Human Health: An In Vitro Investigative Approach | last1 = Lila | first1 = MA| doi-access = free }}</ref> This group of approximately 600 bioactive antioxidants carries the strongest physiological effects of any plant compound.<ref name="Sterling">[http://www.biolink.no/anthocyanins/category143.html Link] {{webarchive |url=https://web.archive.org/web/20150402134152/http://www.biolink.no/anthocyanins/category143.html |date=April 2, 2015 }}, Sterling, M. (2001). What are anthocyanins?</ref> These chemicals are also the most visible of the flavonoid phytochemicals because they provide bright blue, red, or purple pigmentation to many plant species.<ref name="Sterling"/> Anthocyanins also serve to protect the photosynthetic tissues from the direct rays of the sun.<ref name="Inno">{{Cite web|last=Wimmer|first=Richard|date=2022-11-15|title=Bioflavonoids - Medicinal Plants|url=https://www.chineseherbshealing.com/medicinal-plants-2/bioflavonoids.html|access-date=2023-02-08|website=Chinese Herbs Healing|language=en}}</ref>
In addition, the [[antioxidant]], anti-inflammatory, and vasoprotective properties of anthocyanins allow them to demonstrate diverse health effects.<ref name="Sterling"/> In humans, anthocyanins are effective for a variety of health conditions including neurological damage, atherosclerosis, diabetes, as well as visual impairment.<ref name="Inno"/> Anthocyanins frequently interact with other phytochemicals to potentiate biological effects; therefore, contributions from individual biomolecules remains difficult to decipher.<ref name="Lila"/>
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.<ref name="Inno"/> The fruits and vegetables produced by such plants are also brightly pigmented attracting animals to eat them and disperse the seeds.<ref name="Inno"/> Due to this natural mechanism anthocyanin-containing plants are widely abundant in most areas of the world. The high abundance and distribution of anthocyanin-containing plants make it a natural food source for many animals. Through fossil evidence it is known that these compounds were eaten in high amounts by primitive hominins.<ref name="Sterling"/>

During World Wars I and II British Air Force aviators were known to consume extensive amounts of bilberry jam. 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.<ref name="Losso">Losso, J. N., Shahidi, F., & Bagchi, D. (2007). Anti-angiogenic functional and medicinal foods. Boca Raton, FL: Taylor & Francis Group.</ref>

==== Food sources ====
[[File:Blackberry fruits.jpg|thumb|Blackberry fruits]]
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.<ref name="Inno"/> Other foods that are rich in this phytochemical include red onions, blueberries, bilberries, red cabbage, and eggplant.<ref name="Inno"/> The ingestion of any of these food sources will yield a variety of phytochemicals in addition to anthocyanins because they naturally exist together.<ref name="Lila"/> 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.<ref name="Lila"/>

==== Effect on dark adaptation====
Anthocyanins accelerate dark adaptation in humans by enhancing the regeneration of the rod photopigment, rhodopsin.<ref name="Tiru">{{cite journal | last1 = Tirupula | first1 = K. C. | last2 = Balem | first2 = F. | last3 = Yanamala | first3 = N. | last4 = Klein-Seetharaman | first4 = J. | year = 2009 | title = ph-dependent interaction of rhodopsin with cyanidin-3-glucoside. 2. functional aspects | journal = Photochemistry and Photobiology | volume = 85 | issue = 2| pages = 463–470 | doi=10.1111/j.1751-1097.2008.00533.x| pmid = 19267871 | s2cid = 23886170 }}</ref> Anthocyanins accomplish this by binding directly to opsin upon the degradation of rhodopsin to its individual constituents by light.<ref name="Tiru"/> Once bound to opsin, the anthocyanin changes its structure thereby accelerating its access to the retinal binding pocket. 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.<ref name="Tiru"/> Through this mechanism an individual is able to accelerate dark adaptation and achieve night vision in a shorter period of time.

==== Supportive evidence ====
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.<ref name="Naka">{{cite journal | last1 = Nakaishi | first1 = H. | last2 = Matsumoto | first2 = H. | last3 = Tominaga | first3 = S. | last4 = Hirayama | first4 = M. | year = 2000 | title = Effects of black currant anthocyanoside intake on dark adaptation and vdt work-induced transient refractive alteration in healthy humans | journal = Alternative Medicine Review | volume = 5 | issue = 6| pages = 553–562 | pmid = 11134978 }}</ref>{{ums|date=December 2017}} 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.<ref name="Naka"/>{{ums|date=December 2017}} Observing the data as a whole Nakaishi et al. concluded that anthocyanins effectively reduced the dark adaptation period in a dose-dependent manner.<ref name="Naka"/>{{ums|date=December 2017}}

==== Contradictory evidence ====
Despite the fact that many scientists believe anthocyanins to be beneficial in accelerating dark adaptation in humans, a study conducted by Kalt et al. in 2014 showed blueberry anthocyanins have no effect. In this study two double-blind, placebo-controlled studies were conducted to examine dark adaptation following the intake of blueberry products.<ref name="Kalt">{{cite journal|doi=10.1021/jf503689c | pmid=25335781 | volume=62 | issue=46 | title=Blueberry Effects on Dark Vision and Recovery after Photobleaching: Placebo-Controlled Crossover Studies | year=2014 | journal=Journal of Agricultural and Food Chemistry | pages=11180–11189 | last1 = Kalt | first1 = Wilhelmina | last2 = McDonald | first2 = Jane E. | last3 = Fillmore | first3 = Sherry A. E. | last4 = Tremblay | first4 = Francois}}</ref> In neither study did the blueberry anthocyanin intake affect the length of dark adaptation.<ref name="Kalt"/> From these results Kalt et al. concluded that blueberry anthocyanins provide no significant difference to the dark adaptation component of human vision.<ref name="Kalt"/>

== 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

=== [[Contrast (vision)#Contrast_threshold|Increment threshold]] ===
[[File:Light adaptation.png|thumbnail|Schematic of the increment threshold curve of the rod system]]
Using increment threshold experiments, light adaptation can be measured clinically.<ref>H Davson. Physiology of the eye. 5th ed. London: Macmillan Academic and Professional Ltd.; 1990.</ref> 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 [[Photoreceptor cell|cones and rods]].

When the threshold curve for a single system (i.e., [[Photoreceptor cell|just cones or just rods]]) is taken in isolation it can be seen to possess four sections:<ref>Aguilar M, Stiles WS. Saturation of the rod mechanism of the retina at high levels of stimulation. Opt Acta (Lond) 1954;1:59–65.</ref>

;1. [[Eigengrau|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. [[De Vries - Rose law|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|Weber's law]]: Threshold increases with background [[luminance]] proportional to the square root of the background.<ref>{{Cite journal|pages=337–350|pmid=13539843|pmc=1358805|year=1958|last1=Barlow|first1=H. B.|title=Temporal and spatial summation in human vision at different background intensities|journal=The Journal of Physiology|volume=141|issue=2|doi=10.1113/jphysiol.1958.sp005978}}</ref>
;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.<ref>H Davson. Physiology of the eye. 5th ed. London: Macmillan Academic and Professional Ltd.; 1990</ref>

== Insufficiency ==
{{main|Nyctalopia}}
[[File:P360 Onderdendam goed nachtzicht ns nachtblind.jpg|thumb|upright=2|Effect of night blindness. Left: good night vision. Right: nightblind.]]
Insufficiency of adaptation most commonly presents as insufficient adaptation to dark environment, called [[night blindness]] or ''nyctalopia''.<ref name="Wolf"/> The opposite problem, known as [[hemeralopia]], that is, inability to see clearly in bright light, is much rarer.

The [[Fovea centralis in macula|fovea]] is blind to dim light (due to its cone-only array) and the rods are more sensitive, so a dim star on a moonless night must be [[Averted vision|viewed from the side]], so it stimulates the rods. This is not due to pupil width since an artificial fixed-width pupil gives the same results.<ref name="eye, human" />

Night blindness can be caused by a number of factors the most common of which being vitamin A deficiency. If detected early enough nyctalopia can be reversed and visual function can be regained; however; prolonged vitamin A deficiency can lead to permanent visual loss if left untreated.<ref name="Clifford">{{cite journal | year = 2013| title = Reversible night blindness – a reminder of the increasing importance of vitamin a deficiency in the developed world | journal = Journal of Optometry | volume = 6 | issue = 3| pages = 173–174 | doi=10.1016/j.optom.2013.01.002 | last1 = Clifford | first1 = Luke J. | last2 = Turnbull | first2 = Andrew M.J. | last3 = Denning | first3 = Anne M.| pmc = 3880510 }}</ref>

Night blindness is especially prominent in developing countries due to [[malnutrition]] and therefore a lack of vitamin A in the diet.<ref name="Clifford"/> 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.<ref name="Clifford"/>

== See also ==
{{col div|colwidth=40em}}
*[[Adaptive system]]
*[[Dark adaptor goggles]]
*[[Human eye]]
*[[Mesopic vision]]
*[[Neural adaptation]]
*[[Purkinje effect]]
*[[Disc shedding]] 
{{colend}}

== References ==
{{reflist}}

== External links ==
* {{MeshName|Adaptation,+Ocular}}
*[https://webvision.med.utah.edu/book/part-viii-psychophysics-of-vision/light-and-dark-adaptation/ Light and Dark Adaptation by Michael Kalloniatis and Charles Luu]

[[Category:Animal physiology]]
[[Category:Eye]]
[[Category:Vision]]