明順応

関: ocular adaptation、photopic

WordNet

very thin and insubstantial; "thin paper"; "light summer dresses"
make lighter or brighter; "This lamp lightens the room a bit" (同)illume, illumine, light_up, illuminate
a condition of spiritual awareness; divine illumination; "follow Gods light" (同)illumination
having abundant light or illumination; "they played as long as it was light"; "as long as the lighting was good" (同)lighting
(physics) electromagnetic radiation that can produce a visual sensation; "the light was filtered through a soft glass window" (同)visible_light, visible_radiation
any device serving as a source of illumination; "he stopped the car and turned off the lights" (同)light_source
the visual effect of illumination on objects or scenes as created in pictures; "he could paint the lightest light and the darkest dark" (同)lightness
a particular perspective or aspect of a situation; "although he saw it in a different light, he still did not understand"
a person regarded very fondly; "the light of my life"
a visual warning signal; "they saw the light of the beacon"; "there was a light at every corner"
an illuminated area; "he stepped into the light"
mental understanding as an enlightening experience; "he finally saw the light"; "can you shed light on this problem?"
public awareness; "it brought the scandal to light"
(used of color) having a relatively small amount of coloring agent; "light blue"; "light colors such as pastels"; "a light-colored powder" (同)light-colored
moving easily and quickly; nimble; "the dancer was light and graceful"; "a lightsome buoyant step"; "walked with a light tripping step" (同)lightsome, tripping
having relatively few calories; "diet cola"; "light (or lite) beer"; "lite (or light) mayonnaise"; "a low-cal diet" (同)lite, low-cal, calorie-free
less than the correct or legal or full amount often deliberately so; "a light pound"; "a scant cup of sugar"; "regularly gives short weight" (同)scant, short
(of sleep) easily disturbed; "in a light doze"; "a light sleeper"; "a restless wakeful night" (同)wakeful
(physics, chemistry) not having atomic weight greater than average; "light water is ordinary water"
(used of soil) loose and large-grained in consistency; "light soil"
characterized by or emitting light; "a room that is light when the shutters are open"; "the inside of the house was airy and light"
demanding little effort; not burdensome; "light housework"; "light exercise"
designed for ease of movement or to carry little weight; "light aircraft"; "a light truck"
easily assimilated in the alimentary canal; not rich or heavily seasoned; "a light diet"
having little importance; "losing his job was no light matter"
intended primarily as entertainment; not serious or profound; "light verse"; "a light comedy"
not great in degree or quantity or number; "a light sentence"; "a light accent"; "casualties were light"; "light snow was falling"; "light misty rain"; "light smoke from the chimney"
of comparatively little physical weight or density; "a light load"; "magnesium is a light metal--having a specific gravity of 1.74 at 20 degrees C"
of little intensity or power or force; "the light touch of her fingers"; "a light breeze"
of the military or industry; using (or being) relatively small or light arms or equipment; "light infantry"; "light cavalry"; "light industry"; "light weapons"
psychologically light; especially free from sadness or troubles; "a light heart"
(physiology) the responsive adjustment of a sense organ (as the eye) to varying conditions (as of light)
a written work (as a novel) that has been recast in a new form; "the play is an adaptation of a short novel" (同)version
the process of adapting to something (such as environmental conditions) (同)adaption, adjustment
apparatus for supplying artificial light effects for the stage or a film
the craft of providing artificial light; "an interior decorator must understand lighting"
set afire or burning; "the lighted candles"; "a lighted cigarette"; "a lit firecracker" (同)lit

PrepTutorEJDIC

〈U〉『光,光線』;明るさ / 〈U〉夜明け;日中;日光 / 〈C〉『明かり』(太陽・灯火など光を出すもの) / 〈U〉《時にa~》光輝(brightness),(目などの)輝き / 〈C〉(点火するための)火,火花 / 〈C〉明かり採り,採光窓 / 〈C〉《通例単数形で》(絵などの)明るい部分 / 〈U〉(…についての)知識,情報,理解《+『on』+『名』》 / 〈C〉(ものを見る)観点,見地;相 / 〈C〉この世に光を与える人;指導的な人物 / …‘に'『火をつける』《+『up』+『名』+『名』+『up』》 / …‘に'『明かりをつける』,‘を'照らす《+『up』+『名,』+『名』+『up』》 / 〈人〉‘を'明かりをつけて案内する / 〈表情など〉‘を'明るくする《+『up』+『名,』+『名』+『up』》 / 〈物が〉火がつく,点火する;明かりがつく《+『up』》 / 明るくなる,晴れ晴れする《+『up』》 / 『明るい』 / (色が)『薄い』
(重量が)『軽い』 / (いろいろな事について,その程座・量が)軽い,小さい / 『軽快な』 / 軽装の / 肩のこらない;気軽な / 手軽な,骨の折れなし / (食べ物が)消化のよい,もたれない / ふらふらする,目まいのする / アルコール分の少ない / (パンなどが)ふんわりした / (重量が)軽く / (程度が)軽く,容易に;軽装で
(車・馬などから)降りる《+『from』+『名』》(alight) / (木などに)〈鳥などが〉止まる《+『on』(『upon』)+『名』》 / (…に)〈打撃・運などが〉突然ふりかかる《+『on』(『upon』)+『名』》
〈U〉適応,順応 / 〈U〉脚色,翻案;〈C〉翻案物,改作物
照明,点火 / 照明方法(装置)
(羊・豚などの)肺臓

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/08/12 22:11:13」(JST)

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In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.

Efficiency[edit source | edit]

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 one thousand. 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 become ten thousand to one million 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 to bright sunlight from darkness. This is due to cones obtaining more sensitivity when first entering the dark for the first five minutes but the rods take over after five or more minutes.^[1]

Ambient Light Response[edit source | edit]

Visual Response to Darkness

A minor mechanism of adaptation is the pupillary light reflex, adjusting the amount of light that reaches the retina.

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/m2), 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^[2]

Dark Adaptation[edit source | edit]

Rhodopsin, a biological pigment in the photoreceptors of the retina immediately photobleaches in response to light.^[3] 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 half an hour.^[1]^[4] Cones take approximately 9–10 minutes to adapt to the dark.^[1] Sensitivity to light is modulated by changes in intracellular calcium ions and cyclic guanosine monophosphate.^[5]

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.^[6]

Four factors affect dark adaptation:

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.^[7]

Size and Location of 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.^[8]

Wavelength of the Threshold Light

Varying the wavelengths of stimuli also effect the dark adaptation curve. Long wavelengths, such as extreme red, create the absence of a distinct rod/cone break as the rod and cones cells have similar sensitives 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.^[7]

Rhodopsin Regeneration

Dark adaptation depends upon photopigment bleaching, which effects the threshold of both cones and rods.^[7]

Inhibition[edit source | edit]

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.^[1]

Light Adaptation[edit source | edit]

Schematic of the increment threshold curve of the rod system

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.

Photochemical Reaction:

Increment Threshold[edit source | edit]

Using increment threshold experiments, light adaptation can be measured clinically.^[9] 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 ie just cones or just rods is taken in isolation it can been seen to possesses four sections:^[10]

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.^[11]

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.^[12]

Insufficiency[edit source | edit]

Insufficiency of adaptation most commonly presents as insufficient adaptation to dark environment, called night blindness or nyctalopia. The opposite problem, known as hemeralopia, that is, inability to see clearly in bright light, is much rarer.

The 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 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.^[1]

References[edit source | edit]

^ ^a ^b ^c ^d ^e "Sensory Reception: Human Vision: Structure and Function of the Human Eye" Encyclopaedia Brtinnica, vol. 27, 1987
^ http://webvision.med.utah.edu/book/part-viii-gabac-receptors/light-and-dark-adaptation/"Light and Dark Adaptation"
^ Stuart JA, Brige RR (1996). "Characterization of the primary photochemical events in bacteriorhodopsin and rhodopsin". In Lee AG. Rhodopsin and G-Protein Linked Receptors, Part A (Vol 2, 1996) (2 Vol Set). Greenwich, Conn: JAI Press. pp. 33–140. ISBN 1-55938-659-2.
^ Passer and Smith (2008). Psychology: The Science of Mind and Behavior (4th ed.). p. 135. ISBN 0-07-256334-6.
^ Hurley, JB (February 2002). "Shedding Light on Adaptation". Journal of General Physiology 119 (2): 125–128. doi:10.1085/jgp.119.2.125. PMC 2233798. PMID 11815663.
^ Aubert H. Physiologie der Netzhaut. Breslau: E. Morgenstern; 1865.
^ ^a ^b ^c Bartlett NR. Dark and light adaptation. In: Graham CH, editor. Vision and visual perception. New York: John Wiley and Sons, Inc.; 1965.
^ Hallett PE. The variations in visual threshold measurement. J Physiol 1969;202:403–419.1351489 [PubMed: 5784294]
^ H Davson. Physiology of the eye. 5th ed. London: Macmillan Academic and Professional Ltd.; 1990.
^ Aguilar M, Stiles WS. Saturation of the rod mechanism of the reina at high levels of stimulation. Opt Acta (Lond) 1954;1:59–65.
^ Barlow HB. Increment thresholds at low intensities considered as signal noise discriminations. J Physiol 1958;141:337–350. [PubMed: 13539843]
^ H Davson. Physiology of the eye. 5th ed. London: Macmillan Academic and Professional Ltd.; 1990

External links[edit source | edit]

Adaptation, Ocular at the US National Library of Medicine Medical Subject Headings (MeSH)
http://webvision.med.utah.edu/light_dark.html

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