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Sensitivity of human eye to luminance

Sensitivity of human eye to luminance


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I heard once that the human eye has a logarithmic scale for luminance, e.g. to "feel" that a surface is three times as luminous compared to another, the former emits a light 8 times more powerful than the latter. Definitions are indeed vague as my memory doesn't serve me well.

Is this based on anything scientific?


From Stevens & Galanter (1957)

Although an extensive investigation of the subjective scale of brightness is still in progress in this laboratory, enough has been learned to show that, for patches of white light viewed in a dark room, subjective brightness is a power function of luminance. Moreover, the exponent is of the order of one-third which is in reasonable agreement with results obtained by Hanes.

  • Stevens, S. S., & Galanter, E. H. (1957). Ratio scales and category scales for a dozen perceptual continua. Journal of Experimental Psychology, 54(6), 377.

and the related study that is mentioned:

  • Hanes, R. M. (1949). The construction of subjective brightness scales from fractionation data: a validation. Journal of experimental psychology, 39(5), 719.

This logarithmic increase in order to produce a just noticeable difference between stimuli of two different intensities is in fact a general property or the sensory system. It is known as (Weber-) Fechner's law:

Weber's law states that the just-noticeable difference between two stimuli is proportional to the magnitude of the stimuli. Gustav Theodor Fechner (1801-1887), a scholar of Weber, later used Weber's findings to construct a psychophysical scale in which he described the relationship between the physical magnitude of a stimulus and its (subjectively) perceived intensity. Fechner's law (better referred to as Fechner's scale) states that subjective sensation is proportional to the logarithm of the stimulus intensity.


Trichromatic Theory and Opponent Process Theory

In the past, the trichromatic theory was often presented as competing with the opponent-process theory for dominance in explaining color vision. Today, it is believed that both theories can be used to explain how the color vision system operates and that each theory applies to a different level of the visual process.  

  • Opponent process theory: Color vision at the neural level
  • The trichromatic theory: Color vision at the receptor level

Shedding light on how the human eye perceives brightness

Brightness perception can be explained by the summation of a non-linear luminance term and a linear melanopsin term, suggesting that melanopsin signal may express the absolute brightness level. Credit: Yokohama National University

Japanese scientists are shedding new light on the importance of light-sensing cells in the retina that process visual information. The researchers isolated the functions of melanopsin cells and demonstrated their crucial role in the perception of the visual environment. This ushers in a new understanding of the biology of the eye and how visual information is processed.

The findings could contribute to more effective therapies for complications that relate to the eye. They can also serve as the basis for developing lighting and display systems. The research was published in Scientific Reports on May 20th, 2019.

The back of the human eye is lined with the retina, a layer of various types of cells called photoreceptors that respond to different amounts of light. The cells that process a lot of light are called cones and those that process lower levels of light are rods.

Until recently, researchers thought that when light struck the retina, rods and cones were the only two kinds of cells that reacted. Recent discoveries have revealed an entirely new type of cells called intrinsically photosensitive retinal ganglion cells (ipRGCs). Unlike rods and cones, ipRGCs contain melanopsin, a photopigment that is sensitive to light. While it has been established that ipRGCs are involved in keeping the brain's internal clock in sync with changes in daylight, their importance in the detection of the amount of light had not yet been well understood.

"Until now, the role of retinal melanopsin cells and how they contribute to the perception of the brightness of light have been unclear," said Katsunori Okajima, a professor at the Faculty of Environment and Information Sciences, Yokohama National University and one of the authors of the study.

"We've found that melanopsin plays a crucial role on the human ability to see how well-lit the environment is. These findings are redefining the conventional system of light detection that so far has only taken into consideration two variables, namely brightness and the amount of incoming light. Our results suggest that brightness perception should rely on a third variable—the intensity of a stimulus that targets melanopsin."

In the study, the authors showed how cones and melanopsin combine to allow the perception of brightness. In order to better assess the contribution of melanopsin to the detection of light, the melanopsin's signals were isolated from cones and rods. This separation allowed for more accurate observation of the melanopsin signal alone. Visual stimuli were carefully designed and positioned in order to specifically stimulate the light-sensitive chemical. Also, the researchers used tracking software to measure study participants' pupil diameters under each visual stimulus. This served as a way to determine the relationship between brightness perception and the actual visual stimulus intensity on the retina.

The researchers were able to show that the varying brightness levels of an image that was perceived is a sum of the melanopsin response and the response that is generated by the cones. The former is a linear readout and the latter is not. The results also show that melanopsin is not a minor contributor in brightness perception. Rather, it is a crucial player in brightness perception.


Shedding light on how the human eye perceives brightness

Japanese scientists are shedding new light on the importance of light-sensing cells in the retina that process visual information. The researchers isolated the functions of melanopsin cells and demonstrated their crucial role in the perception of visual environment. This ushers in a new understanding of the biology of the eye and how visual information is processed.

The findings could contribute to more effective therapies for complications that relate to the eye. They can also serve as the basis for developing lighting and display systems.

The research was published in Scientific Reports on May 20th, 2019.

The back of the human eye is lined with the retina, a layer of various types of cells, called photoreceptors, that respond to different amounts of light. The cells that process a lot of light are called cones and those that process lower levels of light are named rods.

Up until recently, researchers have thought that when light struck the retina, rods and cones were the only two kinds of cells that react. Recent discoveries have revealed an entirely new type of cells, called intrinsically photosensitive retinal ganglion cells (ipRGCs). Unlike rods and cones, ipRGCs contain melanopsin, a photopigment that is sensitive to light. While it has been established that ipRGCs are involved in keeping the brain's internal clock in sync with changes in daylight, their importance in the detection of the amount of light had not yet been well understood.

"Until now, the role of retinal melanopsin cells and how they contribute to the perception of the brightness of light have been unclear," said Katsunori Okajima, a professor at the Faculty of Environment and Information Sciences, Yokohama National University and one of the authors of the study.

"We've found that melanopsin plays a crucial role on the human ability to see how well-lit the environment is. These findings are redefining the conventional system of light detection that so far has only taken into consideration two variables, namely brightness and the amount of incoming light. Our results suggest that brightness perception should rely on a third variable -- the intensity of a stimulus that targets melanopsin."

In the study, the authors showed how cones and melanopsin combine to allow the perception of brightness. In order to better assess the contribution of melanopsin to the detection of light, the melanopsin's signals were isolated from cones and rods. This separation allowed for more accurate observation of the melanopsin signal alone. Visual stimuli were carefully designed and positioned in order to specifically stimulate the light-sensitive chemical. Also, the researchers used tracking software to measure study participants' pupil diameters under each visual stimulus. This served as a way to determine the relationship between brightness perception and the actual visual stimulus intensity on the retina.

The researchers were able to show that the varying brightness levels of an image that was perceived is a sum of the melanopsin response and the response that is generated by the cones. The former is a linear readout and the latter is not. The results also show that melanopsin is not a minor contributor in brightness perception. Rather, it is a crucial player in brightness perception.

This work was supported by the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (Grant Numbers 15H05926 and 18H04111).


Parts of the Eye and Their Functions

The eye is one of the most complex parts of the body. The different parts of the eye allow the body to take in light and perceive objects around us in the proper color, detail and depth. This allows people to make more informed decisions about their environment. If a portion of the eye becomes damaged, you may not be able to see effectively, or lose your vision all together. What are the parts of the eye? Which part is not functioning properly when we suffer different vision problems like myopia and glaucoma? Which part produces tears?

Parts of the Eye and Their Functions

There are several physical and chemical elements that make up the eye. The eye is linked together with the nervous system, which allows the brain to take in information from the eyes and make the appropriate decisions on how to act upon this information. The nerves must be kept in prime condition or the brain may start to receive false images, or you will not take in enough information to get an accurate perception of your environment.


Effect of Varying Levels of Glare on Contrast Sensitivity Measurements of Young Healthy Individuals Under Photopic and Mesopic Vision

Contrast sensitivity (CS), the ability to detect small spatial changes of luminance, is a fundamental aspect of vision. However, while visual acuity is commonly measured in eye clinics, CS is often not assessed. At issue is that tests of CS are not highly standardized in the field and that, in many cases, optotypes used are not sensitive enough to measure graduations of performance and visual abilities within the normal range. Here, in order to develop more sensitive measures of CS, we examined how CS is affected by different combinations of glare and ambient lighting in young healthy participants. We found that low levels of glare have a relatively small impact on vision under both photopic and mesopic conditions, while higher levels had significantly greater consequences on CS under mesopic conditions. Importantly, we found that the amount of glare induced by a standard built-in system (69 lux) was insufficient to induce CS reduction, but increasing to 125 lux with a custom system did cause a significant reduction and shift of CS in healthy individuals. This research provides important data that can help guide the use of CS measures that yield more sensitivity to characterize visual processing abilities in a variety of populations with ecological validity for non-ideal viewing conditions such as night time driving.

Keywords: contrast sensitivity function glare effect mesopic vision photopic vision visual function measurement.

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Custom modified CSV-1000E contrast sensitivity…

Custom modified CSV-1000E contrast sensitivity test with LED lights placed at each of…

Contrast sensitivity function in photopic…

Contrast sensitivity function in photopic viewing for glare and no glare condition (69…

Contrast sensitivity function in mesopic…

Contrast sensitivity function in mesopic light condition for no glare, low glare (69…


Darwin's Greatest Challenge Tackled: The Mystery Of Eye Evolution

October 28, 2004 -- When Darwin's skeptics attack his theory of evolution, they often focus on the eye. Darwin himself confessed that it was "absurd" to propose that the human eye evolved through spontaneous mutation and natural selection. Scientists at the European Molecular Biology Laboratory (EMBL) have now tackled Darwin's major challenge in an evolutionary study published this week in the journal Science. They have elucidated the evolutionary origin of the human eye.

Researchers in the laboratories of Detlev Arendt and Jochen Wittbrodt have discovered that the light-sensitive cells of our eyes, the rods and cones, are of unexpected evolutionary origin &ndash they come from an ancient population of light-sensitive cells that were initially located in the brain.

"It is not surprising that cells of human eyes come from the brain. We still have light-sensitive cells in our brains today which detect light and influence our daily rhythms of activity," explains Wittbrodt. "Quite possibly, the human eye has originated from light-sensitive cells in the brain. Only later in evolution would such brain cells have relocated into an eye and gained the potential to confer vision."

The scientists discovered that two types of light-sensitive cells existed in our early animal ancestors: rhabdomeric and ciliary. In most animals, rhabdomeric cells became part of the eyes, and ciliary cells remained embedded in the brain. But the evolution of the human eye is peculiar &ndash it is the ciliary cells that were recruited for vision which eventually gave rise to the rods and cones of the retina.

So how did EMBL researchers finally trace the evolution of the eye?

By studying a "living fossil," Platynereis dumerilii, a marine worm that still resembles early ancestors that lived up to 600 million years ago. Arendt had seen pictures of this worm's brain taken by researcher Adriaan Dorresteijn (University of Mainz, Germany). "When I saw these pictures, I noticed that the shape of the cells in the worm's brain resembled the rods and cones in the human eye. I was immediately intrigued by the idea that both of these light-sensitive cells may have the same evolutionary origin."

To test this hypothesis, Arendt and Wittbrodt used a new tool for today's evolutionary biologists &ndash "molecular fingerprints". Such a fingerprint is a unique combination of molecules that is found in a specific cell. He explains that if cells between species have matching molecular fingerprints, then the cells are very likely to share a common ancestor cell.

Scientist Kristin Tessmar-Raible provided the crucial evidence to support Arendt's hypothesis. With the help of EMBL researcher Heidi Snyman, she determined the molecular fingerprint of the cells in the worm's brain. She found an opsin, a light-sensitive molecule, in the worm that strikingly resembled the opsin in the vertebrate rods and cones. "When I saw this vertebrate-type molecule active in the cells of the Playtnereis brain &ndash it was clear that these cells and the vertebrate rods and cones shared a molecular fingerprint. This was concrete evidence of common evolutionary origin. We had finally solved one of the big mysteries in human eye evolution."


Photopic Vision

The Photopic efficacy curve was extrapolated from testing done on 'Standard Observers'. This was done by taking a person with normal vision, and having them compare the brightness of monochromatic light at 555 nm, where the eye is most sensitive, with the brightness of another monochromatic source of differing wavelength. To achieve a balance, the brightness of the 555 nm source was reduced until the observer felt that the two sources were equal in brightness. The fraction by which the 555 nm source is reduced measures the observer's sensitivity to the second wavelength. This exercise is repeated through many wavelengths and many observers. The average of the results gives us the relative sensitivity of the eye at various wavelengths. In 1924, the International Commission on Illumination adopted the "relative sensitivity curve for the C.I.E. Standard Observer".

Each wavelength has a relative value for the Standard Observer's sensitivity, the luminous efficacy at that wavelength, Vλ. The value of Vλ is designated as unity at 555 nm and decreases to zero at the ends of the visible spectrum. This is associated with the daylight vision of the human eye, also known as photopic vision. In lowlight conditions, the efficacy curve shifts toward the blue end of the spectrum due to the sensitivity of the eye. Chemical changes in the eye at night shift our vision to the scotopic range. This differentiation between light and dark vision is caused by the activity of the rods and cones in the retina, and their sensitivity to light.

At 555 nm, this efficacy translates to a luminous flux of 683 lumens/W, and thus a fraction of that value at wavelengths to either side of the visible spectrum. This value is derived from the definition of the candela directly.


9 Health Issues That Can Cause Sensitivity to Light

If stepping outside or flicking on a light makes your eyes want to duck for cover, you could be dealing with sensitivity to light. This basically means that light really bothers your eyes, possibly making it tempting to wear sunglasses 24/7. A little sensitivity to light when going from relative darkness to a bright surrounding is normal, and as you’ve probably experienced, typically fades quickly as your eyes adjust. But if you have photophobia—the medical term for extreme sensitivity to light—light can actually hurt your eyes.

Several health issues can cause sensitivity to light, and they really run the gamut. Here are the most common ones to keep on your radar.

Dry eye is a condition that happens when your eyes can’t lubricate themselves properly because of an issue with your tears, according to the National Eye Institute (NEI). Your tears are vital for keeping your eyes healthy, which is why having inadequate tears in some fashion can be horribly uncomfortable.

This discomfort stems from the way dry eye impacts your corneas, the clear, protective outer layers of your eyes. Your corneas have a lot of nerves, so any kind of problem with them can result in a range of bothersome signs that something’s wrong, JP Maszczak, O.D., assistant professor of clinical optometry at the Ohio State University College of Optometry, tells SELF.

Sensitivity to light is a classic dry eye symptom, as are dryness (obviously), stinging, burning, pain, redness, discharge, scratchiness, and feeling like something is in your eye even if there’s nothing there, the NEI says.

While you can wear your sunglasses to help you deal with sensitivity to light, treating your dry eye is really the only way to make this better. That usually includes using over-the-counter medications like artificial tears, the NEI says. (Make sure to get the simple ones solely meant to wet your eyes, not any with eye-whiteners—those can just cause more irritation.) If you’re grappling with a more severe case of dry eye, your doctor might recommend other treatment, like corticosteroid drops to reduce inflammation or little plugs made of silicone or collagen that can help block your tear ducts and keep moisture from draining away too quickly. You’ll only know what’s best for you if you ask.

Ah, good old allergies. If you have them, you may very well know how badly they can mess with your eyes. You can thank allergic conjunctivitis for that.

Allergic conjunctivitis is actually a form of pink eye, which happens when something irritates your conjunctiva, the delicate membrane that covers your eyes and insides of your eyelids. While bacteria and viruses can cause pink eye, the allergic form of the condition comes about when your body overreacts to an allergen like pollen, dust mites, mold, or animal dander. In an attempt to protect you, your immune system produces antibodies that travel to different cells in your body, causing them to release chemicals that prompt an allergic reaction, according to the American Academy of Allergy Asthma & Immunology (AAAAI). If this process affects your eyes, it’s called allergic conjunctivitis, and you can wind up with symptoms like sensitivity to light, itchiness, excessive tearing, redness, and a burning sensation.

If you have allergic conjunctivitis, your doctor will probably tell you to do what you can to avoid your triggers (we know, we know—easier said than done). If that doesn’t help, things like antihistamines and allergy shots might minimize your symptoms—talk to your doctor to figure out what makes the most sense.

Migraines can feel soul-crushing. Not only is the head pain sometimes debilitating, migraines can also cause symptoms like severe sensitivity to light, nausea and vomiting, blurred vision, and lightheadedness, the Mayo Clinic says.

Migraines are one of those health conditions experts are still working to fully understand. The thinking is that activity in certain nerve cells makes blood vessels in your brain dilate and also causes a release of inflammatory substances like prostaglandins, which can create pain.

The mechanism behind the light sensitivity specifically may be related to irritation of the trigeminal nerve, a cranial nerve that’s responsible for sensation in your face, Ilan Danan, M.D., M.Sc., a sports neurologist at the Center for Sports Neurology and Pain Medicine at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles, tells SELF. All light can be tough to deal with when you have a migraine, but you might find that specific types, like fluorescent light, are particularly hard to take, Dr. Danan says.

It’s not just that having a migraine can induce sensitivity to light—it can kind of work the other way around, too. Bright lights are a well-known migraine trigger, along with a multitude of other things like fluctuations in estrogen levels, foods like aged cheeses, alcohol and caffeine, stress, and changes in your sleep pattern, according to the Mayo Clinic.

If you struggle with migraines, talk to your doctor about treatment options. The right migraine treatment is so individual for each person, but yours could include pain medications to get through migraines as they happen along with preventive ones to avoid them in the first place.

A concussion is a traumatic brain injury that impacts the way your brain functions and is usually caused by a blow to the head, according to the Mayo Clinic. The effects are typically temporary, but they can be subtle and may not show up immediately. Then, they can last for days, weeks, or even longer.

Some symptoms might show up soon after the head injury, including a headache, temporary loss of consciousness, confusion, amnesia about what caused the concussion, dizziness, nausea, vomiting, slurred speech, appearing dazed, and being tired, but some people may have delayed symptoms, like having trouble concentrating or remembering things, trouble sleeping, personality changes, depression, issues smelling or tasting things, and, yup, sensitivity to light, the Mayo Clinic says. It’s pretty rare for someone with a concussion to just have sensitivity to light without the headache—the two usually go together, Dr. Danan says.

Experts typically recommend resting—both physically and mentally—after you get a concussion, since it will help your brain heal more quickly. Beyond that, if you have a concussion, your doctor can recommend treatment for your specific symptoms, like pain relievers if your headaches refuse to GTFO.

Keratitis is corneal inflammation that can come with a whole host of signs that your eyes are crying out for help, according to the Mayo Clinic. There are various forms, like bacterial keratitis, viral keratitis, fungal keratitis, keratitis from a parasite called Acanthamoeba, and non-infectious keratitis. Most of those are self-explanatory save for that last one non-infectious keratitis describes corneal inflammation that happens due to something like wearing your contacts for too long or making other common contact lens mistakes.

No matter the cause, corneal inflammation can distort light that enters your eye, causing sensitivity, Christopher J. Rapuano, M.D., chief of the cornea service at Wills Eye Hospital in Philadelphia, tells SELF. Other symptoms of keratitis include eye pain, redness, blurred vision, excessive tearing, feeling like something is in your eye, and eye discharge, the Mayo Clinic says.

Proper keratitis treatment really depends on the cause. Using an antibiotic won’t help a case of viral keratitis, for example. That’s why it’s so important to see your eye doctor if you think you’re dealing with keratitis. They can prescribe antibiotics if your case is bacterial or due to Acanthamoeba, antifungals if a fungus is to blame, or antivirals if those are necessary. They can also recommend lifestyle treatments that can help with discomfort, like not wearing contacts until your keratitis clears up.


Light and Dark Adaptation by Michael Kalloniatis and Charles Luu

The eye operates over a large range of light levels. The sensitivity of our eye can be measured by determining the absolute intensity threshold, that is, the minimum luminance of a test spot required to produce a visual sensation. This can be measured by placing a subject in a dark room, and increasing the luminance of the test spot until the subject reports its presence. Consequently, dark adaptation refers to how the eye recovers its sensitivity in the dark following exposure to bright lights. Aubert (1865) was the first to estimate the threshold stimulus of the eye in the dark by measuring the electrical current required to render the glow on a platinum wire just visible. He found that the sensitivity had increased 35 times after time in the dark, and also introduce for the term “adaptation”.

Dark adaptation forms the basis of the Duplicity Theory which states that 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.

The dark adaptation curve shown below depicts this duplex nature of our visual system (figure 1). The first curve reflects the cone mechanism. The sensitivity of the rod pathway improves considerably after 5-10 minutes in the dark and is reflected by the second part of the dark adaptation curve. One way to demonstrate that the rod mechanism takes over at low luminance level, is to observe the colour of the stimuli. When the rod mechanism takes over, coloured test spots appear colourless, as only the cone pathways encode colour. This duplex nature of vision will affect the dark adaptation curve in different ways and is discussed below.

To produce a dark adaptation curve, subjects gaze at a pre-adapting light for about five minutes, then absolute threshold is measured over time (figure 1). Pre-adaptation is important for normalisation and to ensure a bi-phasic curve is obtained.

From the above curve, it can be seen that initially there is a rapid decrease in threshold, then it declines slowly. After 5 to 8 minutes, a second mechanism of vision comes into play, where there is another rapid decrease in threshold, then an even slower decline. The curve asymptotes to a minimum (absolute threshold) at about 10 -5 cd/m 2 after about forty minutes in the dark.

Factors Affecting Dark Adaptation.

Intensity and duration of pre-adapting light:
Different intensities and duration of the pre-adapting light will affect dark adaptation curve in a number of areas. With increasing levels of pre-adapting luminances, the cone branch becomes longer while the rod branch becomes more delayed. Absolute threshold also takes longer to reach. At low levels of pre-adapting luminances, rod threshold drops quickly to reach absolute threshold (figure 2).

The shorter the duration of the pre-adapting light, the more rapid the decrease in dark adaptation (figure 3). For extremely short pre-adaptation periods, a single rod curve is obtained. It is only after long pre-adaptation that a bi-phasic, cone and rod branches are obtained.

Size and location of the retina used:
The retinal location used to register the test spot during dark adaptation will affect the dark adaptation curve due to the distribution of the rod and cones in the retinal (figure 4).

When a small test spot is located at the fovea (eccentricity of 0 o ), only one branch is seen with a higher threshold compared to the rod branch. When the same size test spot is used in the peripheral retina during dark adaptation, the typical break appears in the curve representing the cone branch and the rod branch (figure 5).

A similar principle applies when different size of the test spot is used. When a small test spot is used during dark adaptation, a single branch is found as only cones are present at the fovea. When a larger test spot is used during dark adaptation, a rod-cone break would be present since the test spot stimulates both cones and rods. As the test spot becomes even larger, incorporating more rods, the sensitivity of the eye in the dark is even greater (figure 6), reflecting the larger spatial summation characteristics of the rod pathway.

Wavelength of the threshold light:
When stimuli of different wavelengths are used, the dark adaptation curve is affected. From figure 7 below, a rod-cone break is not seen when using light of long wavelengths such as extreme red. This occurs due to rods and cones having similar sensitivities to light of long wavelengths (figure 8). Figure 8 depicts the photopic and scotopic spectral sensitivity functions to illustrate the point that the rod and cone sensitivity difference is dependent upon test wavelength (although normalization of spatial, temporal and equivalent adaptation level for the rod and cones is not present in this figure). On the other hand, when light of short wavelength is used, the rod-cone break is most prominent as the rods are much more sensitive than the cones to short wavelengths once the rods have dark adapted.

Rhodopsin regeneration

Dark adaptation also depends upon photopigment bleaching. Retinal (or reflection) densitometry, which is a procedure based on measuring the light reflected from the fundus of the eye, can be used to determine the amount of photopigment bleached. Using retinal densitometry, it was found that the time course for dark adaptation and rhodopsin regeneration was the same. However, this does not fully explain the large increase in sensitivity with time. Bleaching rhodopsin by 1% raises threshold by 10 (decreases sensitivity by 10). In figure 9, it can be seen that, bleaching 50% of rhodopsin in rods raises threshold by 10 log units while the bleaching 50% of cone photopigment raises threshold by about one and a half log units. Therefore, rod sensitivity is not fully accounted for at the receptor level and may be explained by further retinal processing. The important thing to note is that bleaching of cone photopigment has a smaller effect on cone thresholds.

Light Adaptation.

With dark adaptation, we noticed that there is progressive decrease in threshold (increase in sensitivity) with time in the dark. With light adaptation, the eye has to quickly adapt to the background illumination to be able to distinguish objects in this background. Light adaptation can be explored by determining increment thresholds. In an increment threshold experiment, a test stimulus is presented on a background of a certain luminance. The stimulus is increased in luminance until detection threshold is reached against the background (figure 10) Therefore, the independent variable is the luminance of the background and the dependent variable is the threshold intensity or luminance of the incremental test required for detection. Such an approach is used when visual fields are measured in clinical practice.

The experimental conditions shown in figure 10, can be repeated by changing the background field luminance. Depending upon the choice of test and background wavelength, the test size and retinal eccentricity, a monophasic or biphasic threshold versus intensity (tvi) curve is obtained. Figure 11 illustrates such a curve for parafoveal presentation of a yellow test field on a green background. This stimulus choice leads to two branches. A lower branch belonging to the rod system. As the background light level increases, visual function shifts from the rod system to the cone system. A dual-branched curve reflects the duplex nature of vision, similar to the bi-phasic response in the dark adaptation curve.

When a single system (eg. the rod system) is isolated under certain experimental conditions, four sections of the curve is apparent. These experiment conditions involve using a red background to suppress the cone photoreceptors and a green test spot to stimulate the rod photoreceptors (Aguilar and Stiles, 1954). The curve in figure 12 can also be obtained by performing increment threshold experiments on rod monochromats who lack cone photoreceptors. When the rod system is isolated using the conditions of Aguilar and Stiles, four sections are obtained:

The threshold in the linear portion of the tvi curve is determined by the dark light level. As background luminance is increased, the curve remains constant (and equal to the absolute threshold). Sensitivity in this section is limited by neural (internal) noise, the so called “dark light”. The background field is relatively low and does not significantly affect threshold. This neural noise is internal to the retina and examples of these include thermal isomerisations of photopigment, spontaneous opening of photoreceptor membrane channels and spontaneous neurotransmitter release.

The second part of the tvi curve is called the square root law or (de Vries-Rose Law) region. This part of the curve is limited by quantal fluctuation in the background. Rose (1948) proposed that visual threshold would be quantal limited. The visual system is usually compared to a theoretical construct, an ideal light detector. An ideal detector can detect and encode each absorbed quantum of light and is limited only by the noise due to quantal fluctuations in the source. To detect the stimulus, the stimulus must be sufficiently exceed the fluctuations of the background (noise).

Because the variability in quanta increases with the number of quanta absorbed, threshold would increase with background luminance. In fact, the increase in threshold should be proportional to the square root of the background luminance hence the slope of one half in a log-log plot. For the rod pathway a slope of 0.6 is often found (Hallett, 1969). Barlow (1958) explored the conditions which influenced the transition from the square root law to Weber’s law (see below). He concluded that for brief, small test spots, increment thresholds rise as the square root of the background over the entire photopic range. Spots of large areas and long durations have slopes close to Weber’s law. Other spatio-temporal configurations result in different proportions of each region.

When plotted using log D L versus log L coordinates, the Weber law section ideally has a slope 1. For the rod pathway, a slope 0.8 or less is found, implying that the rod pathway does operate under true Weber conditions. This section of the curve demonstrates an important aspect of our visual system. Our visual system is designed to distinguish objects from its background. In the real world, objects have contrast, which is constant and independent of ambient luminance. Therefore, the principle of Weber’s law can be applied to contrast which remains constant regardless of illumination changes. This is called contrast constancy or contrast invariance, with this contrast level defined as Webers constant. Contrast constancy can be mathematically expressed as D L /L = constant. D L is the increment threshold on a background L. The constant is also known as the Weber constant or Weber fraction. The Weber constant for the rod and cone is 0.14 (Cornsweet, 1970) and 0.02 to 0.03 (Davson, 1990) respectively. Within the cone pathways, the S-cone pathway, again has a different characteristics to those of the longer-wavelength pathway with a Weber constant of around 0.09 (Stiles, 1959).

Section 4 of the curve (figure 12) shows rod saturation at high background luminance. The slope begins to increase rapidly and the rod system starts to become unable to detect the stimulus. This section of the curve occurs for cone mechanism under high background light levels.

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Pirenne MH (1962) Rods and Cones. Chapter 2. In: Davson, H. (ed), The Eye, vol 2. London, Academic Press.

Rose A. The sensitivity performance of the human eye on a absolute scale. J Opt Soc Am. 194838:196–208. [PubMed]

Stiles WS. Colour vision: the approach through increment threshold sensitivity. Proc Natl Acad Sci U S A. 195975:100–114.

Michael Kalloniatis was born in Athens Greece in 1958. He received his optometry degree and Master’s degree from the University of Melbourne. His PhD was awarded from the University of Houston, College of Optometry, for studies investigating colour vision processing in the monkey visual system. Post-doctoral training continued at the University of Texas in Houston with Dr Robert Marc. It was during this period that he developed a keen interest in retinal neurochemistry, but he also maintains an active research laboratory in visual psychophysics focussing on colour vision and visual adaptation. He was a faculty member of the Department of Optometry and Vision Sciences at the University of Melbourne until his recent move to New Zealand. Dr. Kalloniatis is now the Robert G. Leitl Professor of Optometry, Department of Optometry and Vision Science, University of Auckland. e-mail: [email protected]

The author

Charles Luu was born in Can Tho, Vietnam in 1974. He was educated in Melbourne and received his optometry degree from the University of Melbourne in 1996 and proceeded to undertake a clinical residency within the Victorian College of Optometry. During this period, he completed post-graduate training and was awarded the post-graduate diploma in clinical optometry. His areas of expertise include low vision and contact lenses. During his tenure as a staff optometrist, he undertook teaching of optometry students as well as putting together the “Cyclopean Eye”, in collaboration with Dr Michael Kalloniatis. The Cyclopean Eye is a Web based interactive unit used in undergraduate teaching of vision science to optometry students. He is currently in private optometric practice as well as a visiting clinician within the Department of Optometry and Vision Science, University of Melbourne.


Effect of Varying Levels of Glare on Contrast Sensitivity Measurements of Young Healthy Individuals Under Photopic and Mesopic Vision

Contrast sensitivity (CS), the ability to detect small spatial changes of luminance, is a fundamental aspect of vision. However, while visual acuity is commonly measured in eye clinics, CS is often not assessed. At issue is that tests of CS are not highly standardized in the field and that, in many cases, optotypes used are not sensitive enough to measure graduations of performance and visual abilities within the normal range. Here, in order to develop more sensitive measures of CS, we examined how CS is affected by different combinations of glare and ambient lighting in young healthy participants. We found that low levels of glare have a relatively small impact on vision under both photopic and mesopic conditions, while higher levels had significantly greater consequences on CS under mesopic conditions. Importantly, we found that the amount of glare induced by a standard built-in system (69 lux) was insufficient to induce CS reduction, but increasing to 125 lux with a custom system did cause a significant reduction and shift of CS in healthy individuals. This research provides important data that can help guide the use of CS measures that yield more sensitivity to characterize visual processing abilities in a variety of populations with ecological validity for non-ideal viewing conditions such as night time driving.

Keywords: contrast sensitivity function glare effect mesopic vision photopic vision visual function measurement.

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Photopic Vision

The Photopic efficacy curve was extrapolated from testing done on 'Standard Observers'. This was done by taking a person with normal vision, and having them compare the brightness of monochromatic light at 555 nm, where the eye is most sensitive, with the brightness of another monochromatic source of differing wavelength. To achieve a balance, the brightness of the 555 nm source was reduced until the observer felt that the two sources were equal in brightness. The fraction by which the 555 nm source is reduced measures the observer's sensitivity to the second wavelength. This exercise is repeated through many wavelengths and many observers. The average of the results gives us the relative sensitivity of the eye at various wavelengths. In 1924, the International Commission on Illumination adopted the "relative sensitivity curve for the C.I.E. Standard Observer".

Each wavelength has a relative value for the Standard Observer's sensitivity, the luminous efficacy at that wavelength, Vλ. The value of Vλ is designated as unity at 555 nm and decreases to zero at the ends of the visible spectrum. This is associated with the daylight vision of the human eye, also known as photopic vision. In lowlight conditions, the efficacy curve shifts toward the blue end of the spectrum due to the sensitivity of the eye. Chemical changes in the eye at night shift our vision to the scotopic range. This differentiation between light and dark vision is caused by the activity of the rods and cones in the retina, and their sensitivity to light.

At 555 nm, this efficacy translates to a luminous flux of 683 lumens/W, and thus a fraction of that value at wavelengths to either side of the visible spectrum. This value is derived from the definition of the candela directly.


Shedding light on how the human eye perceives brightness

Brightness perception can be explained by the summation of a non-linear luminance term and a linear melanopsin term, suggesting that melanopsin signal may express the absolute brightness level. Credit: Yokohama National University

Japanese scientists are shedding new light on the importance of light-sensing cells in the retina that process visual information. The researchers isolated the functions of melanopsin cells and demonstrated their crucial role in the perception of the visual environment. This ushers in a new understanding of the biology of the eye and how visual information is processed.

The findings could contribute to more effective therapies for complications that relate to the eye. They can also serve as the basis for developing lighting and display systems. The research was published in Scientific Reports on May 20th, 2019.

The back of the human eye is lined with the retina, a layer of various types of cells called photoreceptors that respond to different amounts of light. The cells that process a lot of light are called cones and those that process lower levels of light are rods.

Until recently, researchers thought that when light struck the retina, rods and cones were the only two kinds of cells that reacted. Recent discoveries have revealed an entirely new type of cells called intrinsically photosensitive retinal ganglion cells (ipRGCs). Unlike rods and cones, ipRGCs contain melanopsin, a photopigment that is sensitive to light. While it has been established that ipRGCs are involved in keeping the brain's internal clock in sync with changes in daylight, their importance in the detection of the amount of light had not yet been well understood.

"Until now, the role of retinal melanopsin cells and how they contribute to the perception of the brightness of light have been unclear," said Katsunori Okajima, a professor at the Faculty of Environment and Information Sciences, Yokohama National University and one of the authors of the study.

"We've found that melanopsin plays a crucial role on the human ability to see how well-lit the environment is. These findings are redefining the conventional system of light detection that so far has only taken into consideration two variables, namely brightness and the amount of incoming light. Our results suggest that brightness perception should rely on a third variable—the intensity of a stimulus that targets melanopsin."

In the study, the authors showed how cones and melanopsin combine to allow the perception of brightness. In order to better assess the contribution of melanopsin to the detection of light, the melanopsin's signals were isolated from cones and rods. This separation allowed for more accurate observation of the melanopsin signal alone. Visual stimuli were carefully designed and positioned in order to specifically stimulate the light-sensitive chemical. Also, the researchers used tracking software to measure study participants' pupil diameters under each visual stimulus. This served as a way to determine the relationship between brightness perception and the actual visual stimulus intensity on the retina.

The researchers were able to show that the varying brightness levels of an image that was perceived is a sum of the melanopsin response and the response that is generated by the cones. The former is a linear readout and the latter is not. The results also show that melanopsin is not a minor contributor in brightness perception. Rather, it is a crucial player in brightness perception.


Parts of the Eye and Their Functions

The eye is one of the most complex parts of the body. The different parts of the eye allow the body to take in light and perceive objects around us in the proper color, detail and depth. This allows people to make more informed decisions about their environment. If a portion of the eye becomes damaged, you may not be able to see effectively, or lose your vision all together. What are the parts of the eye? Which part is not functioning properly when we suffer different vision problems like myopia and glaucoma? Which part produces tears?

Parts of the Eye and Their Functions

There are several physical and chemical elements that make up the eye. The eye is linked together with the nervous system, which allows the brain to take in information from the eyes and make the appropriate decisions on how to act upon this information. The nerves must be kept in prime condition or the brain may start to receive false images, or you will not take in enough information to get an accurate perception of your environment.


9 Health Issues That Can Cause Sensitivity to Light

If stepping outside or flicking on a light makes your eyes want to duck for cover, you could be dealing with sensitivity to light. This basically means that light really bothers your eyes, possibly making it tempting to wear sunglasses 24/7. A little sensitivity to light when going from relative darkness to a bright surrounding is normal, and as you’ve probably experienced, typically fades quickly as your eyes adjust. But if you have photophobia—the medical term for extreme sensitivity to light—light can actually hurt your eyes.

Several health issues can cause sensitivity to light, and they really run the gamut. Here are the most common ones to keep on your radar.

Dry eye is a condition that happens when your eyes can’t lubricate themselves properly because of an issue with your tears, according to the National Eye Institute (NEI). Your tears are vital for keeping your eyes healthy, which is why having inadequate tears in some fashion can be horribly uncomfortable.

This discomfort stems from the way dry eye impacts your corneas, the clear, protective outer layers of your eyes. Your corneas have a lot of nerves, so any kind of problem with them can result in a range of bothersome signs that something’s wrong, JP Maszczak, O.D., assistant professor of clinical optometry at the Ohio State University College of Optometry, tells SELF.

Sensitivity to light is a classic dry eye symptom, as are dryness (obviously), stinging, burning, pain, redness, discharge, scratchiness, and feeling like something is in your eye even if there’s nothing there, the NEI says.

While you can wear your sunglasses to help you deal with sensitivity to light, treating your dry eye is really the only way to make this better. That usually includes using over-the-counter medications like artificial tears, the NEI says. (Make sure to get the simple ones solely meant to wet your eyes, not any with eye-whiteners—those can just cause more irritation.) If you’re grappling with a more severe case of dry eye, your doctor might recommend other treatment, like corticosteroid drops to reduce inflammation or little plugs made of silicone or collagen that can help block your tear ducts and keep moisture from draining away too quickly. You’ll only know what’s best for you if you ask.

Ah, good old allergies. If you have them, you may very well know how badly they can mess with your eyes. You can thank allergic conjunctivitis for that.

Allergic conjunctivitis is actually a form of pink eye, which happens when something irritates your conjunctiva, the delicate membrane that covers your eyes and insides of your eyelids. While bacteria and viruses can cause pink eye, the allergic form of the condition comes about when your body overreacts to an allergen like pollen, dust mites, mold, or animal dander. In an attempt to protect you, your immune system produces antibodies that travel to different cells in your body, causing them to release chemicals that prompt an allergic reaction, according to the American Academy of Allergy Asthma & Immunology (AAAAI). If this process affects your eyes, it’s called allergic conjunctivitis, and you can wind up with symptoms like sensitivity to light, itchiness, excessive tearing, redness, and a burning sensation.

If you have allergic conjunctivitis, your doctor will probably tell you to do what you can to avoid your triggers (we know, we know—easier said than done). If that doesn’t help, things like antihistamines and allergy shots might minimize your symptoms—talk to your doctor to figure out what makes the most sense.

Migraines can feel soul-crushing. Not only is the head pain sometimes debilitating, migraines can also cause symptoms like severe sensitivity to light, nausea and vomiting, blurred vision, and lightheadedness, the Mayo Clinic says.

Migraines are one of those health conditions experts are still working to fully understand. The thinking is that activity in certain nerve cells makes blood vessels in your brain dilate and also causes a release of inflammatory substances like prostaglandins, which can create pain.

The mechanism behind the light sensitivity specifically may be related to irritation of the trigeminal nerve, a cranial nerve that’s responsible for sensation in your face, Ilan Danan, M.D., M.Sc., a sports neurologist at the Center for Sports Neurology and Pain Medicine at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles, tells SELF. All light can be tough to deal with when you have a migraine, but you might find that specific types, like fluorescent light, are particularly hard to take, Dr. Danan says.

It’s not just that having a migraine can induce sensitivity to light—it can kind of work the other way around, too. Bright lights are a well-known migraine trigger, along with a multitude of other things like fluctuations in estrogen levels, foods like aged cheeses, alcohol and caffeine, stress, and changes in your sleep pattern, according to the Mayo Clinic.

If you struggle with migraines, talk to your doctor about treatment options. The right migraine treatment is so individual for each person, but yours could include pain medications to get through migraines as they happen along with preventive ones to avoid them in the first place.

A concussion is a traumatic brain injury that impacts the way your brain functions and is usually caused by a blow to the head, according to the Mayo Clinic. The effects are typically temporary, but they can be subtle and may not show up immediately. Then, they can last for days, weeks, or even longer.

Some symptoms might show up soon after the head injury, including a headache, temporary loss of consciousness, confusion, amnesia about what caused the concussion, dizziness, nausea, vomiting, slurred speech, appearing dazed, and being tired, but some people may have delayed symptoms, like having trouble concentrating or remembering things, trouble sleeping, personality changes, depression, issues smelling or tasting things, and, yup, sensitivity to light, the Mayo Clinic says. It’s pretty rare for someone with a concussion to just have sensitivity to light without the headache—the two usually go together, Dr. Danan says.

Experts typically recommend resting—both physically and mentally—after you get a concussion, since it will help your brain heal more quickly. Beyond that, if you have a concussion, your doctor can recommend treatment for your specific symptoms, like pain relievers if your headaches refuse to GTFO.

Keratitis is corneal inflammation that can come with a whole host of signs that your eyes are crying out for help, according to the Mayo Clinic. There are various forms, like bacterial keratitis, viral keratitis, fungal keratitis, keratitis from a parasite called Acanthamoeba, and non-infectious keratitis. Most of those are self-explanatory save for that last one non-infectious keratitis describes corneal inflammation that happens due to something like wearing your contacts for too long or making other common contact lens mistakes.

No matter the cause, corneal inflammation can distort light that enters your eye, causing sensitivity, Christopher J. Rapuano, M.D., chief of the cornea service at Wills Eye Hospital in Philadelphia, tells SELF. Other symptoms of keratitis include eye pain, redness, blurred vision, excessive tearing, feeling like something is in your eye, and eye discharge, the Mayo Clinic says.

Proper keratitis treatment really depends on the cause. Using an antibiotic won’t help a case of viral keratitis, for example. That’s why it’s so important to see your eye doctor if you think you’re dealing with keratitis. They can prescribe antibiotics if your case is bacterial or due to Acanthamoeba, antifungals if a fungus is to blame, or antivirals if those are necessary. They can also recommend lifestyle treatments that can help with discomfort, like not wearing contacts until your keratitis clears up.


Trichromatic Theory and Opponent Process Theory

In the past, the trichromatic theory was often presented as competing with the opponent-process theory for dominance in explaining color vision. Today, it is believed that both theories can be used to explain how the color vision system operates and that each theory applies to a different level of the visual process.  

  • Opponent process theory: Color vision at the neural level
  • The trichromatic theory: Color vision at the receptor level

Darwin's Greatest Challenge Tackled: The Mystery Of Eye Evolution

October 28, 2004 -- When Darwin's skeptics attack his theory of evolution, they often focus on the eye. Darwin himself confessed that it was "absurd" to propose that the human eye evolved through spontaneous mutation and natural selection. Scientists at the European Molecular Biology Laboratory (EMBL) have now tackled Darwin's major challenge in an evolutionary study published this week in the journal Science. They have elucidated the evolutionary origin of the human eye.

Researchers in the laboratories of Detlev Arendt and Jochen Wittbrodt have discovered that the light-sensitive cells of our eyes, the rods and cones, are of unexpected evolutionary origin &ndash they come from an ancient population of light-sensitive cells that were initially located in the brain.

"It is not surprising that cells of human eyes come from the brain. We still have light-sensitive cells in our brains today which detect light and influence our daily rhythms of activity," explains Wittbrodt. "Quite possibly, the human eye has originated from light-sensitive cells in the brain. Only later in evolution would such brain cells have relocated into an eye and gained the potential to confer vision."

The scientists discovered that two types of light-sensitive cells existed in our early animal ancestors: rhabdomeric and ciliary. In most animals, rhabdomeric cells became part of the eyes, and ciliary cells remained embedded in the brain. But the evolution of the human eye is peculiar &ndash it is the ciliary cells that were recruited for vision which eventually gave rise to the rods and cones of the retina.

So how did EMBL researchers finally trace the evolution of the eye?

By studying a "living fossil," Platynereis dumerilii, a marine worm that still resembles early ancestors that lived up to 600 million years ago. Arendt had seen pictures of this worm's brain taken by researcher Adriaan Dorresteijn (University of Mainz, Germany). "When I saw these pictures, I noticed that the shape of the cells in the worm's brain resembled the rods and cones in the human eye. I was immediately intrigued by the idea that both of these light-sensitive cells may have the same evolutionary origin."

To test this hypothesis, Arendt and Wittbrodt used a new tool for today's evolutionary biologists &ndash "molecular fingerprints". Such a fingerprint is a unique combination of molecules that is found in a specific cell. He explains that if cells between species have matching molecular fingerprints, then the cells are very likely to share a common ancestor cell.

Scientist Kristin Tessmar-Raible provided the crucial evidence to support Arendt's hypothesis. With the help of EMBL researcher Heidi Snyman, she determined the molecular fingerprint of the cells in the worm's brain. She found an opsin, a light-sensitive molecule, in the worm that strikingly resembled the opsin in the vertebrate rods and cones. "When I saw this vertebrate-type molecule active in the cells of the Playtnereis brain &ndash it was clear that these cells and the vertebrate rods and cones shared a molecular fingerprint. This was concrete evidence of common evolutionary origin. We had finally solved one of the big mysteries in human eye evolution."


Shedding light on how the human eye perceives brightness

Japanese scientists are shedding new light on the importance of light-sensing cells in the retina that process visual information. The researchers isolated the functions of melanopsin cells and demonstrated their crucial role in the perception of visual environment. This ushers in a new understanding of the biology of the eye and how visual information is processed.

The findings could contribute to more effective therapies for complications that relate to the eye. They can also serve as the basis for developing lighting and display systems.

The research was published in Scientific Reports on May 20th, 2019.

The back of the human eye is lined with the retina, a layer of various types of cells, called photoreceptors, that respond to different amounts of light. The cells that process a lot of light are called cones and those that process lower levels of light are named rods.

Up until recently, researchers have thought that when light struck the retina, rods and cones were the only two kinds of cells that react. Recent discoveries have revealed an entirely new type of cells, called intrinsically photosensitive retinal ganglion cells (ipRGCs). Unlike rods and cones, ipRGCs contain melanopsin, a photopigment that is sensitive to light. While it has been established that ipRGCs are involved in keeping the brain's internal clock in sync with changes in daylight, their importance in the detection of the amount of light had not yet been well understood.

"Until now, the role of retinal melanopsin cells and how they contribute to the perception of the brightness of light have been unclear," said Katsunori Okajima, a professor at the Faculty of Environment and Information Sciences, Yokohama National University and one of the authors of the study.

"We've found that melanopsin plays a crucial role on the human ability to see how well-lit the environment is. These findings are redefining the conventional system of light detection that so far has only taken into consideration two variables, namely brightness and the amount of incoming light. Our results suggest that brightness perception should rely on a third variable -- the intensity of a stimulus that targets melanopsin."

In the study, the authors showed how cones and melanopsin combine to allow the perception of brightness. In order to better assess the contribution of melanopsin to the detection of light, the melanopsin's signals were isolated from cones and rods. This separation allowed for more accurate observation of the melanopsin signal alone. Visual stimuli were carefully designed and positioned in order to specifically stimulate the light-sensitive chemical. Also, the researchers used tracking software to measure study participants' pupil diameters under each visual stimulus. This served as a way to determine the relationship between brightness perception and the actual visual stimulus intensity on the retina.

The researchers were able to show that the varying brightness levels of an image that was perceived is a sum of the melanopsin response and the response that is generated by the cones. The former is a linear readout and the latter is not. The results also show that melanopsin is not a minor contributor in brightness perception. Rather, it is a crucial player in brightness perception.

This work was supported by the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (Grant Numbers 15H05926 and 18H04111).


Light and Dark Adaptation by Michael Kalloniatis and Charles Luu

The eye operates over a large range of light levels. The sensitivity of our eye can be measured by determining the absolute intensity threshold, that is, the minimum luminance of a test spot required to produce a visual sensation. This can be measured by placing a subject in a dark room, and increasing the luminance of the test spot until the subject reports its presence. Consequently, dark adaptation refers to how the eye recovers its sensitivity in the dark following exposure to bright lights. Aubert (1865) was the first to estimate the threshold stimulus of the eye in the dark by measuring the electrical current required to render the glow on a platinum wire just visible. He found that the sensitivity had increased 35 times after time in the dark, and also introduce for the term “adaptation”.

Dark adaptation forms the basis of the Duplicity Theory which states that 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.

The dark adaptation curve shown below depicts this duplex nature of our visual system (figure 1). The first curve reflects the cone mechanism. The sensitivity of the rod pathway improves considerably after 5-10 minutes in the dark and is reflected by the second part of the dark adaptation curve. One way to demonstrate that the rod mechanism takes over at low luminance level, is to observe the colour of the stimuli. When the rod mechanism takes over, coloured test spots appear colourless, as only the cone pathways encode colour. This duplex nature of vision will affect the dark adaptation curve in different ways and is discussed below.

To produce a dark adaptation curve, subjects gaze at a pre-adapting light for about five minutes, then absolute threshold is measured over time (figure 1). Pre-adaptation is important for normalisation and to ensure a bi-phasic curve is obtained.

From the above curve, it can be seen that initially there is a rapid decrease in threshold, then it declines slowly. After 5 to 8 minutes, a second mechanism of vision comes into play, where there is another rapid decrease in threshold, then an even slower decline. The curve asymptotes to a minimum (absolute threshold) at about 10 -5 cd/m 2 after about forty minutes in the dark.

Factors Affecting Dark Adaptation.

Intensity and duration of pre-adapting light:
Different intensities and duration of the pre-adapting light will affect dark adaptation curve in a number of areas. With increasing levels of pre-adapting luminances, the cone branch becomes longer while the rod branch becomes more delayed. Absolute threshold also takes longer to reach. At low levels of pre-adapting luminances, rod threshold drops quickly to reach absolute threshold (figure 2).

The shorter the duration of the pre-adapting light, the more rapid the decrease in dark adaptation (figure 3). For extremely short pre-adaptation periods, a single rod curve is obtained. It is only after long pre-adaptation that a bi-phasic, cone and rod branches are obtained.

Size and location of the retina used:
The retinal location used to register the test spot during dark adaptation will affect the dark adaptation curve due to the distribution of the rod and cones in the retinal (figure 4).

When a small test spot is located at the fovea (eccentricity of 0 o ), only one branch is seen with a higher threshold compared to the rod branch. When the same size test spot is used in the peripheral retina during dark adaptation, the typical break appears in the curve representing the cone branch and the rod branch (figure 5).

A similar principle applies when different size of the test spot is used. When a small test spot is used during dark adaptation, a single branch is found as only cones are present at the fovea. When a larger test spot is used during dark adaptation, a rod-cone break would be present since the test spot stimulates both cones and rods. As the test spot becomes even larger, incorporating more rods, the sensitivity of the eye in the dark is even greater (figure 6), reflecting the larger spatial summation characteristics of the rod pathway.

Wavelength of the threshold light:
When stimuli of different wavelengths are used, the dark adaptation curve is affected. From figure 7 below, a rod-cone break is not seen when using light of long wavelengths such as extreme red. This occurs due to rods and cones having similar sensitivities to light of long wavelengths (figure 8). Figure 8 depicts the photopic and scotopic spectral sensitivity functions to illustrate the point that the rod and cone sensitivity difference is dependent upon test wavelength (although normalization of spatial, temporal and equivalent adaptation level for the rod and cones is not present in this figure). On the other hand, when light of short wavelength is used, the rod-cone break is most prominent as the rods are much more sensitive than the cones to short wavelengths once the rods have dark adapted.

Rhodopsin regeneration

Dark adaptation also depends upon photopigment bleaching. Retinal (or reflection) densitometry, which is a procedure based on measuring the light reflected from the fundus of the eye, can be used to determine the amount of photopigment bleached. Using retinal densitometry, it was found that the time course for dark adaptation and rhodopsin regeneration was the same. However, this does not fully explain the large increase in sensitivity with time. Bleaching rhodopsin by 1% raises threshold by 10 (decreases sensitivity by 10). In figure 9, it can be seen that, bleaching 50% of rhodopsin in rods raises threshold by 10 log units while the bleaching 50% of cone photopigment raises threshold by about one and a half log units. Therefore, rod sensitivity is not fully accounted for at the receptor level and may be explained by further retinal processing. The important thing to note is that bleaching of cone photopigment has a smaller effect on cone thresholds.

Light Adaptation.

With dark adaptation, we noticed that there is progressive decrease in threshold (increase in sensitivity) with time in the dark. With light adaptation, the eye has to quickly adapt to the background illumination to be able to distinguish objects in this background. Light adaptation can be explored by determining increment thresholds. In an increment threshold experiment, a test stimulus is presented on a background of a certain luminance. The stimulus is increased in luminance until detection threshold is reached against the background (figure 10) Therefore, the independent variable is the luminance of the background and the dependent variable is the threshold intensity or luminance of the incremental test required for detection. Such an approach is used when visual fields are measured in clinical practice.

The experimental conditions shown in figure 10, can be repeated by changing the background field luminance. Depending upon the choice of test and background wavelength, the test size and retinal eccentricity, a monophasic or biphasic threshold versus intensity (tvi) curve is obtained. Figure 11 illustrates such a curve for parafoveal presentation of a yellow test field on a green background. This stimulus choice leads to two branches. A lower branch belonging to the rod system. As the background light level increases, visual function shifts from the rod system to the cone system. A dual-branched curve reflects the duplex nature of vision, similar to the bi-phasic response in the dark adaptation curve.

When a single system (eg. the rod system) is isolated under certain experimental conditions, four sections of the curve is apparent. These experiment conditions involve using a red background to suppress the cone photoreceptors and a green test spot to stimulate the rod photoreceptors (Aguilar and Stiles, 1954). The curve in figure 12 can also be obtained by performing increment threshold experiments on rod monochromats who lack cone photoreceptors. When the rod system is isolated using the conditions of Aguilar and Stiles, four sections are obtained:

The threshold in the linear portion of the tvi curve is determined by the dark light level. As background luminance is increased, the curve remains constant (and equal to the absolute threshold). Sensitivity in this section is limited by neural (internal) noise, the so called “dark light”. The background field is relatively low and does not significantly affect threshold. This neural noise is internal to the retina and examples of these include thermal isomerisations of photopigment, spontaneous opening of photoreceptor membrane channels and spontaneous neurotransmitter release.

The second part of the tvi curve is called the square root law or (de Vries-Rose Law) region. This part of the curve is limited by quantal fluctuation in the background. Rose (1948) proposed that visual threshold would be quantal limited. The visual system is usually compared to a theoretical construct, an ideal light detector. An ideal detector can detect and encode each absorbed quantum of light and is limited only by the noise due to quantal fluctuations in the source. To detect the stimulus, the stimulus must be sufficiently exceed the fluctuations of the background (noise).

Because the variability in quanta increases with the number of quanta absorbed, threshold would increase with background luminance. In fact, the increase in threshold should be proportional to the square root of the background luminance hence the slope of one half in a log-log plot. For the rod pathway a slope of 0.6 is often found (Hallett, 1969). Barlow (1958) explored the conditions which influenced the transition from the square root law to Weber’s law (see below). He concluded that for brief, small test spots, increment thresholds rise as the square root of the background over the entire photopic range. Spots of large areas and long durations have slopes close to Weber’s law. Other spatio-temporal configurations result in different proportions of each region.

When plotted using log D L versus log L coordinates, the Weber law section ideally has a slope 1. For the rod pathway, a slope 0.8 or less is found, implying that the rod pathway does operate under true Weber conditions. This section of the curve demonstrates an important aspect of our visual system. Our visual system is designed to distinguish objects from its background. In the real world, objects have contrast, which is constant and independent of ambient luminance. Therefore, the principle of Weber’s law can be applied to contrast which remains constant regardless of illumination changes. This is called contrast constancy or contrast invariance, with this contrast level defined as Webers constant. Contrast constancy can be mathematically expressed as D L /L = constant. D L is the increment threshold on a background L. The constant is also known as the Weber constant or Weber fraction. The Weber constant for the rod and cone is 0.14 (Cornsweet, 1970) and 0.02 to 0.03 (Davson, 1990) respectively. Within the cone pathways, the S-cone pathway, again has a different characteristics to those of the longer-wavelength pathway with a Weber constant of around 0.09 (Stiles, 1959).

Section 4 of the curve (figure 12) shows rod saturation at high background luminance. The slope begins to increase rapidly and the rod system starts to become unable to detect the stimulus. This section of the curve occurs for cone mechanism under high background light levels.

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Michael Kalloniatis was born in Athens Greece in 1958. He received his optometry degree and Master’s degree from the University of Melbourne. His PhD was awarded from the University of Houston, College of Optometry, for studies investigating colour vision processing in the monkey visual system. Post-doctoral training continued at the University of Texas in Houston with Dr Robert Marc. It was during this period that he developed a keen interest in retinal neurochemistry, but he also maintains an active research laboratory in visual psychophysics focussing on colour vision and visual adaptation. He was a faculty member of the Department of Optometry and Vision Sciences at the University of Melbourne until his recent move to New Zealand. Dr. Kalloniatis is now the Robert G. Leitl Professor of Optometry, Department of Optometry and Vision Science, University of Auckland. e-mail: [email protected]

The author

Charles Luu was born in Can Tho, Vietnam in 1974. He was educated in Melbourne and received his optometry degree from the University of Melbourne in 1996 and proceeded to undertake a clinical residency within the Victorian College of Optometry. During this period, he completed post-graduate training and was awarded the post-graduate diploma in clinical optometry. His areas of expertise include low vision and contact lenses. During his tenure as a staff optometrist, he undertook teaching of optometry students as well as putting together the “Cyclopean Eye”, in collaboration with Dr Michael Kalloniatis. The Cyclopean Eye is a Web based interactive unit used in undergraduate teaching of vision science to optometry students. He is currently in private optometric practice as well as a visiting clinician within the Department of Optometry and Vision Science, University of Melbourne.


Watch the video: Το ξωκκλήσι που έκανε διάσημο η Ντέμι Μουρ και κατέρρευσε με τα 6,3 Ρίχτερ! (July 2022).


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