Information

What photoreceptors are necessary to permit infrared vision?


Humans have red green and blue photoreceptors allowing them to sense colours in the spectrum of about 400-700nm. Certain proteins allow for the extending of wavelength range in the RGB receptors, this however is only perceived as more shades of the colour in question. Additional photoreceptors are necessary to sense a new colour, explicitly separate from RGB.

Organisms such as mantis shrimp can sense infrared light as they have 12-16 cones, allowing them to see into the infrared spectrum.

The issue is that there seems to be no information detailing the the structure of mantis shrimp eyes, specifically regarding the the aspects of which are responsible for vision into the near infrared spectrum (roughly 700-1000nm).

What would the cones of an organism that saw exclusively in infrared look like? (assuming the organism had eyes like a human)

Would they be differently coloured or structurally different to our own?


The photoreceptors of the mantis shrimp (Fig. 1) are quite different from that of the human retina (Fig. 2).

Importantly, when light enters the units in the shrimp's eye, it must first pass through a crystalline cone, which lies over the receptors. These crystalline cones contain frequency-specific light-blocking substances called MAAs (mycosporine-like amino acids). These filters help to filter the spectrum and this means that less different opsins are necessary tod etect different wavelengths. In other words, a mantis shrimp does not feature 16 different opsins. Instead it utilizes the same opsins for various wavelengths by putting a filter on top of the cone (source: National Geographic).


Fig. 1. Mantis shrimp photoreceptors. source: Physiologizing


Fig. 2. Human cone cell. source: Wikipedia


What photoreceptors are necessary to permit infrared vision? - Biology

Vision is the ability to detect light patterns from the outside environment and interpret them into images. Animals are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.


Light

As with auditory stimuli, light travels in waves. The compression waves that compose sound must travel in a medium&mdasha gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no medium light can travel in a vacuum (Figure (PageIndex<1>)). The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light&mdasha packet of electromagnetic radiation called a photon. A glance at the electromagnetic spectrum shows that visible light for humans is just a small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light.

Certain variables are important when discussing perception of light. Wavelength (which varies inversely with frequency) manifests itself as hue. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm) one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers, and some non-avian reptiles sense infrared light (heat that prey gives off).

Figure (PageIndex<1>): In the electromagnetic spectrum, visible light lies between 380 nm and 740 nm. (credit: modification of work by NASA)

Wave amplitude is perceived as luminous intensity, or brightness. The standard unit of intensity of light is the candela , which is approximately the luminous intensity of a one common candle.

Light waves travel 299,792 km per second in a vacuum, (and somewhat slower in various media such as air and water), and those waves arrive at the eye as long (red), medium (green), and short (blue) waves. What is termed &ldquowhite light&rdquo is light that is perceived as white by the human eye. This effect is produced by light that stimulates equally the color receptors in the human eye. The apparent color of an object is the color (or colors) that the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other wavelengths of light.


Nanotechnology makes it possible for mice to see in infrared

This graphical abstract shows how injectable photoreceptor-binding particles with the ability to convert photons from to high-energy forms allow mice to develop infrared vision without compromising their normal vision and associated behavioral responses. Credit: Ma et al./Current Biology

Mice with vision enhanced by nanotechnology were able to see infrared light as well as visible light, reports a study published February 28 in the journal Cell. A single injection of nanoparticles in the mice's eyes bestowed infrared vision for up to 10 weeks with minimal side effects, allowing them to see infrared light even during the day and with enough specificity to distinguish between different shapes. These findings could lead to advancements in human infrared vision technologies, including potential applications in civilian encryption, security, and military operations.

Humans and other mammals are limited to seeing a range of wavelengths of light called visible light, which includes the wavelengths of the rainbow. But infrared radiation, which has a longer wavelength, is all around us. People, animals and objects emit infrared light as they give off heat, and objects can also reflect infrared light.

"The visible light that can be perceived by human's natural vision occupies just a very small fraction of the electromagnetic spectrum," says senior author Tian Xue of the University of Science and Technology of China. "Electromagnetic waves longer or shorter than visible light carry lots of information."

A multidisciplinary group of scientists led by Xue and Jin Bao at the University of Science and Technology of China as well as Gang Han at the University of Massachusetts Medical School, developed the nanotechnology to work with the eye's existing structures.

"When light enters the eye and hits the retina, the rods and cones—or photoreceptor cells—absorb the photons with visible light wavelengths and send corresponding electric signals to the brain," says Han. "Because infrared wavelengths are too long to be absorbed by photoreceptors, we are not able to perceive them."

This video shows how researchers used nanotechnology to give mice near-infrared vision. Credit: Ma et al./Cell

In this study, the scientists made nanoparticles that can anchor tightly to photoreceptor cells and act as tiny infrared light transducers. When infrared light hits the retina, the nanoparticles capture the longer infrared wavelengths and emit shorter wavelengths within the visible light range. The nearby rod or cone then absorbs the shorter wavelength and sends a normal signal to the brain, as if visible light had hit the retina.

"In our experiment, nanoparticles absorbed infrared light around 980 nm in wavelength and converted it into light peaked at 535 nm, which made the infrared light appear as the color green," says Bao.

The researchers tested the nanoparticles in mice, which, like humans, cannot see infrared naturally. Mice that received the injections showed unconscious physical signs that they were detecting infrared light, such as their pupils constricting, while mice injected with only the buffer solution didn't respond to infrared light.

To test whether the mice could make sense of the infrared light, the researchers set up a series of maze tasks to show the mice could see infrared in daylight conditions, simultaneously with visible light.

This image shows nanoparticles, in green, binding to the rods (violet) and cones (red) of the eye's retina. Credit: Ma et al./Current Biology

In rare cases, side effects from the injections such as cloudy corneas occurred but disappeared within less than a week. This may have been caused by the injection process alone because mice that only received injections of the buffer solution had a similar rate of these side effects. Other tests found no damage to the retina's structure following the sub-retinal injections.

"In our study, we have shown that both rods and cones bind these nanoparticles and were activated by the near infrared light," says Xue. "So we believe this technology will also work in human eyes, not only for generating super vision but also for therapeutic solutions in human red color vision deficits."

Current infrared technology relies on detectors and cameras that are often limited by ambient daylight and need outside power sources. The researchers believe the bio-integrated nanoparticles are more desirable for potential infrared applications in civilian encryption, security, and military operations. "In the future, we think there may be room to improve the technology with a new version of organic-based nanoparticles, made of FDA-approved compounds, that appear to result in even brighter infrared vision," says Han.

The researchers also think more work can be done to fine tune the emission spectrum of the nanoparticles to suit human eyes, which utilize more cones than rods for their central vision compared to mouse eyes. "This is an exciting subject because the technology we made possible here could eventually enable human beings to see beyond our natural capabilities," says Xue.


36.5 Vision

By the end of this section, you will be able to do the following:

  • Explain how electromagnetic waves differ from sound waves
  • Trace the path of light through the eye to the point of the optic nerve
  • Explain tonic activity as it is manifested in photoreceptors in the retina

Vision is the ability to detect light patterns from the outside environment and interpret them into images. Animals are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.

Light

As with auditory stimuli, light travels in waves. The compression waves that compose sound must travel in a medium—a gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no medium light can travel in a vacuum (Figure 36.17). The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light—a packet of electromagnetic radiation called a photon. A glance at the electromagnetic spectrum shows that visible light for humans is just a small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light.

Certain variables are important when discussing perception of light. Wavelength (which varies inversely with frequency) manifests itself as hue. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm) one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers, and some non-avian reptiles sense infrared light (heat that prey gives off).

Wave amplitude is perceived as luminous intensity, or brightness. The standard unit of intensity of light is the candela , which is approximately the luminous intensity of one common candle.

Light waves travel 299,792 km per second in a vacuum, (and somewhat slower in various media such as air and water), and those waves arrive at the eye as long (red), medium (green), and short (blue) waves. What is termed “white light” is light that is perceived as white by the human eye. This effect is produced by light that stimulates equally the color receptors in the human eye. The apparent color of an object is the color (or colors) that the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other wavelengths of light.

Anatomy of the Eye

The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the retina (shown in Figure 36.18) on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. It passes through other layers that process it so that it can be interpreted by the retina (Figure 36.18b). The cornea , the front transparent layer of the eye, and the crystalline lens , a transparent convex structure behind the cornea, both refract (bend) light to focus the image on the retina. The iris , which is conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.

Visual Connection

Which of the following statements about the human eye is false?

  1. Rods detect color, while cones detect only shades of gray.
  2. When light enters the retina, it passes the ganglion cells and bipolar cells before reaching photoreceptors at the rear of the eye.
  3. The iris adjusts the amount of light coming into the eye.
  4. The cornea is a protective layer on the front of the eye.

The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and re-focusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called hyperopia caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear.

There are two types of photoreceptors in the retina: rods and cones , named for their general appearance as illustrated in Figure 36.19. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright light, and their primary role is in daytime, color vision.

The fovea is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object’s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea.

Link to Learning

Review the anatomical structure of the eye, clicking on each part to practice identification.

Transduction of Light

The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, rhodopsin , has two main parts (Figure 36.20): an opsin, which is a membrane protein (in the form of a cluster of α-helices that span the membrane), and retinal—a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (cis) form of the molecule to its linear (trans) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na + channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold (Figure 36.21).

Trichromatic Coding

There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive, as shown in Figure 36.22. Some cones are maximally responsive to short light waves of 420 nm, so they are called S cones (“S” for “short”) others respond maximally to waves of 530 nm (M cones, for “medium”) a third group responds maximally to light of longer wavelengths, at 560 nm (L, or “long” cones). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.

The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about 2 million distinct colors.

Retinal Processing

Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.

Photoreceptors in the retina continuously undergo tonic activity . That is, they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at a baseline while some stimuli increase firing rate from the baseline, and other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus, the visual system relies on change in retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells.

You can demonstrate this using an easy demonstration to “trick” your retina and brain about the colors you are observing in your visual field. Look fixedly at Figure 36.23 for about 45 seconds. Then quickly shift your gaze to a sheet of blank white paper or a white wall. You should see an afterimage of the Norwegian flag in its correct colors. At this point, close your eyes for a moment, then reopen them, looking again at the white paper or wall the afterimage of the flag should continue to appear as red, white, and blue. What causes this? According to an explanation called opponent process theory, as you gazed fixedly at the green, black, and yellow flag, your retinal ganglion cells that respond positively to green, black, and yellow increased their firing dramatically. When you shifted your gaze to the neutral white ground, these ganglion cells abruptly decreased their activity and the brain interpreted this abrupt downshift as if the ganglion cells were responding now to their “opponent” colors: red, white, and blue, respectively, in the visual field. Once the ganglion cells return to their baseline activity state, the false perception of color will disappear.

Higher Processing

The myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different qualities of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (Greek, for “crossing”) found at the base of the brain and allows us to coordinate information from both eyes.

Once in the brain, visual information is processed in several places, and its routes reflect the complexity and importance of visual information to humans and other animals. One route takes the signals to the thalamus, which serves as the routing station for all incoming sensory impulses except olfaction. In the thalamus, the magnocellular and parvocellular distinctions remain intact, and there are different layers of the thalamus dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the parietal lobe, in the side of the brain, carries magnocellular (“where”) information. A second stream projects to the temporal lobe and carries both magnocellular (“where”) and parvocellular (“what”) information.

Another important visual route is a pathway from the retina to the superior colliculus in the midbrain, where eye movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina to the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is a cluster of cells that is considered to be the body’s internal clock, which controls our circadian (day-long) cycle. The SCN sends information to the pineal gland, which is important in sleep/wake patterns and annual cycles.

Link to Learning

View this interactive presentation to review what you have learned about how vision functions.


Challenges associated with UV photosensitivity and UV vision

The problem of chromatic aberration

Chromatic aberration arises from a property that essentially all transparent materials (such as those used in biological optics) possess – their refractive index (see Glossary) decreases with wavelength. Consequently, short-wavelength images are focused closer to a lens than longer-wavelength ones. Because UV has unusually short wavelengths, its focal plane lies well in front of those of visible wavelengths, thus blurring the image and decreasing its contrast. The effect of chromatic aberration increases with eye size, so one might expect only animals with small eyes to tolerate it and thus to be UV sensitive. This is generally true, but as we show here, the exceptions are numerous.

Management of chromatic aberration in invertebrates

Given the optics of chromatic aberration, one possible way to manage it is to place UV photoreceptors closer to the lens than longer-wavelength classes. Invertebrates are generally small animals, and those that have compound eyes are essentially immune to the effects of chromatic aberration because the entire length of the photoreceptor acts as a single light guide, and resolution depends only on the separation of independent units. Nevertheless, the UV receptors are almost always found closer to the lens than other receptor types. Here, however, the reason is to boost their sensitivity, not to cope with aberrations. Because all visual pigments absorb fairly well in the UV, placing UV receptors deeper in the retina would put them at the mercy of the overlying receptors, greatly diminishing number of the UV photons that actually reach them.

Still, there are invertebrates with multiple spectral receptor types and single-lens optics. Where these species have been carefully described, they generally do manage chromatic aberration by layering UV receptor classes at the top of the retina (and also the longest-wavelength receptors in the bottom layers). Jumping spiders have large-lensed principal eyes, and their retinal tiers are nicely spaced to correct the chromatic aberration of the lens the UV receptors are on top and the green receptors are lower – both at the correct focal plane for light to which they most strongly respond (Blest et al., 1981). As an aside, jumping spiders use focal plane changes to judge distance, but this apparently involves only green-sensitive receptors, not the UV system (Nagata et al., 2012). Of the relatively small number of other single-lensed, imaging invertebrate eyes that have been well characterized, only those of larvae of the diving beetle Thermonectes marmoratus definitely contain UV photoreceptors. Here, however, the UV receptors lie deeper in the retina than the middle-wavelength class, where they would be both shielded by the overlying retina and well behind the proper focal plane (Maksimovic et al., 2009). This counterintuitive arrangement has yet to be explained.

Management of chromatic aberration in vertebrates

Vertebrates have simple eyes, and nearly always large ones. Consequently, many species with UV photoreceptors potentially face chromatic aberration difficulties. In aquatic species, UV photosensitivity is mainly correlated with habitat, not with eye size. The largest fish eyes occur in high-speed predators such as tuna, swordfish or other billfishes because these hunt away from the surface of the sea, the UV flux they experience is not strong, and they tend to be dichromats with blue and green receptor types. The lenses of most billfishes, in fact, block the entry of UV light into the eye (Fritsches et al., 2000). Amphibians tend to be small animals with rather poor spatial resolution, so they need not bother with correcting for chromatic issues.

Most terrestrial animals, however, live in a world drenched with UV photons. If they have large eyes, they must face the issues caused by chromatic defocus. Birds, as mentioned already, have two types of SWS1 cones: UVS and violet-sensitive (VS). The corresponding opsins vary at a single critical amino acid residue (Wilkie et al., 2000 Yokoyama et al., 2000 Carvalho et al., 2007). This makes it relatively easy to categorize a given species as UVS or VS using genetic approaches. It turns out that the evolution of avian UV sensitivity is complex and chaotic, and there is no clear pattern to be discerned (Ödeen et al., 2011 Ödeen and Håstad, 2013). Nevertheless, the largest birds, including cranes and ratites (ostriches and emus) are VS, consistent with their very large eyes (Wright and Bowmaker, 2001 Ödeen and Håstad, 2013 Porter et al., 2014). A potential solution to the chromatic aberration challenge that UV sensitivity imposes is the use of multifocal lenses in many avian species (Lind et al., 2008). Such lenses have the ability to focus both short- and medium-wavelength images simultaneously. Another solution is to remove UV light by filtering it out. A comprehensive study of ocular media among birds did show decreasing UV transmittance with increasing eye size, a finding consistent with controlling chromatic aberration at very short wavelengths (Lind et al., 2014). This same study found that raptors have among the least UV-transmissive eyes of all birds, which strongly suggests that their eye designs provide very high acuity without the contamination of out-of-focus light on the retina.

Turning to terrestrial mammals, we already know from earlier sections that human lenses block UV entry very effectively. What about other species? All mammals known to have a designated UV receptor class (based, as in birds, on an SWS1 opsin) are small and/or short-lived – either rodents (mice, rats, gophers, gerbils Jacobs et al., 1991) (Fig. 3A) or microbats (Feller et al., 2009 Müller et al., 2009). The microbat Glossophaga soricina is an exception that lacks a UV cone class but has UV-transmissive optics it derives UV sensitivity from the β-band of its green-sensitive visual pigment (Winter et al., 2003). Until recently, it was assumed that larger mammals were generally similar to humans, using yellow lenses to block UV. Marine mammals lack even blue-sensitive cones (much less UV types), although this is not an adaptation for chromatic aberration (Peichl et al., 2001 Levenson and Dizon, 2003). However, in 2011, Hogg et al. published their discovery of UV sensitivity in Arctic reindeer. This came as something of a shock, because reindeer obviously have very large eyes and must view bright (and potentially photodamaging) UV-reflective snow in winter. The sensitivity is based on a standard mammalian SWS1 cone pigment absorbing maximally in the blue (Hogg et al., 2011) and being excited simply from the transparency of the optics. The finding seemed to be a strange exception until the publication of a major comparative study of mammalian ocular media (Douglas and Jeffery, 2014) revealing that the lenses of many good-sized mammals – including, surprisingly, dogs, cats and pigs – admit a sizable fraction of environmental UVA light into the eye, thus conferring UV sensitivity (albeit with the probable use in most cases of the β-bands of typical SWS or MWS cone pigments). It was subsequently found that even a deep-diving seal (Crystophora cristata), an animal that presumably has no UVS or blue-sensitive cones, still has the ability to detect UV light in fact, the ability is improved by the presence of a UV-reflecting tapetum (see Glossary Hogg et al., 2015). Clearly, the wash of largely unfocused UV images on some mammalian retinas apparently is tolerable, although it is also true that most UV-sensitive mammals have relatively poor acuity in any case in addition, many mammals have multifocal lenses that could partially alleviate this problem (Kröger et al., 1999). Nevertheless, some mammalian groups, including primates and a few rodents, do have UV-blocking lenses. This may be related to visual acuity, or have another basis answering this question requires yet more comparative data.

The problem of UV-associated photodamage

As noted above, it is thought that high-energy UV light is potentially damaging to retinal tissues. The energy in UV photons can break chemical bonds, potentially producing free radicals and mutating DNA, thus interfering with cellular function. At present, it is not at all obvious how large and long-living animals manage the cumulative photodamage expected from UV irradiation. Carvalho et al. (2011) noted that – like the mammals just discussed – parrots, which are very long-lived birds (commonly attaining 50 years), also have continuous, bright UV irradiance on their corneas, lenses, ocular humors and retinas, and seem to tolerate this well. They suggest that protection from oxidative radicals produced by UV absorption may be offset by the action of carotenoid pigments in the eye. Similar mechanisms may act in mammals (Douglas and Cronin, 2016). Clearly, the costs of accepting visual damage are manageable given the widespread appearance of UV transmission in mammalian eyes.


Why animals don't have infrared vision: Source of the visual system's 'false alarms' discovered

On rare occasion, the light-sensing photoreceptor cells in the eye misfire and signal to the brain as if they have captured photons, when in reality they haven't. For years this phenomenon remained a mystery. Reporting in the June 10 issue of Science, neuroscientists at the Johns Hopkins University School of Medicine have discovered that a light-capturing pigment molecule in photoreceptors can be triggered by heat, as well, giving rise to these false alarms.

"A photon, the unit of light, is just energy, which, when captured by the pigment rhodopsin, most of the time causes the molecule to change shape, then triggering the cell to send an electrical signal to the brain to inform about light absorption," explains King-Wai Yau, Ph.D., professor of neuroscience at Johns Hopkins and member of its Center for Sensory Biology. "If rhodopsin can be triggered by light energy," says Yau, "it may also be occasionally triggered by other types of energy, such as heat, producing false alarms. These fake signals compromise our ability to see objects on a moonless night. So we tried to figure it out namely, how the pigment is tripped by accident."

"Thermal energy is everywhere, as long as the temperature is above absolute zero," says neuroscience research associate Dong-Gen Luo, Ph.D. "The question is: How much heat energy would it take to trigger rhodopsin and enable it to fire off a signal, even without capturing light?" says Johns Hopkins Biochemistry, Cellular and Molecular Biology graduate student Wendy Yue.

For 30 years, the assumption was that heat could trigger a pigment molecule to send a false signal, but through a mechanism different from that of light, says Yau, because it seemed, based on theoretical calculations: that very little thermal energy was required compared to light energy.

But the theory, according to Yau, was based mainly on the pigment rhodopsin. However, rhodopsin is mainly responsible for seeing in dim light and is not the only pigment in the eye other pigments are present in red-, green- and blue-sensitive cone photoreceptors that are used for color and bright-light vision. Although researchers are able to measure the false events of rhodopsin from a single rhodopsin-containing cell, a long-standing challenge has been to take measurements of the other pigments. "The electrical signal from a single cone pigment molecule is so small in a cone cell that it is simply not measurable," says Luo. "So we had to figure out a new way to measure these false signals from cone pigments."

By engineering a rod cell to make human red cone pigment, which is usually only found in cone cells, Yau's team was able to measure the electrical output from an individual cell and calculate this pigment's false signals by taking advantage of the large and detectable signals sent out from the cell.

As for blue cone pigment, "Nature did the experiment for us," says Yau. "In many amphibians, one type of rod cells called green rods naturally express a blue cone pigment, as do blue cones." So to determine whether heat can cause pigment cells to misfire, the team, working in the dark, first cooled the cells, and then slowly returned the cells to room temperature, measuring the electrical activity of the cells as they warmed up. They found that red-sensing pigment triggers false alarms most frequently, rhodopsin (bluish-green-sensing pigment) triggers falsely less frequently, and blue-sensing pigment does so even less.

"This validates the 60-year-old Barlow's hypothesis that suggested the longer wavelength the pigment senses -- meaning the closer to the red end of the spectrum -- the noisier it is," says Yau. And this finding led the team to develop and test a new theory: that heat can trigger pigments to misfire, by the same mechanism as light.

Pivotal to this theory is that visual pigment molecules are large, complex molecules containing many chemical bonds. And since each chemical bond has the potential to contain some small amount of thermal energy, the total amount of energy a pigment molecule could contain can, in theory, be enough to trigger the false alarm.

"For a long time, people assumed that light and heat had to trigger via different mechanisms, but now we think that both types of energy, in fact, trigger identical changes in the pigment molecules," says Yau. Moreover, since longer wavelength pigments have higher rates of false alarms, Yau says this may explain why animals never evolved to have infrared-sensing pigments.

"Apart from putting to rest a long-standing debate, it's a wake-up call for researchers to realize that biomolecules in general have more potential thermal energy than previously thought," says Luo.

This study was funded by the National Institutes of Health, the Antonio Champalimaud Vision Award and The Academy of Finland.

Story Source:

Materials provided by Johns Hopkins Medical Institutions. Note: Content may be edited for style and length.


Nanoparticles give mice infrared vision

Invisible to the naked eye, infrared light is nevertheless felt as heat, and we can see it with the aid of special infrared-sensitive cameras.

But now a team led by Yuqian Ma, from the University of Science and Technology of China, have created a new technology that gives mice 10 weeks of infrared vision with little side-effects.

“In order to see infrared light, it must first be converted to visible light,” explains Tijmen Euser, from the Nanophotonics centre at the University of Cambridge, who is not an author of the work. “They applied nanoparticles that are able to absorb infrared light and convert it to visible light [then] they have injected these nanoparticles into the mice eyes and have shown that the mice can, indeed, see infrared light.”

The process spans the fields of physics, chemistry and biology. The novel step was that the researchers coated the off-the-shelf nanoparticles with a molecular glue to enable them to attach to the light-sensitive photoreceptor "cone" cells in the retina.

Once in position, the nanoparticles absorb two photons of infrared light and combine their energies to produce a single photon of visible, green light. “The mice would see both the visible light that is always there, but on top of that, they would see this additional contribution of converted infrared light, and they would experience this as green light,” Euser explains.

To test that mice could, indeed, see infrared, the researchers measured the electrical response of the cells in their eyes, as well as their pupil responses, and, finally, performed a behavioural conditioning test where the animals received a food reward for responding to patterns of shapes shown to them. Crucially, the mice were trained using shapes displayed in visible light. Then the team switched to showing the shapes only in infrared and the animals continued to respond, proving that they could still see the presented shapes.

The possibilities arising from this research are many and varied, including treatment of some vision impairments, development of enhanced human vision for military applications, and human-machine interactions.

According to the study's co-author Tian Xue, "we believe this technology will also work in human eyes, not only for generating 'super-vision' but also for therapeutic solutions in human red colour vision deficits. This is an exciting subject, because the technology we made possible here could eventually enable human beings to see beyond our natural capabilities."


Photoreceptor transplant restores vision in mice

Scientists from the UCL Institute of Ophthalmology have shown for the first time that transplanting light-sensitive photoreceptors into the eyes of visually impaired mice can restore their vision.

The research, published in Nature, suggests that transplanting photoreceptors -- light-sensitive nerve cells that line the back of the eye -- could form the basis of a new treatment to restore sight in people with degenerative eye diseases.

Scientists injected cells from young healthy mice directly into the retinas of adult mice that lacked functional rod-photoreceptors. Loss of photoreceptors is the cause of blindness in many human eye diseases including age-related macular degeneration, retinitis pigmentosa and diabetes-related blindness.

There are two types of photoreceptor in the eye -- rods and cones. The cells transplanted were immature (or progenitor) rod-photoreceptor cells. Rod cells are especially important for seeing in the dark as they are extremely sensitive to even low levels of light.

After four to six weeks, the transplanted cells appeared to be functioning almost as well as normal rod-photoreceptor cells and had formed the connections needed to transmit visual information to the brain.

We've shown for the first time that transplanted photoreceptor cells can integrate successfully with the existing retinal circuitry and truly improve vision. We're hopeful that we will soon be able to replicate this success with photoreceptors derived from embryonic stem cells and eventually to develop human trials.

The researchers also tested the vision of the treated mice in a dimly lit maze. Those mice with newly transplanted rod cells were able to use a visual cue to quickly find a hidden platform in the maze whereas untreated mice were able to find the hidden platform only by chance after extensive exploration of the maze.

Professor Robin Ali at UCL Institute of Ophthalmology, who led the research, said: "We've shown for the first time that transplanted photoreceptor cells can integrate successfully with the existing retinal circuitry and truly improve vision. We're hopeful that we will soon be able to replicate this success with photoreceptors derived from embryonic stem cells and eventually to develop human trials.

"Although there are many more steps before this approach will be available to patients, it could lead to treatments for thousands of people who have lost their sight through degenerative eye disorders. The findings also pave the way for techniques to repair the central nervous system as they demonstrate the brain's amazing ability to connect with newly transplanted neurons."

Dr Rachael Pearson from UCL Institute of Ophthalmology and principal author, said: "We are now finding ways to improve the efficiency of cone photoreceptor transplantation and to increase the effectiveness of transplantation in very degenerate retina. We will probably need to do both in order to develop effective treatments for patients."

Dr Rob Buckle, head of regenerative medicine at the MRC said:"This is a landmark study that will inform future research across a wide range of fields including vision research, neuroscience and regenerative medicine. It provides clear evidence of functional recovery in the damaged eye through cell transplantation, providing great encouragement for the development of stem cell therapies to address the many debilitating eye conditions that affect millions worldwide."

The researchers demonstrated previously, in another study published in Nature, that it is possible to transplant photoreceptor cells into an adult mouse retina, provided the cells from the donor mouse are at a specific stage of development -- when the retina is almost, but not fully, formed. In this study they optimised the rod transplantation procedure to increase the number of cells integrated into the recipient mice and so were able to restore vision.

The research was funded by the MRC, the Wellcome Trust, the Royal Society, the British Retinitis Pigmentosa Society, Alcon Research Institute and The Miller's Trust. Robin Ali is a senior investigator of the National Institute for Health Research and carries out research at the NIHR Biomedical Research Centre at Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology. Rachael Pearson is a Royal Society University Research Fellow.


Why animals don't have infrared vision

On rare occasion, the light-sensing photoreceptor cells in the eye misfire and signal to the brain as if they have captured photons, when in reality they haven't. For years this phenomenon remained a mystery. Reporting in the June 10 issue of Science , neuroscientists at the Johns Hopkins University School of Medicine have discovered that a light-capturing pigment molecule in photoreceptors can be triggered by heat, as well, giving rise to these false alarms. "A photon, the unit of light, is just energy, which, when captured by the pigment rhodopsin, most of the time causes the molecule to change shape, then triggering the cell to send an electrical signal to the brain to inform about light absorption," explains King-Wai Yau, Ph.D., professor of neuroscience at Johns Hopkins and member of its Center for Sensory Biology. "If rhodopsin can be triggered by light energy," says Yau, "it may also be occasionally triggered by other types of energy, such as heat, producing false alarms. These fake signals compromise our ability to see objects on a moonless night. So we tried to figure it out namely, how the pigment is tripped by accident."

"Thermal energy is everywhere, as long as the temperature is above absolute zero," says neuroscience research associate Dong-Gen Luo, Ph.D. "The question is: How much heat energy would it take to trigger rhodopsin and enable it to fire off a signal, even without capturing light?" says Johns Hopkins Biochemistry, Cellular and Molecular Biology graduate student Wendy Yue.

For 30 years, the assumption was that heat could trigger a pigment molecule to send a false signal, but through a mechanism different from that of light, says Yau, because it seemed, based on theoretical calculations: that very little thermal energy was required compared to light energy.

But the theory, according to Yau, was based mainly on the pigment rhodopsin. However, rhodopsin is mainly responsible for seeing in dim light and is not the only pigment in the eye other pigments are present in red-, green- and blue-sensitive cone photoreceptors that are used for color and bright-light vision. Although researchers are able to measure the false events of rhodopsin from a single rhodopsin-containing cell, a long-standing challenge has been to take measurements of the other pigments. "The electrical signal from a single cone pigment molecule is so small in a cone cell that it is simply not measurable," says Luo. "So we had to figure out a new way to measure these false signals from cone pigments."

By engineering a rod cell to make human red cone pigment, which is usually only found in cone cells, Yau's team was able to measure the electrical output from an individual cell and calculate this pigment's false signals by taking advantage of the large and detectable signals sent out from the cell.

As for blue cone pigment, "Nature did the experiment for us," says Yau. "In many amphibians, one type of rod cells called green rods naturally express a blue cone pigment, as do blue cones." So to determine whether heat can cause pigment cells to misfire, the team, working in the dark, first cooled the cells, and then slowly returned the cells to room temperature, measuring the electrical activity of the cells as they warmed up. They found that red-sensing pigment triggers false alarms most frequently, rhodopsin (bluish-green-sensing pigment) triggers falsely less frequently, and blue-sensing pigment does so even less.

"This validates the 60-year-old Barlow's hypothesis that suggested the longer wavelength the pigment senses—meaning the closer to the red end of the spectrum—the noisier it is," says Yau. And this finding led the team to develop and test a new theory: that heat can trigger pigments to misfire, by the same mechanism as light.

Pivotal to this theory is that visual pigment molecules are large, complex molecules containing many chemical bonds. And since each chemical bond has the potential to contain some small amount of thermal energy, the total amount of energy a pigment molecule could contain can, in theory, be enough to trigger the false alarm.

"For a long time, people assumed that light and heat had to trigger via different mechanisms, but now we think that both types of energy, in fact, trigger identical changes in the pigment molecules," says Yau. Moreover, since longer wavelength pigments have higher rates of false alarms, Yau says this may explain why animals never evolved to have infrared-sensing pigments.

"Apart from putting to rest a long-standing debate, it's a wake-up call for researchers to realize that biomolecules in general have more potential thermal energy than previously thought," says Luo.