Posts tagged color vision
Articles and news from the latest research reports.
Birds evolved ultraviolet vision several times
Ultraviolet vision evolved at least eight times in birds from a common violet sensitive ancestor finds a study published in BioMed Central’s open access journal BMC Evolutionary Biology. All of these are due to single nucleotide changes in the DNA.
Modern daytime birds either have violet sensitive or ultraviolet sensitive vision. Being ultraviolet sensitive alters visual cues used to select a mate, avoiding predators, and in finding food. Researchers from Uppsala University and the Swedish University of Agricultural Sciences sequenced the genes responsible for producing the light sensitive pigment (SWS1 opsin) from 40 species of birds, in 29 families.
Generating a phylogenetic tree from these sequences shows that there have been at least 14 shifts between violet and ultraviolet sensitive colour vision and back. An ancestor of Passeriformes (perching birds including larks, swallows, blackbirds, finches, birds of paradise, and crows) and Psittaciformes (parrots and allies) changed from the ancestral violet sensitive colour vision to ultraviolet and, in some cases passerines have reverted back to violet vision.
Anders Ödeen and Olle Håstad, who performed this research commented, “There are two different amino acid alterations that can each change bird colour vision from violet to ultraviolet. One particular single nucleotide change has occurred at least 11 separate times. In general during evolution once a colour shift has occurred all species from this ancestor keep it meaning that the rest of the eye and physiology, must also evolved to ‘cement’ in the new colour sensitivity.”
(Image: webexhibits.org)
New understanding of how we see colors
Scientists have until now not fully understood how animals see in color, since visual pigments in eyes contain exactly the same chromophore (light absorbing segment of the molecule) and yet can absorb different wavelengths of light.
The chromophore retinal (Vitamin A aldehyde or retinaldehyde) is used by all animals but, depending on the photoreceptor proteins (opsins) associated with it, the same molecule can absorb a spectrum of colors from blues or even ultraviolet to reds. How a single molecule can do this has until now been uncertain.
Now researchers, led by Prof. Babak Borhan of Michigan State University at East Lansing, set out to try to understand the mechanism by which the opsins change the light absorption spectrum of the chromophore retinal. They concentrated their efforts on a pigment found in human retinal photoreceptor cells, rhodopsin, which consists of opsin and chromophore components.
One of the major theories about how retinal works is that because it is strongly positively charged at one end it could distribute this electrostatic charge across the chromophore molecule, and this would enable it to absorb the longer wavelengths at the red end of the spectrum. Another theory held that a change in shape of the chromophore-opsin complex could alter the absorption capabilities.
The problem with testing the theories, Borhan said, is that the visual pigments have proved difficult to work with. So instead, Borhan and colleagues used human cellular retinol binding protein II, (hCRBPII), a gut protein that binds retinol, which is closely related to retinal but which tolerates mutations more readily.
The team first created a mutation of hCRPBII that could bind retinal. They then changed the distribution of the electrostatic charge on the retinal molecule by replacing amino acids at the binding site retinal uses on hCRPBII in various ways, and in so doing created a range of pigment proteins.
The team then used spectrophotometry to compare the light entering and leaving the proteins to determine which wavelengths were being absorbed. Using this approach they were able to prove the charge distribution theory was correct and that no change in shape was necessary.
A by-product of the new research is the production of the 11 new artificial pigments, which could be used in tracking proteins or cell types being studied, as well as other possible applications such as in food dyes. One of the new pigments could absorb a red wavelength of 644 nanometers (nm), which is above the theoretical maximum wavelength retinal can absorb (560 nm) and is close to infrared (750 nm +).
The paper was published in the journal Science.
(Source: medicalxpress.com)
Sharks see world as 50 shades of grey
Sharks are colour blind, a new molecular study by Australian scientists has confirmed, filling a gap in our knowledge about the evolution of colour vision. Dr Susan Theiss, from the University of Queensland, and colleagues, report their findings in the journal Biology Letters.
The evolution of colour vision has been studied in most vertebrates, but until recently, elasmobranchs (sharks, skates and rays) had been overlooked. Previous physiological research has shown some rays have colour vision but it suggested sharks were colour blind.
These previous studies looked at opsins, which are light-sensitive proteins found in the photoreceptor cells of the retina. Rod opsins are used in low light and produce a black and white image, while cone opsins are used in bright light, and often to see colours. Two or more different types of cone opsins are needed for colour vision.
While some ray species have multiple cone opsins as well as rods, studies in various shark species suggested they had only a single cone visual pigment.
To check whether this really was the case, Theiss and colleagues isolated the visual opsin genes from two wobbegong shark species: the spotted wobbegong Orectolobus maculatus and the ornate wobbegong O. ornatus.
Their findings confirm that wobbegongs possess only one cone opsin, meaning they see the world in shades of grey. The findings help fill in the picture of how colour vision evolved in different species.
"We know the earliest vertebrates had colour vision, but it has been lost by some groups over the course of evolution," says co-author Associate Professor Nathan Hart, a neuroecologist at the University of Western Australia.
Vision cells, not brain, to blame for colour blindness
The real culprits of colour blindness are vision cells rather than unusual wiring in the eye and brain, recent research has shown.
The discovery brings scientists a step closer to restoring full colour vision for people who are colour blind – a condition that affects close to two million Australians, says Professor Paul Martin from The Vision Centre and The University of Sydney.
It may also help pave the way for an answer to one of the most common causes of blindness – age-related macular degeneration (AMD), which accounts for half of the legal blindness cases in Australia.
“There are millions of cones in our eyes – vision cells that pick up bright light and allow us to see colour,” Prof. Martin says. “They are nicknamed red, green and blue cones because they are sensitive to different wavelengths of light.
“We now know that in the macular region of the eye, each cone has its own ’private line’ into the optic nerve and the brain. Just as a painter can mix from three tubes of paint to produce a wide and vivid palette, our brain uses the ‘private lines’ from the three cone types to create thousands of colour sensations.
Scientists previously thought that full colour vision depends on specialised nerve wiring in the eye and brain, but animal studies show that the wiring is identical for monkeys whether they have normal or abnormal colour vision, Prof. Martin says.
“This tells us that there’s nothing wrong in the brain – it’s only working with the signals that it receives on the ‘private lines’,” he says. “So the only difference in normal and abnormal colour vision is caused by the first stage of sight, which points to faulty cones. Either they have failed to develop, or else they are picking up abnormal wavelengths.
“Now that we know faulty wiring isn’t the cause, we can concentrate on fixing the cones, which are controlled by genes – and thus prone to mutation or mistakes during cell replication. There are already promising results from gene therapy as a way to restore full colour vision in colour blind monkeys.”
“While we have still have some way to go, the benefits of this gene therapy – if successful – can potentially extend beyond providing complete colour vision,” he says.
“If we can get these genes to work in human eyes, it means that the same approach might be possible for other visual problems – including blinding diseases such as macular degeneration.”
"In macular degeneration, energy supplies to the macula can’t keep up with demand. So the ‘private line’ system must be very energy-intensive. Gene therapy could be used to turn down the cones’ energy demand, or to increase energy supply from supporting cells to cone cells,” Prof. Martin says.
“Together with clinical researchers at the Save Sight Institute, we are now working hard to find out exactly how many ‘private lines’ there are in humans. That can point us to where energy demand is highest and we can target gene therapy to the right place.
"So animal research on ‘private lines’ for colour vision has given new clues for understanding one of the most important visual diseases – macular degeneration – in humans."
(Source: scinews.com.au)
EnChroma glasses designed to compensate for color-blindness
While many people may think that being color blind means seeing everything in black-and-white, such a condition is in fact quite rare. Instead, the majority of people who are classified as color blind are capable of color vision, but they have difficulty distinguishing red and green as distinct colors. EnChroma’s Cx sunglasses are designed to help in these cases, by selectively reducing the transmission of given wavelengths of light, thus allowing red and green to stand out.
The key to the sunglasses’ performance is a proprietary coating on the lenses. Said to be harder and more scratch-resistant than glass, it can be tweaked in production to filter certain wavelengths that cause “color confusion.” The result is an improved signal-to-noise ratio in the perception of colors, in which red and green don’t just appear as variations of yellowy-brown – as an example.
Depending on their specific type of red-green color vision deficiency, users can choose between two different models of the sunglasses, designed to filter different wavelengths of light. There are also models that simply boost the intensity of all colors (for use by normally-sighted users), and that boost colors while also blocking UV rays.
People who are completely incapable of seeing any colors will unfortunately not be helped by any of the models. Also, because they are sunglasses, their color correction feature only works in bright light.
EnChroma’s Cx sunglasses should be available as of the middle of next month. Expect to pay at least US$800 for a complete set of glasses, or $700 for the lenses alone.
What good is color vision in the dark of the deep sea? For some crabs, an ability to see blue and ultraviolet light may mean the difference between chowing down on a good meal versus a toxic one.
A new study published in the Journal of Experimental Biology finds that some seafloor, or benthic, crabs can see in color. But the crustaceans live in darkness of the deep Caribbean where sunlight does not penetrate, making their sensitivity to blue and ultraviolet light mysterious.
(Source: livescience.com)