BIL260 Color & Vision Notes
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This 2 page Bundle was uploaded by Ashi Ma on Tuesday February 10, 2015. The Bundle belongs to BIL360 at a university taught by Dr. DuBois in Fall. Since its upload, it has received 53 views.
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Date Created: 02/10/15
The basic way that color vision works is to use a small number of receptor types that have different but overlapping wavelength sensitivities These are dispersed across the retina in a mosaic pattern and the outputs of the receptors can be combined and compared to produce a sensation of color differences Animals can generate photopigments with different absorption peaks meaning what wavelengths of light they ll readily absorb Humans do this by associating the retinal chromophore with different opsin proteins Animals living in dark poorly lit environments must use all of their photoreceptors for spatial pattern analysis and brightness contrasts they can39t afford the luxury of color vision Such animals are monochromats and they see the world in shades of gray Examples are nocturnal deepsea and cavedwelling vertebrates that typically have high proportions of rod receptors in their eyes Once a species has evolved two cone pigments with different spectral sensitivities a form of color vision becomes possible This animal is a dichromat With three photopigment types the animal is a trichromat Humans are trichromatic we have a color vision system based on three types of cones The three cone types are maximally sensitive to different wavelengths of light see absorbance spectrum We judge colors by comparing the output of the different cone types For example stimulation of the long wavelength red cone type by itself does not produce the sensation of red rather it is strong output by the LW cone compared to weak output by the MW cone that causes a red sensation Strong stimulation of both LW and MW compared to weak stimulation of SW causes a sensation of yellow Strong stimulation of all three cone types causes a sensation of white etc Mammals outside the primates tend to have poor color vision Dogs are a familiar example they are often said to lack color vision altogether Dogs actually do have cones as well as rods so they are capable of seeing colors However they don39t have as many cones as some other animals such as humans Dogs are dichromatic they only have two cone types What does this mean for their ability to perceive colors One effect is that dogs perceive fewer categories ofcoorthan we do Remember color is perceived based on comparisons of the output of the various cone types with 2 cones instead of 3 there are a lot fewer combinations that are possible The slide shows one hypothesis about the colors that can be discriminated by dogs You shouldn39t interpret this as saying that dogs see as yellow what we see as red even within humans we cannot say that one person39s sensation of red is the same as another39s What you can get from this slide is an impression of what colors dogs might be able to tell apart thus colors in the long wavelengths reds greens and yellows are likely to look similar to dogs because they don39t have any cone type in this region Now before you start to feel very superior to dogs and other mammals that are worse than us at color vision there are other animals that are better notably birds and sh In the graph you can see the wavelength sensitivities of the photoreceptors of a pigeon One thing to notice immediately is that the pigeon has four cone types to our three it is therefore said to be tetrachromatic Remember that I said that dogs with two cone types would see fewer colors than we can with three cone types because there are fewer possible combinations of comparisons with two types than with three With four the number of possible combinations is again greatly increased In particular tetrachromatic color vision allows a great increase in the number of nonspectral colors A nonspectral color is one that is not in the rainbow and which arises from the stimulation of two or more cone types that are sensitive to nonadjacent wavebands Purple is the only nonspectral color that humans are sensitive to Purple is perceived when the short wavelength and the long wavelength cones are stimulated and the medium wavelength cones are not A tetrachromat would be able to perceive several nonspectral colors instead ofjust one This increase in the dimensionality of color must produce a change in how these animals perceive color that we as humans cannot fully understand Another major difference between our color vision and that of birds is that one of the birds cone types extends down into the UV Because of this difference birds are seeing a range of re ected light that is invisible to us Birds use this ability to see patterns that we cannot For example the hill mynah looks to us as in the lefthand photograph mostly all black With a spectrophotometer you can show that some of these black areas are re ecting light in the UV the righthand picture gives an idea of what the bird might look like to other birds Many insects can also perceive light in the UV range and their view of a ower will be different they can perceive markings like these nectar guides on this primrose And last the pinnacle of photopigment complexity is found in the stomatopod shrimp like the mantis shrimp which have 16 different photoreceptor types 12 of which are specialized for color vision It39s not clear whether these shrimp use a coloropponent system as we do but it39s possible that they perceive many more colors beyond the understanding of human experience
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