Introduction to the Biology of Vision

An Introduction to the Biology of Vision


This main goals of this text are to provide undergraduates with a working vocabulary and knowledge of the biology of vision and to acquaint them with the major themes in biological vision research. Part I treats the eye as an image-forming organ and provides an overview of the projections from the retina to key visual structures of the brain. The second part builds on this material, examining the functions of the retina and its central projections in greater detail. Part III addresses certain…

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Is it true that animals with more types of color receptive cones can see colors we cannot conceive of?(r/askscience)

It is reasonable to suppose that animals with more cone types perceive the visible spectrum as being divided up into more distinguishable hues than it appears to us.

We perceive the greatest number of distinguishable hues in the areas where our cones overlap in sensitivity. Basically our brain has no a-priori way to measure a wavelength; what our brain does is more like: “ok, this light here stimulated our L cones 20% more than it stimulated our M cones. Let’s call that ‘orange’.” or “This light only stimulated our S cones and didn’t stimulate the other two types at all. I’m going to call that ‘blue’.” (vastly oversimplifying, and actually the visual cortex is doing something more complex, but that’s the idea).

More specifically – there’s a relatively large region of wavelengths that all appears to us as just “blue” because they all only stimulate the S (short wavelength) cones. Then there is a relatively small set of wavelengths where we perceive a whole bunch of different hues ranging across “blue-green-yellow-orange-red”. Most of what we think of as the “rainbow” is actually stuffed into a small range of wavelengths. (warning: before anybody replies to say “But figure X in this textbook doesn’t show it like that”, most biology textbooks display this incorrectly. I believe this is due to copying figures from older textbooks that date to before we knew the actual sensitivity of the M and L cones). The reason we perceive a lot of hues here is because those wavelengths stimulate our cones to different degrees, because the S and especially the M and L cones overlap in sensitivity here. Then, going to longer wavelengths, there’s a large set of wavelengths that only stimulate the red (long wavelength) cone, and we perceive those all as “red”.

Ergo: we perceive more hues in regions where cones overlap in sensitivity.

Reasonable conclusion: Animals with multiple cones probably perceive a variety of hues where those cone sensitivities overlap. Specifically: Most non-mammalian vertebrates have a UV cone that has a large region of overlap with the S cone, and so they probably perceive a variety of hues that we perceive as all just “blue”.

I’m not even getting into the issue of having infrared or ultraviolet sensitivity.

source: I wrote the sensory biology chapter for one of the major bio textbooks on the market. Try here, here to dig in deeper.

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James T. McIlwain

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Cambridge University Press

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An Introduction to the Biology of Vision

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Is it true that animals with more types of color receptive cones can see colors we cannot conceive of?

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