Psychology professor Howard Hughes teaches Psychology 21, “Perception,” as well as Psychology 51.05, “The History of Psychology and Neuroscience.” His award-winning book, “Sensory Exotica: A World beyond Human Experience,” explores the fascinating sensory systems found in the animal kingdom.
For our “Vision” issue, the Mirror spoke with Hughes — who has spent his career learning about vision, both in human and in the animal kingdom — about honeybee dances, mantis shrimp and the capabilities and limitations of human vision.
You mentioned Psychology 21 covers vision, hearing and touch, with the greatest emphasis on vision. Why is vision so important?
HH: As much as half of your cerebral cortex is in some way devoted to vision. I think that’s one indication of how important vision is to us. That’s the reason why Psychology 21 is half vision and half everything else. We consider these sensory systems as devices that have been amazingly well engineered and honed by our evolution, in different ways than other animals.
How does color vision work?
HH: We have what is called “trichromatic vision.” With three primary colors — red, green and blue — you can mix them in various proportions and produce any color you want. All the light that enters your eyes has some proportion of red, green or blue light, and you have these red, green or blue cones that tell you what proportion of red, green and blue is in this little patch of your visual field, and that’s how you recognize the color. Color vision is based on, in our case, having three different kinds of photoreceptors, and each of the photoreceptors has a kind of a pigment molecule that absorbs light. That’s like the secret of light — if you want to detect it, you have to absorb some of it. And that’s what those pigment molecules do. They vary in peak sensitivity to different wavelengths. So, we have a long-wavelength cone, and those are really sensitive to red light, and then we have a middle-wavelength cone, that’s most sensitive to green light, and then the step-child in this color system is the blue-sensitive cones, which are not very sensitive and very rare. So, we’re actually not very sensitive to blue light.
Every single object in the world gets illuminated by light, and then, depending on the nature of the material, it absorbs light in different wavelengths. That green sofa reflects primarily green light, and it absorbs primarily red and blue. The sky on a sunny day looks blue because the molecules in the atmosphere reflect primarily blue light. Of course, we can only see part of the electromagnetic spectrum. We don’t have the receptors for it, so we can’t see ultraviolet light. But it turns out that lots of animals can see ultraviolet light. A lot of insects and some birds can see ultraviolet light, as well as some fish, because ultraviolet light is translated better in water than red light.
So, the sky would look different to the animals that can see ultraviolet light?
HH: Yes, for example, honeybees can see ultraviolet light. A honeybee can tell where the sun is even if the sun isn’t directly visible. The ultraviolet sky light is polarized, and the pattern is symmetric around the sun, so the honeybee only needs to have two patches of blue sky to tell where the sun is. It uses this to communicate with other members of the hive where the good sources of nectar are. So, if you’ve ever heard about the waggle dance of honeybees, that dance is telling the other bees there’s really good nectar 30 degrees from the sun. The vigorousness of the waggle indicates how good the source is. The direction relative to vertical is the way they’d fly relative to the sun, so when they come out of the hive they can say, “Well, the sun’s over there, so I need to go that way.” The amazing thing is that the flowers have co-evolved with the bees. Flowers have evolved ultraviolet markers to direct bees to pollinate them. I believe that if you talk to a botanist, you’d find that not all flowers depend on bees for pollination, but those that do have these ultraviolet flower markers. We can’t see those markers, but the bees can.
What is it like to be colorblind?
HH: Really, people who are colorblind are not incapable of seeing colors; they do not see the world like a black and white movie. Colorblind people can see colors. There’s just certain mixtures of colors that look the same to them but look different to us.
So that’s why people who have, specifically, red-green colorblindness can see some colors, but have trouble distinguishing between those two?
HH: Right. There’s really three different variants of colorblindness, because you can be missing either the red, the green or the blue cone. That’s why they can use those little colored plates where if you can’t see the number inside — Ishihara tests — to diagnose which type of colorblindness you have. Now here’s the really wild thing: there’s a type of person that’s called an anomalous trichromat, which is someone that has three different color cones, but one of them is a little different than the others. You still have three, but the peak of what wavelength of light that pigment can absorb is not in the same place as it is for most of us. You will not be considered colorblind, but you are considered an anomalous trichromat because your color matching is a little different than the rest of us.
Trichromatic vision is actually very rare in the primate class of mammals. Almost all the New World monkeys are dichromatic, missing the blue cone. Now, in certain primate species that are usually dichromats, within their gene pool there are a population of trichromats. Because all this color vision stuff is sex-linked (it’s very rare for females to be colorblind), trichromats are always females. So, it’s possible that we could have tetrachromatic [human] females. There is a search going on for tetrachromatic females, and it would be a very subtle thing, because they would have to be the offspring of an anomalous trichromat and a regular trichromat and then they could, conceivably, get both versions of the gene — for the anomalous pigment and the other pigment — and they could both be expressed in the photopigments in the cones. If that happened, they would be females with better color vision.
Is there a specific advantage to having trichromatic vision?
HH: You know, the first thing in the evolution of color vision may have been the development of the red and green first. The idea there would be that if we are descended from arboreal animals, fruit eating animals, and we used to spend time in trees, we’d want to be able to pick out the red fruit from the green leaves. For food and foraging, that was a big advantage.
Color vision is wild in the animal kingdom. Look at tropical fish. DayGlo blue with yellow spots? I’d think they have color vision — there’d be no point if they looked grey to the other fish. Have you ever seen the mating dances of a bird of paradise? They’re crazy, and they evolved like this! It’s sexual selection. There’s a creature called the mantis shrimp, and the mantis shrimp looks like a psychedelic lobster. They are amazing looking. They live in the South Pacific. They have 11 — or 13, they’re still counting — different cones. I don’t even know how to imagine what the world looks like to them, but it goes way into the ultraviolet, because that travels well in water. These kinds of creatures can move their eyes individually. They’re like two separate visual systems. But they still want to see in stereoscopic depth.
Stereoscopic depth?
HH: Stereoscopic depth involves, for us, a comparison between the two eyes. So, if you look at me with one eye, and then look at me with the other…
You’re in slightly different positions.
HH: And what your brain does is take those two images and combine them and recover depth information from that. So, you go and you see “Gravity” or “Avatar” in the movie theater, and you have to wear the glasses. If you take the glasses off, when things move, you see the two images seperate. I can’t imagine a Dartmouth student who hasn’t noticed when they’re trying to read — not my book or the Psychology 21 textbook, but something boring — and they’re tired, the words kind of drift apart. Through effort of will you can kind of snap them back together again. You’re not focused on the plane of the book, but you’re focused in front of it or behind it.
Put your index finger in front of you and look at my nose. How many fingers do you see?
It looks like two.
HH: If you alternately close each eye, you’ll see that the two fingers you’re seeing are from each eye. You can’t fuse that image because the images are so different. Only within a certain zone can your brain combine these. There are also people who are stereoblind.
How does that happen?
HH: It happens usually because you have something wrong with the alignment of the two eyes. If you’re cross-eyed when you’re born (in Psychology 21 we don’t talk about “cross-eyed,” we say “convergence strabismus”), your eyes aren’t aligned, and when that happens you would see in double vision all the time because there are two different images on your retina. Stereovision is critically important, although about 10 percent of the population has some kind of stereo anomaly, and it almost always has something to do with either a bad refractive error in one eye or misalignment of the two eyes. You go into a Psychology 1 class — and I’ve done it — that has, say, 180 students in it, and if you ask for a show of hands of people that can’t get anything out of the magic eye books or viewmaster viewers [which have 2D images that appear 3D]. There’ll be about 10 percent of the people that say, “I just don’t see that.” And if you see double all the time, there is no way that your brain can combine these radically different images into one. So, what happens in these circumstances is that during development, the one eye is going to lose its connection to the brain.
So you’d end up only seeing one of the images?
HH: You only see out of one eye. It’s amazing that they can even fix this, but if you don’t get it fixed before the critical period is up, it can be fixed for cosmetic value, but one eye will have lost access to the brain. That’s called amblyopia [or “lazy eye”]. During development there’s a competition between the two eyes. In the normal course of development, each eye is an equal competitor, and so the eyes share the cortical tissue. Each has equal access to the brain. But if one eye is at a really bad disadvantage, even if there’s nothing wrong with it optically, it will lose, and then even after restoring the proper alignment, you’ll never be able to see out of that eye.
Everyone actually has a dominant eye. If you were to look into a microscope with only one ocular, you would want to use that eye. If you were going to shoot a bow and arrow, or look in a peephole, you’d use that eye. So, that doesn’t have major perceptual consequences because we both have normal binocular vision, but we still end up with this little preference for one eye over the other. To determine your dominant eye, sit directly across from someone. Look at their nose. Raise your hands, palms facing your partner, to either side of them. Bring your hands together until they block out the entirety of your partner’s face except for their nose. Have your partner indicate which of your eyes, right or left, they can see between your hands from their vantage point. That is your dominant eye.
This interview has been edited and condensed for clarity and length.
