02/11/2013 04:55 pm ET Updated Apr 13, 2013

Feel Like a Butterfly, See Like a Bee? The Mystery of Perception

Co-authored with Murali Doraiswamy, M.D.; Rudolph E. Tanzi, Ph.D.; and Menas Kafatos, Ph.D.

In any collection of quotations by Albert Einstein, one of the most intriguing is this: "Reality is merely an illusion, albeit a very persistent one." Anyone who is interested in going beyond the illusion to find a more satisfying reality would pay attention, but in actuality we all have made ourselves comfortable with the illusion that convinces us. Even physicists, who know with certainty that seemingly solid objects are actually constructed of invisible energy clouds, treat tables, chairs and cars stalled in traffic like solid, tangible things. The field of quantum mechanics has shown for more than 80 years now that the perceived reality of hard objects that senses give us is an illusion.

Yet if things lost their thing-ness, which our senses make us believe in, we'd have little choice but to reevaluate what is real and what isn't. For example, what if you had the following experience, recently related on the opinion page of The New York Times:

First, riding in the passenger seat of the car, I started noticing, from the corner of my eye, vistas opening in the landscape out the window. I saw little woodland trails I could follow dreamily, even as we drove down city streets after dark. Once we were out of the city, trees by the side of the road morphed into towering brick walls suggestive of a maximum-security prison, sometimes intricately patterned with what looked like droplets of colored sugar.

The writer, Maxine Kumin, wasn't having a psychotic break. She was suffering from "non-psychiatric" hallucinations caused by a loss of sight in her central visual field because of macular degeneration. When the central field of vision deteriorates, the person normally experiences blurry sight eventually followed by blindness that moves from the middle of the visual field outward. In this case, however, the brain's visual cortex, unable to decode the signals arriving from the eye, substituted its own fully formed images. The brain's best guess wasn't just a filled-in version of what was actually there. It replaced a blurry tree with a brick wall, a bare patch of woodland ground with a winding trail.

This raises a startling possibility: How do we know that our own "normal" sight isn't a "fictive visual percept," to use the medical term for hallucinated images? Actually, we don't. No philosopher using deduction or researcher using brain scans has been able to prove with certainty that our perception of the world matches "reality as it is out there," whatever that may mean. Hallucinations are identified medically because they usually arrive with other symptoms, like those associated with schizophrenia, and because if one person sees a brick wall where everyone else sees a tree, the outsider must be hallucinating.

If we follow the mystery of perception, many more issues arise than the fairly simple one of hallucination. Hallucinations are rare, but the brain's ability to turn electrical impulses and chemical reactions into a world we see, hear, touch, taste and smell is incredibly baffling. There is no light in the brain. Yet the light of the sun is blinding. This disparity is crucial, because without someone to see it, the sun is invisible. There is no visible light in nature without an eye to perceive it. What if your brains, having taken a totally different evolutionary path, didn't "see" light but "heard" it? There's no obstacle to such a development. (A phenomenon known as synesthesia, in which the senses get mixed up, is well known to neuroscience. It became much more familiar during the LSD 1960s, when trippers discovered that they could taste colors or see music.)

The fact that our senses don't match reality can't be taken for granted, even though we do that all the time. We want to discuss the profound implications of perception versus reality, but first let's look at the best proof we have. You and I may agree that the street is lined with trees rather than prison walls, but other species live in the world with us and do not agree. All living creatures sense "reality" through specific filters, and all assume that their filters (i.e., their senses) have perfect fidelity. (Cats, dogs, bees and butterflies can't tell us what their assumptions are, but we will accept that they don't think they're hallucinating. Every species operates efficiently in the world it perceives.) We take it for granted that every species using its filters sees a common reality, but what is "common" among all these perceptions is much harder to pin down.

In fact, sensory abilities differ vastly among the millions of species on the planet. What is real to one species (like a bat's sonar) is hidden to another (a deaf paramecium). Even among 7 billion humans, every person has a different "mix" of reality, depending on personal acuity, predispositions, habits, memories and upbringing (the child of a horticulturalist might automatically see 20 different wildflowers in a meadow where you see a blur of color). We tend to ignore the fact that sensory abilities differ from one person to another, unless the difference is striking, as between one person who is tone-deaf and another who has perfect pitch. Yet the larger truth is that each of us uses the brain like a personal CGI factory, creating a 3D movie of the world unlike anyone else's. Are we illusion makers or reality makers? That's the big question.

Let's start with visual systems first, given that color and imagery affect much of how we perceive reality. Humans have one lens in each eye, and our eyes are trichromatic: We have three types of color-sensing cells, or "cones," which allows us to distinguish a million or so colors. But even within humans, there is a twofold or threefold physical difference from one person to another in every aspect of our visual system (e.g., the size of the optic nerve, lateral geniculate nucleus and primary visual cortex, etc.). Variation in cone pigment genes is very widespread, particularly between genders. Recent evidence suggests that somewhere between 2 percent and 50 percent of women may have four cones, giving them super color vision (tetrachromacy), while colorblindness is a male trait.

However, color vision and eyesight vary even more dramatically among different species, many of which are monochromatic, such as seals, sea lions and owl monkeys. If a species is a "rod monochromat," then for that species, the world is free of all colors other than shades of gray. If a species in a "cone monochromat" (e.g., it only has one type of cone), then it can see about 100 shades of a single color or its combinations. Some species, such as cats, are dichromatic, which means they can see only about 10,000 colors. There are also surprising gender differences in animals; among many New World monkey species, for example, males are dichromatic, but many females are trichromatic, like humans. Honey bees are trichromatic, but not quite like humans; they cannot see red, but they can see ultraviolet frequencies.

Evolution hasn't ordered living creatures in a straight line from "crude" sight (as we humans would judge it) to more "evolved" sight (as we would call our own). Many birds, insects and fish are tetrachromatic, and some spiders and birds can see ultraviolet colors, which humans cannot. This would make insect prey glow green in the dark. The reason that we cannot see UV colors is that our lens blocks them from striking the retina, but people whose lenses have been removed in a cataract procedure or who were born without a lens (aphakia) have been reported to detect UV light.

As evolution has developed different sensory systems, reality shifted. There is no "normal" way to decode photos of invisible light. Pigeons and some butterflies are actually pentachromats; in theory such creatures could distinguish up to 10 billion colors, even though we have no way to prove this. The mantis shrimp probably has the most amazing eyes in the animal kingdom, with 16 different receptor types, including four types of receptors just for seeing UV light, and four others for polarized light. A human would need many distinct kinds of sunglasses to duplicate the sensation. Many snakes can also "see" infrared, or heat radiation, using special detectors that send thermal information through their visual system. Even the density of rods and cones differs greatly. Humans have about 200,000 per square millimeter, whereas sparrows have 400,000 and buzzards 1 million in the same tiny area. Giant squid, who live at ocean depths that are inky black, outdo all other species with a sensitivity that is several thousand times that of humans (possibly a billion total receptors, though the density is not well known).

It's hard to escape our assumption that eyesight connects us to the real world, but every living thing is connected to a created world. The question of matching our creation to a possible "real reality" will come next. The conclusions of quantum mechanics will certainly have to be brought in. For the moment, we need to realize that the world created by other species is inconceivable to us. Humans have one lens in each eye. Insects have more complex compound eyes, with individual components that resemble a single human eye. Depending on species, flies can have 3,000 to 25,000 lenses; bees have 5,500. Some box jellyfish have 24 eyes; some scorpions have 12 eyes, including several pairs in different locations on their body; scallops have 100 or so eyes along the edges of their mantle, with a special reflector lens and two types of retinas; and some spiders have eight eyes, including some with special telephoto-like lenses. No doubt you've seen multi-lens photos that attempt to show the world through a fly's many eyes, but they are misleading, because the fly has yet to process all those snapshots into a coherent world, which may have dozens of facets or only one. Are those snapshots still or moving? Another mystery, because humans have a flicker fusion rate of 50 images per second, which means that anything slower is captured one image at a time, whereas anything moving faster appears as continuous motion. But chickens don't see continuous motion until the speed reaches 100 flickers per second, and flies don't see it until 300 flickers per second, so for these creatures, the world doesn't turn into a movie until long after it does for us.

Finally, the mystery of perception must be sorted out from defective perception. Humans suffer from certain peculiar visual defects. For example, we can fill in information that we partially see (e.g., if an edge is blocked out), but some animals don't do that. Optical illusions have proven that our visual system is often wrong in its detection accuracy for size, shape, color, motion and depth. (Think of desert mirages where shimmering hot air looks like water.) Yet in a sense, confining our examples to eyesight is misleading, because the world is created by blending all the senses, and variations in touch, taste, hearing and smell lead to bewildering riddles. The Indian elephant hears better than humans at lower ranges, bats at much higher ranges. Cats cannot taste sweetness. Cows have about 25,000 taste buds, pigs about 15,000 and catfish about 150,000, outstripping a gourmet chef operating with the human complement of 10,000, but this, too, varies two- to threefold, as in eyesight.

So while feeling superior to chickens with their 100 taste buds and ignoring bees, which can smell something miles away, or sharks, which can detect faint, distant electrical impulses, humans must take advantage of one extrasensory gift -- our ability to reason -- in order to find out where we stand in the shadowy realm of illusion versus reality.

To be continued...

Murali Doraiswamy, M.D., is a professor of psychiatry at Duke University Medical Center in Durham, N.C. Rudolph E. Tanzi, Ph.D. is the Joseph P. and Rose F. Kennedy Professor of Neurology at Harvard University and the director of the Genetics and Aging Research Unit at Massachusetts General Hospital (MGH). Menas Kafatos, Ph.D., is the Fletcher Jones Endowed Professor in Computational Physics at Chapman University.

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