These posts make more sense when read in order.
Please click here for the first article in this series to enter the rabbit hole.
Depth perception
Much of our perception is created by our brains. Each of our retinas sees the world as two-dimensional—since we have a flat layer of receptor cells—and, interestingly enough, our brains originally process it that way. Using visual cues and information from the other eye, the brain gradually builds a three-dimensional view of the world a layer at a time. Depth is not something your eyes can see. It’s constructed in your brain.
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The top illusion by Kanizsa and D. Varin shows how visual cues make us see a rectangle in front of the four targets, while the addition of caps on the upper right image destroy the illusion. The rectangle no longer floats above the semicircles. This illusion works because experience tells us that most objects are opaque like the imaginary square and often parts of objects are blocked from view as the targets appear to be. The four lower drawings illustrate how binocular vision converts the two dimensions each eye sees into three dimensions, even though the images themselves are actually flat. Look at either the top or bottom pair and cross your eyes until the two dots become one and you see three boxes on each line. The upper middle image projects back, while the lower one extends towards you. For these images reality is two dimensional, yet we see the illusion of depth. These images were redesigned by John Richard Stephens and are based on older versions.
Using binocular vision is not the only way you do it, as the two sets of illusions I’ve included show. When looking with one eye, you can also move your head up and down or left and right to discern which objects are closer and which are farther, as closer objects move and they block the view of farther objects. We see our entire visual field as three dimensional, but this is an illusion. If you close one of your eyes and look at your nose, you’ll see that it and part of your face blocks part of your view in that eye. By closing that eye and opening the other, you can see how much of your peripheral vision is blocked. Since we rely largely on both eyes to see depth, even with both eyes open we can only see those blocked areas in two dimensions—yet everything looks 3D to us. It’s thought this is because our brains’ predictions seamlessly add dimension to those areas. Of course you can gain full depth perception for those areas by turning your head to bring them into view of both eyes. |
Our hearing is also three dimensional, indicating which direction and how far away the source of a sound is. Using a number of clues, our brains piece this information together to construct our hearing’s dimensionality.
Colors only exist in your mind
Unlike depth, which actually exists but has to be recreated by our brains, colors are an illusion and they don’t exist in the real world outside of our brains. It’s an interpretation, just as we interpret tastes to be bitter or sweet, or the vibrations we hear as discordant or harmonious. Colors are a sensation that, as I mentioned earlier, we don’t see as constant. Our brains vary them with the environment, similar to the Checker-Shadow Illusion.
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| In the Checker-Shadow Illusion, checkerboard squares A and B are identical shades of gray, as can be seen on the right where they have been extracted and placed on a more neutral background. The illusion of lighting and a shadow tricks our brains. Edward H. Adelson, Massachusetts Institute of Technology, © 1995. |
Another example is that our cone receptors become less sensitive to colors as we age and the lenses of our eyes yellow, but we don’t notice these changes because our brains make adjustments for it.[1]
Our perception of colors also changes with the seasons. Researchers think this keeps our color perceptions consistent in spite of changes in the environment.[2] In addition, there are illusions that can make a single color appear to be two different colors.Colors are in a continuum, blending from one to the next along a line extending from ultraviolet to infrared, but these are just electromagnetic waves—like microwaves and FM radio waves, only of a different frequency. What we see when we perceive a color is a wave of light vibrating a specific frequency. There is no color in the wave itself. Increase or decrease the frequency and you’ll perceive a different color, but except for that change, the waves are identical.
Your retina’s three types of color receptor cells, which are called cones because of their shape, signal their reception of photons of a particular range of wavelengths, which we call green, blue, and red, although red cones primarily respond in the yellow range. The next level of cells take the information from these cones and use it to add red and brightness, but colors aren’t coded by single cells, rather by patterns of cells. Our brains take this information and blend it together to create the range of colors we see. Even though the lenses of our eyes yellow after the age of forty, our brains make adjustments so we continue to see the same colors even though the electromagnetic waves that reach our retinas have changed.
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| Three of the many variations of Benham’s disks. |
Evolutionary biologist Richard Dawkins wrote, “I used to think false colour images were a kind of cheat. I wanted to know what the scene ‘really’ looked like. I now realize that everything I think I see, even the colours of my own garden through the window, are ‘false’ in the same sense: arbitrary conventions used, in this case by my brain, as convenient labels for wavelengths of light.”[3]
Since colors are created in our brains, scientists can turn them on or off by applying magnetic fields to the lower center of our brains towards the back of our heads. They can also use this technique to make us see unusual colors.[4] While there are some people who don’t need magnetic fields as they can hallucinate colors at will.[5]
Over and under the rainbow
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| A double rainbow in Alma, Michigan. Tom (adjusted). |
By looking at a rainbow you can see the complete spectrum of the pure colors that we’re able to see—from red along the outer edge to violet on the inner one, sometimes with a couple of bands of repeated colors on the lower edge (a supernumerary bow). Blended colors—such as purple, pink, brown, and olive—are a mix of two or more pure colors, while black, white, and gray are considered non-colors. None of those are in a rainbow. Neither are wavelengths we can’t see, such as ultraviolet, infrared, radio waves, microwaves, etcetera.
Most Americans see five or six bands of color in rainbows, but that’s largely cultural. We say there are seven, but that’s because Issac Newton added a couple of them to reach that number because he thought it was mystical. You’ll commonly see seven bands in artwork of rainbows because that’s how many the artists think rainbows are supposed to have. Some cultures see different ones, with several not recognizing blue as a color. Rainbows are actually a spectrum of uncountable colors.
Rainbows are usually opposite the sun because the sunlight has to go past you and reflect back at a specific angle from the droplets of water in the air, which act as prisms, splitting the wavelengths. The height of the rainbow is determined by how high the sun is in the sky—the lower the sun, the higher the rainbow. Red rainbows are seen at sunrise and sunset when longer wavelengths of light are dominant.
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| Various sun halos and arcs, with sun dogs off to each side of the sun as seen from the South Pole. Lieutenant Commander Heather Moe, NOAA Corps (adjusted). |
Here in Hawai‘i—I live on Maui—we often get double rainbows. The second rainbow is a bit fainter and well above the first. And the colors are reversed, with red at the bottom and violet on top. You can also see rainbows when you’re looking down at clouds, sprinklers, or mist from waterfalls, but generally the sun has to be at your back.
While rainbows look real, they’re just reflections—a trick of the sunlight and water or ice—which is why you can’t go over the rainbow, or under it, or get to the end of one, where all the gold is supposedly hidden.
The Falling Tree
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| © John Richard Stephens, 2024. |
Colors and much of our vision are creations of our minds and the same is true of our other senses. There’s the oft quoted question, “If a tree falls in a forest and no one is there to hear it, does it make a sound?” This philosophical question was raised as part of the subjectivist versus objectivist debate. We’ll get to the subjectivist argument shortly. Here I want to take the objectivist view.
The answer to the question depends on your definition of sound. If sound is the pressure waves the tree makes that pass through the air, then it does make a sound. But that’s not what we experience as sound, which is the sensation created by your brain when vibrations move the tiny hairs inside your inner ears—more specifically in your
cochlea. The hairs turn the pressure frequencies into electrical signals, which your brain interprets as sound. If that’s your definition, then the falling tree doesn’t make a sound without a listener, whether it’s Alice or a stink bug.Hearing is rather like a radio picking up a radio signal. Like the pressure waves, the radio waves are there whether a radio is tuned to them or not. Is that sound? Perhaps, but you won’t be able to hear it without the radio. The radio tunes into a tiny band of electromagnetic waves and converts them into mechanical pressure waves, much as our brain then converts these pressure waves into what we hear.
If your definition requires the perception of sound, then a tree could fall, creating pressure waves that strike against the eardrums of a deaf person and there would be no sound. But as long as a person or some forest creature hears it, then their ears will perceive a sound. Sound, by this definition, is a sensation—an interpretation in the brain. The falling tree exists. The sound might or might not.
Your perception of sight, sound, touch, taste, and smell are creations of your mind and they don’t exist outside of your mind except in completely different forms. They are neurophysiological processes that your brain uses to make you aware of something in the environment. It’s part of how your brain creates your perception of the world around you.
While our senses aren’t very accurate, that might not be such a bad thing. With all their flaws, they are efficient. There’s just no way our brains could handle the torrent of information constantly flooding our senses, so we have to take short cuts.
Our brains predict, enhance, and alter our perceptions in order to make them more efficient and useful to us. Most of the time we never notice because we generally have no way to compare our perceptions to reality. Human evolution has traded accuracy for efficiency and for the most part it serves us well. While they’re not completely accurate, they’re accurate enough for you to survive...at least, most of the time.We rely on our senses and we’re not used to thinking of them as being wrong. Some aircraft crashes are caused by pilots who are convinced they are flying level, when actually they’re diving. When their instruments tell them something different from their senses, they can mistakenly think the instrument has malfunctioned and they’ll fly an aircraft in excellent shape right into a mountain, the ocean, or the ground, especially at night or in fog. Such accidents are categorized as “Controlled Flight into Terrain”. This is apparently what happened to John F. Kennedy Jr., Stevie Ray Vaughan, Buddy Holly, and probably Kobe Bryant.
Similar illusions can cause driving accidents. Fog can make you drive faster than you think are going, as can the night in certain circumstances. The flashing lights of emergency vehicles can seem farther away than they are, causing you to crash into them. Also, if you try to follow someone at night who only has one working taillight or if you focus on just one of the lights, you’ll think it’s going somewhere it isn’t, causing you to crash into something or drive into a ditch. This was a big problem for truck convoys driving at night during the air raid blackouts of World War II.
One night I was following two vehicles that were kicking up clouds of dust on a dirt road. The driver of the truck in front of me was trying to closely follow the front vehicle by watching its taillights alone, but I held back far enough to see the road. It was only a minute or so before I saw the truck drop towards the right and flip over onto its roof, as it rolled down the side of a hill. The driver was okay, but his truck wasn’t. Sometimes you can’t trust your senses.
Click here for the next article in this series:
[1] Public Library of Science, "Brain, not eye mechanisms keep color vision constant across lifespan", ScienceDaily, May 8, 2013, http://www.sciencedaily.com/releases/2013/05/130508172135.htm, citing Sophie Wuerger, "Colour Constancy Across the Life Span: Evidence for Compensatory Mechanisms", PLoS ONE, 2013; 8 (5): e63921 https://doi.org/10.1371/journal.pone.0063921.
[2] Carl Engelking, “Your Color Perception Changes With the Seasons”, Discover Magazine, August 18, 2015, https://www.discovermagazine.com/mind/your-color-perception-changes-with-the-seasons.
[3] Richard Dawkins, Unweaving the Rainbow, New York: Houghton Mifflin Co., 1998.
[4] Donald D. Hoffman, Visual Intelligence, New York: W. W. Norton & Co., 1998, pp. 108-10.
[5] University of Hull, “Some people can hallucinate colors at will”, ScienceDaily, November 30, 2011,
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