These posts make more sense when read in order.
Please click here for the first article in this series to enter the rabbit hole.
Our brains don’t just construct, modify, distort, and predict our perceptions, they also create bits of it.
Our eyes were honed by evolution to be efficient and to use the minimal amount of resources. Cutting costs is of high importance. This is one reason why our eyes are not like video cameras. For video cameras, the entire field of view has to be in focus. With our eyes, we use our fovea to focus, but it can only look at a tiny spot at a time—about the size of the moon as it appears to us in the sky. Everything else is out of focus, but it doesn’t seem that way to us. Part of this is because we move our foveae around very rapidly. Another is because our vision is processed to make it appear sharp to us. Our brains have tools like those in PhotoShop that clean up our perceptions.
Part of the cleaning process involves filling in missing data, such as discerning objects in low light or through fog, or understanding sentences at a crowded, noisy cocktail party. Our brains are able to restore the missing bits. Optical illusions show us that we fill in the gaps in lines, circles, rectangles, and other objects, even filling in color where there is none.
Most people know they have two blind spots—one in each eye. They’re there because somewhere in our evolutionary history, one of our ancient ancestors was born with his or her retinas inside out. It was probably one of the first vertebrates, since all animals with backbones have this condition. It must have made the creature’s vision much worse than others its species, but it probably had other features that made it successful, since its genes spread throughout the species and carried on to their descendants.
Along the line some improvements occurred, which is a good thing for us. Still, our light receptors are at the back of our retinas, so light has to pass through about half-a-dozen layers of cells to reach our rods and cones. In addition, all of the nerve cables (axons and dendrites) and blood vessels pass forward to the front of our retinas, with the nerve bundles running across the surface of the retina to a hole they pass through, out the back of the retina, and off towards the visual cortex. We have to see through all of this stuff.
Somehow the deformity survived. Over millennia some work-arounds developed, so today we can see relatively well. Squids, octopuses, and other invertebrates, on the other hand, still have their retinas the right way around.
Anyway, because of this problem we have a hole in each of our retinas where the optic nerves pass through it. We don’t see it because our brains fill in these visual gaps. One way to see what your brain’s doing is to extend your arms in front of you with your thumbs touching and your index fingers pointing up. Close your right eye and look at your right fingertip with your left eye. With your eye still on that fingertip, see whether your left index finger is still there.
This is tricky to do because you’re staring at one thing while trying to look at another. You may have to adjust your left hand higher or lower, but part of your finger will vanish. Reverse the procedure using the opposite hands to see whether that works better. It’s best if you look with your dominant eye.
Your blind spot blocks out an area about the size of a golf ball at that distance. Notice how in place of your finger you appear to see the background behind it. It’s as if your finger really is gone. With both eyes open, your two eyes fill in each other’s blind spot. And your eyes are constantly moving which also helps hide the spots, but when you stare with only one eye, your brain fills the blind spot with whatever it sees around the hole. If you slowly move your invisible finger back and forth, you’ll see how wide your blind spot is.
[1]If you hold your arm outstretched again with your thumb up, your thumbnail will cover the area we can see clearly when we stare at something. Outside of that small area things quickly get blurry and go to black and white. This is because most of our seven million cone cells are concentrated in our fovea. The rest of the retina is mostly rod receptors—around 120 million of them. Only a few cones report to a single neuron, while many rods pool their input to one of their neurons. As a result, our eyes can see better in the dark, but our vision is blurrier outside of our fovea. In fact, if we only had our peripheral vision to see with, we’d be legally blind.
We use our cones to see during the day. They give us color vision, they focus better, giving us detail, and they work in bright light. Rods on the other hand allow us to see at night. They’re sensitive in low light, but they only respond in black, white, and gray scale.
When you look at the stars at night, you’ll notice they disappear when you look directly at them. This is because they don’t give off enough light to activate the cones in our fovea, but if you look slightly to the side of the stars, they reappear since their light is now reaching your rods. Cones, on the other hand, stop working at about the level of moonlight. Since our foveae only have cones, on dark nights they become additional blind spots which our brains fill in. There is around a billion times less light on a moonless night as there is during the day. This is bad for night driving since they block whatever you look at in the dark that’s out of the range of your headlights.
So, whatever we focus on should be sharp and in color, while everything else should be blurry and almost completely in gray scale, black, and white. This isn’t what we see though, and that’s because our brains are using predictions to create the missing colors and details. Our eyes rapidly flit around three or four times a second, focusing on whatever catches our interest, picking up further bits of detail, but our foveae only record about 0.1% of our visual field. Our brains fill in the rest.[2] Also we have a small spot in the center of our fovea that can’t see blue. Our brains fix that as well.
But our brains don’t always fill in spaces with the surroundings. Sometimes it fills them with hallucinations.
Late one night I was walking by the window of a closed clothing store when, out of the corner of my eye, I saw a largish man in his late-40s looking over a short stack of clothes on a table. Knowing the store had been closed for hours, I went back to check. It turned out to be two headless mannequins. They were facing left with a tall rack at their backs, while the man I originally saw was facing right towards a table. He was also wearing different clothes and he had a head. This is an example of my brain taking liberties with a scene when filling in my peripheral vision.
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Looking at the photograph it’s hard to imagine how the two mannequins could be seen as a man, but our brains create hallucinations to fill in our peripheral vision all the time. We just rarely notice. © John Richard Stephens, 2024. |
Neuroscientist Nadine Dijkstra of University College London, UK, writes, “It turns out, reality and imagination are completely intermixed in our brain, which means that the separation between our inner world and the outside world is not as clear as we might like to think.”
[3]We all appear to have top-down corrections to our vision, we just don’t realize it because we’re used to them and they show us what we would expect. It’s only when top-down corrections dominate and become unexpected that we’re forced to take notice.
This has prompted neuroscientists Vilayanur S. Ramachandran and Diane Rogers-Ramachandran of the University of California, San Diego—two of the leading researchers in this field—to write, “It is almost as though we are all hallucinating all the time and what we call object perception merely involves selecting the one hallucination that best matches the current sensory input, however fragmentary. Vision, in short, is controlled hallucination.”[4]
Your mind is playing tricks on you
The toys in the attic come out to play.
They hate it when you lock them away.
—lyrics from the song “Ave Dementia” by The Marionettes
In dreams and your mind’s eye, everything is created by your imagination, whereas in hallucinations—including those from drugs, hypnosis, and Charles Bonnet syndrome—imagination is generally overlaid on normal waking consciousness.
Charles Bonnet syndrome is a very interesting condition that occurs when people begin to lose their vision. Because they’re receiving degraded and incomplete input, their brains have to fill in larger gaps, while having less information to fill it with, so they begin seeing things like fairies, gnomes, and cute furry animals, if they’re fortunate, or horrifying faces, scary animals, and threatening creatures, if they’re not. One man saw a spider that was trying to kill him. Outside they may see old architecture on new buildings. These hallucinations are usually just visual and aren’t accompanied by any sounds.
This syndrome is actually quite common, although many who have it are afraid to admit it for fear of being thought insane. But generally having hallucinations alone are not an indication of insanity, irrational thoughts are. And in this case, these particular ones stop once the person is blind.
It should be noted that these hallucinations fit in seamlessly with actual vision, just as we don’t see a seam between our foveal and peripheral vision. As unusual as they might be, they are as vivid as any normal perception. There’s no way to distinguish them from regular vision, other than their unusual nature.
As vision fades, there’s less information for the brain to base its predictions on, so the predictions begin to drift, relying more heavily on memories. With little sensory input, the brain is unable to make corrections, so the predictions become wilder.
This is similar to sensory deprivation, which is known to induce hallucinations
and is often experienced by people in monotonous environments, such as drivers traveling through featureless deserts, pilots flying through empty skies, sailors sitting on becalmed seas, arctic dog sledders traversing snow-coated landscapes, and long-haul truckers rolling along endless roads.Hallucinations often appear to be as real as anything else. You can have them and never know you had them. Often the only thing that makes you suspect they aren’t real is their weirdness, but sometimes even that isn’t enough to make you question it. For example, most people who see a strange spacecraft or an extraterrestrial would immediately think that such things do exist, rather than suspect it was a hallucination, even though that might be the more logical and likely explanation.
Since our perceptions of the world are put together by our brains and hallucinations are also constructed by our brains, both can seem real. Researchers at King’s College London used brain scans to show that people used the same parts of the visual cortex when seeing visual hallucinations as when seeing actual objects.[5] You often can’t tell them apart, except that logic might suggest that the former is probably not real.
Neurologist Oliver Sacks once hallucinated he had a conversation with a spider on “rather technical matters of analytic philosophy”, insisting, “It did not seem at all strange to me that a spider should say hello (any more than it seemed strange to Alice when the White Rabbit spoke).”[6] He admitted, “With the spider, I should have known that it’s impossible. That’s one of the few times when I was completely taken in.”[7]
Hallucinations can seem convincingly real, even when they don’t make sense. Sometimes they’re so vivid and detailed that they can seem even more real than reality. History is full of examples of people who were convinced their hallucinations were messages from gods or the dead.
Oliver Sacks, an expert in hallucinations, added, “I want to say, strongly, hallucinations aren’t evidence of anything, let alone heaven.[...] Hallucinations can be very powerful and very persuasive. I think one may have to fight to deny them weight.”[8]
We tend to overemphasize the connection between hallucinations and mental illness, although there are some mental illnesses that can induce hallucinations.
According to neuroscientist Flavie Waters of the University of Western Australia in Perth, the sensory cortices of schizophrenics are overactive, but are poorly connected to the frontal lobes, which are associated with higher cognitive functions, such as attention, thought, problem-solving, and decision-making. Thus their brains make many predictions that aren’t compared to reality before reaching consciousness as hallucinations.[9]While such medical issues can produce hallucinations, there are many other causes. The better known are drugs, alcohol, and aging. Lesser known are caffeine, sleep deprivation, sedative drug withdrawal, ocular migraines, fatigue, and stress—particularly after the death of a loved one. And sometimes they just happen with no apparent cause.
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| One of several types of hallucinations caused by ocular migraines. |
The first time I had an ocular migraine, it was quite startling. It looked like a giant jagged crescent-shaped geometric design floating in the middle of my living room. It was like something straight out of science fiction. Fortunately I had seen a painting of this before in Scientific American[10], so I knew what it was and that it was harmless. It moved with my eyes. It gradually grew bigger and more complex before fading away. It lasted for about half an hour and was shocking to see.
Over the past decade I’ve seen two others—one with just one eye and the other with both. They looked quite different from the first and not as striking. Apparently ocular migraines can appear in many ways. My second one was black and initially thought there was a tear in my retina, then it began to look like a dead wingless crane fly. The third one was similar to the first, but smaller and rapidly shimmering in bright psychedelic colors. I have no idea what triggered my ocular migraines and they weren’t associated with headaches, but they are a strange experience.
Hallucinations are not all visual, but can be sounds, smells, tastes, and touch. Most hallucinations are unexpected, transitory, and very personal—only the person experiencing them knows what he or she is seeing, hearing, or feeling, making them very hard to study, but scientists have devised ways of doing it. They can even induce hallucinations by applying electrical stimulation to parts of the brain or by using strobe lights at certain frequencies.
Our neurons are constantly firing. Sometimes a pattern can form at random out of the background noise and manifest as a hallucination. The type of hallucination can depend on the brain area where this arises. Some of the lower visual areas can generate geometric patterns, while higher functions can manifest complex visions that are enhanced as the cognitive areas draw upon memories in attempts to make sense of it all.
Bloody Mary in the mirror
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| Conjuring up Bloody Mary. © John Richard Stephens, 2024. |
Pre-teens and teens sometimes use the Bloody Mary illusion to scare themselves, each other, and to challenge their courage. There are a number of variations on this, but the basic idea is to go into a very dark room that has a mirror. By candlelight, you spin around three times (or seven or thirteen times) while chanting “Bloody Mary” or “Bloody Bones” over and over, or saying, “I believe in Mary Worth”, or shouting “Bloody Mary, I killed your baby!” Then you stare at your reflection in the mirror and Bloody Mary’s pale face with glowing eyes will appear to you. And if she’s seeking revenge, she might just scratch your face off. It’s enough to scare the bejeebers out of any self-respecting tike.
But the thing is, the illusion actually works. You don’t need the spinning or the chanting. You just need a dark room with low light and a mirror you can stare into for ten minutes or longer. Eventually your face fades and your brain fills in the blanks with other things, many of which aren’t very pleasant, especially if you prime yourself to see something horrible.
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| Unchanging stimuli fades from your attention. Stare at the X and the Cheshire Cat’s face fades away leaving only it’s smile. Eventually the smile will fade away as well. The Illusions Index (CC BY-NC_SA 4.0). |
Psychologist Giovanni Caputo of the University of Urbino in Italy, who studied this illusion, sometimes doing it himself, said that “You are suddenly conscious that there is another person behind the mirror.” Adding that the perception of time can speed up or slow down, while everything begins to feel unreal. But he found that some people find it psychologically beneficial, saying it helped them see the face of something they have internalized.[12]
Running on autopilot
Most of our perception of the world runs on autopilot. Much of the information we receive is sparse and distorted. For our primary sense we rely on data coming from the waves of photons that are able to make it through several layers of cells to produce an upside-down and backwards reflection on our curved retina. As we have seen so far, your brain takes the blips of information shuttled to it and enhances, modifies, and distorts it, while casting aside most of what it receives. Using just electromagnetic waves, it creates light, colors, and sounds, none of which exist in the real world. It gauges distances, analyzes motions, and calculates speeds. Then it throws in some expectations and imagination.
Our brains subconsciously figure out what we’re going to perceive by sorting through the possibilities and coming up with its best interpretation, presenting what’s left as a three-dimensional dishonestly honest representation for us to respond to. And, oddly, research shows we believe our brains’ inferences more than what we actually see.[13]
I like the way science writer Sally Adee put it. “We are hallucinating all the time, according to neuroscience research.[...] we are like passengers facing backwards on a train—the world, and all its possible futures, can only be pieced together from what has recently passed.”[14]
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| These are the amazing things personal computers could do in 1983. |
Cognitive scientist Donald Hoffman likens our perceptions to a computer interface. Before graphical user interfaces (GUIs) like Microsoft’s Windows were developed, we had to type in code words and sentences telling the computer what we wanted it to do. Perhaps you’re not old enough to remember C: Prompts, the Three-Fingered Salute, or the Blue Screen of Death. If not, then you’re fortunate. I remember working on a computer in 1977 and ’78 that was housed in its own building that had to be refrigerated to keep the computer cool. Before that I had some punch cards that data used to be stored on. Back in the old days—the 1980s and ’90s—computers had advanced enough that could easily make hours of work vanish...forever... and would often do it when you least expected it.
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| The machine code behind the pictures. John Richard Stephens, 2024, using machine code from BigDumbDinosaur. |
Now we have laptops, cell phones, and smartwatches with icons and images. We move the cursor image and click on the image of what we want, but computers can’t see images. Programmers write source code and then use an app to translate into the machine code that computers can understand. We never see the coded instructions that actually run the computer—just the images and text that we understand.
Hoffman sees perception like this. Reality is the code and what we see are the icons and images. What we see are projections from reality we can understand, but they have no real basis in reality, just as sound is actually waves of vibrations. Our senses evolved as a simplified species-specific GUI, because reality—like computer codes—is way too complicated for us to process. This is why he believes everything we sense is actually an illusion.[15]
Anna Wang Roe, a neuroscientist at Harvard, MIT, and Zhejiang University in Hangzhou, China, largely agrees, saying, “Illusions are not unusual or strange—they are how we interpret the world. We think we know what’s out there in the physical world, but it’s all interpreted by our brains. Everything we sense is an illusion to a degree.”[16]
Neuroscientist Beau Lotto, in his book on perception Deviate: The Science of Seeing Differently, writes:
Alice follows a white rabbit down a hole and ends up in a world in which fantastical things happen. She grows in size; time is eternally stopped for the Mad Hatter at 6 p.m.; the Cheshire Cat’s grin floats in the air, sans the cat. Alice must navigate this bizarre new environment and at the same time maintain her sense of self, no easy task for anyone, let alone a child. The book Alice in Wonderland underscores the virtue of being adaptive when confronting shifting circumstances. From the perspective of neuroscience, however, there is a much more powerful lesson: We’re all like Alice all the time[...] except that we didn’t have to drop through the rabbit hole. We’re already deep inside it.[17]
Click here for the next article in this series:
[1] eLife. “Humans rely more on ‘inferred’ visual objects than ‘real’ ones”, ScienceDaily, www.sciencedaily.com/releases/2017/05/170516080752.htm, citing Benedikt V. Ehinger, Katja Häusser, José P Ossandón, and Peter König, “Humans treat unreliable filled-in percepts as more real than veridical ones”, eLife, 2017; 6, https://doi.org/10.7554/eLife.21761.
[2] Sally Adee, “Bring on the new”, New Scientist, no. 3393, July 2, 2022, p. 36, and as “The Men review: Gender dystopia in a world where men have vanished”, June 29, 2022, https://www.newscientist.com/article/mg25533932-300-the-men-review-gender-dystopia-in-a-world-where-men-have-vanished/.
[3] Donald Hoffman, The Case Against Reality, New York: W.W. Norton & Co., 2019.
[4] Vanderbilt University, “Brain Maps Perceptions, Not Reality”, ScienceDaily, November 4, 2003, http://www.sciencedaily.com/releases/2003/11/031104063920.htm.
[7] Bielefeld University. “How the brain leads us to believe we have sharp vision” ScienceDaily, October 17, 2014. www.sciencedaily.com/releases/2014/10/141017101339.htm, citing Arvid Herwig and Werner X. Schneider, “Predicting object features across saccades: Evidence from object recognition and visual search”, Journal of Experimental Psychology: General, 2014; 143 (5), 1903, https://doi.org/10.1037/a0036781.
[8] Nadine Dijkstra, “The Fine Line Between Reality and Imaginary”, Nautilus, July 28, 2021, https://nautil.us/issue/104/harmony/the-fine-line-between-reality-and-imaginary.
[9] Vilayanur S. Ramachandran and Diane Rogers-Ramachandran, “Ambiguities and Perception”, Scientific American Mind, November/December 2007, pp. 18-20.
[10] D.H. ffytche, R.J. Howard, M.J. Brammer, A. David, P. Woodruff, and S. Williams, “The Anatomy of Conscious Vision: An fMRI Study of Visual Hallucinations”, Nature Neuroscience, December 1998, vol. 1, no. 8, pp. 738-42, https://www.nature.com/articles/nn1298_738, https://doi.org/10.1038/37.
[11] Oliver Sacks, Hallucinations, New York: Alfred A. Knopf, 2012.
[12] Tiffany O'Callaghan, “Oliver Sacks: I want to de-stigmatise hallucinations” (interview), New Scientist, no. 2889, October 31, 2012, pp. 28-29,
[13] Tiffany O'Callaghan, “Oliver Sacks: I want to de-stigmatise hallucinations” (interview), New Scientist, no. 2889, October 31, 2012, pp. 28-29,
[14] Helen Thomson, “Making things up”, New Scientist, no. 3098, November 5, 2016, pp. 29-32, and as “You are hallucinating right now to make sense of the world”, https://www.newscientist.com/article/mg23230980-400-you-are-hallucinating-right-now-to-make-sense-of-the-world/.
[15] “Letters”, Scientific American, December 2008, p. 16, and without the picture at https://www.scientificamerican.com/article/water-cycle-fighting-hunger-migra/.
[16] Susana Martinez-Conde and Stephen L. Macknik, “Illusory Scenes Fade into and out of View”, Scientific American Mind, July 31, 2013,
[17] Douglas Heaven, “Mirror, mirror”, New Scientist, no. 2941, November 2, 2013, pp. 39-41, and as “The Halloween trick that conjures ghosts of the mind”, www.newscientist.com/article/dn24466-the-halloween-trick-that-conjures-ghosts-of-the-mind.html.
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