“Reality” is constructed by your brain. Here’s what that means, and why it matters.
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Fix your gaze on the black dot on the left side of this image. But wait! Finish reading this paragraph first. As you gaze at the left dot, try to answer this question: In what direction is the object on the right moving? Is it drifting diagonally, or is it moving up and down?
Remember, focus on the dot on the left.Courtesy of Patrick Cavanagh
It appears as though the object on the right is moving diagonally, up to the right and then back down to the left. Right? Right?! Actually, it’s not. It’s moving up and down in a straight, vertical line.
See for yourself. Trace it with your finger.
This is a visual illusion. That alternating black-white patch inside the object suggests diagonal motion and confuses our senses. Like all misperceptions, it teaches us that our experience of reality is not perfect. But this particular illusion has recently reinforced scientists’ understanding of deeper, almost philosophical truths about the nature of our consciousness.
“It’s really important to understand we’re not seeing reality,” says neuroscientist Patrick Cavanagh, a research professor at Dartmouth College and a senior fellow at Glendon College in Canada. “We’re seeing a story that’s being created for us.”
Most of the time, the story our brains generate matches the real, physical world — but not always. Our brains also unconsciously bend our perception of reality to meet our desires or expectations. And they fill in gaps using our past experiences.
All of this can bias us. Visual illusions present clear and interesting challenges for how we live: How do we know what’s real? And once we know the extent of our brain’s limits, how do we live with more humility — and think with greater care about our perceptions?
Rather than showing us how our brains are broken, illusions give us the chance to reveal how they work. And how do they work? Well, as the owner of a human brain, I have to say it’s making me a little uneasy.
Where the conflict between perception and reality lies in the brain
My colleague Sigal Samuel recently explored the neuroscience of meditation. During her reporting, she found good evidence that a regular meditation practice is associated with increased compassion. That evidence, she writes, “feel[s] like a challenge, even a dare. If it takes such a small amount of time and effort to get better at regulating my emotions … am I not morally obligated to do it?”
Perception science, for me, provokes a similar question. If the science tells us our brains are making up a “story” about reality, shouldn’t we be curious about, and even seek out the answers to, how that reality might be wrong?
It’s not about doubting everything that comes through our senses. It’s about looking for our blind spots, with the goal of becoming better thinkers. It can also help with empathy. When other people misperceive reality, we may not agree with their interpretation, but we can understand where it comes from.
To approach this challenge, I think it helps to know that the brain is telling us stories about the smallest things we perceive, like the motion of objects. But it also tells us stories about some of the most complex things we think about, creating assumptions about people based on race, among other social prejudices.
Let’s start with the small.
In 2019, Cavanagh and his colleagues Sirui Liu, Qing Yu, and Peter Tse used the above “double drift” illusion of the two dots to probe how our brains generate the illusory diagonal motion. To figure this out, Cavanagh and his colleagues ran a neuroimaging study that compared how a brain processes the illusory animation with how it processes a similar, non-illusory animation. In this second animation, the object on the right really is moving diagonally. Trace it with your finger again.Courtesy of Patrick Cavanagh
With fMRI neuroimaging, which allows researchers to map brain activity, Cavanagh and his team could ask the question: If we perceive each animation similarly, what in our brains makes that happen? What’s the source of the illusion in the first animation? “We want to find where the conscious perception diverges from the physical sensation,” Cavanagh says.
One possibility is that the illusion is generated in the visual cortex. Located at the back of your head, this is the part of your brain that directly processes the information coming from your eyes. Maybe the visual system “sees” it wrong. The alternative is that the visual system “sees” it just fine, but some other part of the brain overrides it, creating a new reality.
The experiment included only nine participants but collected a lot of data on each of them. Each participant completed the experiment (and was run through the brain scan) 10 times.
Here’s what the analysis found. That visual system in the back of the brain? It doesn’t seem fooled by the illusion. Each animation produces a different pattern of activation in the visual cortex. In other words, “the visual system thinks they are different,” Cavanagh says.
Okay, the visual system correctly “sees” these two animations differently. Then why do we perceive them as being the same?
The patterns of activation in the frontal lobes of the participants’ brains — the higher-level thinking area dedicated to anticipation and decision-making — were similar. That is: The front of the brain thinks both animations are traveling in a diagonal direction.
“There’s a whole world of visual analysis and computation and prediction that is happening outside of the visual system, happening in the frontal lobes,” Cavanagh says. That’s where the “story” of reality is constructed — at least in this one example, as evidenced by this one small study. (To be sure: Vision is a vastly complex system involving around 30 areas of the brain. There are other illusions that do seem to “fool” the visual cortex, because no story about the brain can be simple.)
But you don’t need an fMRI to conclude that some part of your brain is overriding the plain truth about the path of the object. You can see it for yourself. “The remarkable thing is that — even when you are told what is happening — you still see it in the illusory form,” Justin Gardner, a Stanford University neuroscientist who wasn’t involved in this study, said in an email. “You can’t seem to consciously override the ‘wrong’ interpretation.”So many illusions work like this: Even when you’re told about the trick, you can’t unsee the illusion. Take the classic checker-shadow illusion by Edward Adelson. Squares A and B are the exact same shade of gray when seen side by side. But when B is cast in an apparent shadow and surrounded by apparently darker tiles, it just looks lighter. There’s nothing about the physical construction of our eyes that would cause this effect, I’m told. The apparent lightening of tile B is a story told by our brains. Courtesy of Edward H. Adelson
The lesson: The stories our brains tell us about reality are extremely compelling, even when they are wrong.
We’re not seeing reality. Our vision runs 100 milliseconds behind the real world.
Why are we seeing a story about the world — a story — and not the real deal? It’s not because evolution made our minds flawed. It’s actually an adaptation.
“We don’t have the necessary machinery, and we wouldn’t even want it, to process carefully all of the amount of information that we’re constantly bombarded with,” says Susana Martinez-Conde, a neuroscientist and illusion researcher at SUNY Downstate Medical Center.
Think about what it takes to perceive something move, like the objects in the above animations. Once light hits the retinas at the back of our eyeballs, it’s converted into an electrical signal that then has to travel to the visual processing system at the back of our brains. From there, the signal travels forward through our brains, constructing what we see and creating our perception of it. This process just takes time.
“The dirty little secret about sensory systems is that they’re slow, they’re lagged, they’re not about what’s happening right now but what’s happening 50 milliseconds ago, or, in the case for vision, hundreds of milliseconds ago,” says Adam Hantman, a neuroscientist at Howard Hughes Medical Institute’s Janelia Research Campus.
If we relied solely on this outdated information, though, we wouldn’t be able to hit baseballs with bats, or swat annoying flies away from our faces. We’d be less coordinated, and possibly get hurt more often.
So the brain predicts the path of motion before it happens. It tells us a story about where the object is heading, and this story becomes our reality. That’s what’s likely happening with Cavanagh’s illusion. It happens all the time.
Don’t believe it? See for yourself. Here’s a simple illusion that reveals our visual system is a bit lagged.
It’s called the flash-lag illusion. The red dot is moving across the screen, and the green dot flashes exactly when the red dot and green dot are in perfect vertical alignment. Yet it’s incredibly hard to see the red dot and the green dot as being vertically aligned. The red dot always seems a little bit farther ahead.LaurentPerrinet/Wikimedia Commons
This is our brain predicting the path of its motion, telling us a story about where it ought to be and not where it is. “For moving things — we see them ahead on their path of motion,” Cavanagh explains, “by just enough.” The illusion, he says, “is actually functional. It helps us overcome these delays and see things … where they will be when we get there.”Cavanagh and Stuart Anstis of UCSD have designed a more elaborate version of the flash-lag illusion. In the above GIF, you’ll see flashing red and blue boxes. The boxes are the same size and positioned in the same place, yet the red box seems smaller. It’s the motion of the background that confuses us. “The visual system assumes [the boxes] are moving too, and we get to see them where they would be if they had continued with the motion of the background,” Cavanagh says. Courtesy of Stuart Anstis
In Hantman’s view, what we experience as consciousness is primarily the prediction, not the real-time feed. The actual sensory information, he explains, just serves as error correction. “If you were always using sensory information, errors would accumulate in ways that would lead to quite catastrophic effects on your motor control,” Hantman says. Our brains like to predict as much as possible, then use our senses to course-correct when the predictions go wrong.
This is true not only for our perception of motion but also for so much of our conscious experience.