how much of this reality is a construction of your brain, taking place only inside your head?
Consider the rotating snakes, below. Although nothing is actually moving on the page, the snakes appear to be slithering. How can your brain perceive motion when you know that the figure is fixed in place?
Nothing moves on the page, but you perceive motion. Rotating Snakes illusion by Akiyoshi Kitaoka.
Compare the color of the squares marked A and B. Checkerboard illusion by Edward Adelson.
Or consider the checkerboard above.
Although it doesn’t look like it, the square marked A is exactly the same color as the square marked B. Prove this to yourself by covering up the rest of the picture. How can the squares look so different, even though they’re physically identical?
Illusions like these give us the first hints that our picture of the external world isn’t necessarily an accurate representation. Our perception of reality has less to do with what’s happening out there, and more to do with what’s happening inside our brain.
Your experience of reality
It feels as though you have direct access to the world through your senses. You can reach out and touch the material of the physical world – like this book or the chair you’re sitting on. But this sense of touch is not a direct experience. Although it feels like the touch is happening in your fingers, in fact it’s all happening in the mission control center of the brain. It’s the same across all your sensory experiences. Seeing isn’t happening in your eyes; hearing isn’t taking place in your ears; smell isn’t happening in your nose. All of your sensory experiences are taking place in storms of activity within the computational material of your brain.
Here’s the key: the brain has no access to the world outside. Sealed within the dark, silent chamber of your skull, your brain has never directly experienced the external world, and it never will.
Instead, there’s only one way that information from out there gets into the brain. Your sensory organs – your eyes, ears, nose, mouth, and skin – act as interpreters. They detect a motley crew of information sources (including photons, air compression waves, molecular concentrations, pressure, texture, temperature) and translate them into the common currency of the brain: electrochemical signals.
These electrochemical signals dash through dense networks of neurons, the main signaling cells of the brain. There are a hundred billion neurons in the human brain, and each neuron sends tens or hundreds of electrical pulses to thousands of other neurons every second of your life.
Neurons communicate with one another via chemical signals called neurotransmitters. Their membranes carry electrical signals rapidly along their length. Although artistic renditions like this one show empty space, in fact there is no room between cells in the brain – they are packed tightly against one another.
Everything you experience – every sight, sound, smell – rather than being a direct experience, is an electrochemical rendition in a dark theater.
How does the brain turn its immense electrochemical patterns into a useful understanding of the world? It does so by comparing the signals it receives from the different sensory inputs, detecting patterns that allow it to make its best guesses about what’s “out there”. Its operation is so powerful that its work seems effortless. But let’s take a closer look.
Let’s begin with our most dominant sense: vision. The act of seeing feels so natural that it’s hard to appreciate the immense machinery that makes it happen. About a third of the human brain is dedicated to the mission of vision, to turning raw photons of light into our mother’s face, or our loving pet, or the couch we’re about to nap on. To unmask what’s happening under the hood, let’s turn to the case of a man who lost his vision, and then was given the chance to get it back.
I was blind but now I see
Mike May lost his sight at the age of three and a half. A chemical explosion scarred his corneas, leaving his eyes with no access to photons. As a blind man, he became successful in business, and also became a championship paralympic skier, navigating the slopes by sound markers.
Then, after over forty years of blindness, Mike learned about a pioneering stem cell treatment that could repair the physical damage to his eyes. He decided to undertake the surgery; after all, the blindness was only the result of his unclear corneas, and the solution was straightforward.
But something unexpected happened. Television cameras were on hand to document the moment the bandages came off. Mike describes the experience when the physician peeled back the gauze: “There’s this whoosh of light and bombarding of images on to my eye. All of a sudden you turn on this flood of visual information. It’s overwhelming.”
SENSORY TRANSDUCTION
Biology has discovered many ways to convert information from the world into electrochemical signals. Just a few of the translation machines that you own: hair cells in the inner ear, several types of touch receptors in the skin, taste buds in the tongue, molecular receptors in the olfactory bulb, and photoreceptors at the back of the eye.
Signals from the environment are translated into electrochemical signals carried by brain cells. It is the first step by which the brain taps into information from the world outside the body. The eyes convert (or transduce) photons into electrical signals. The mechanisms of the inner ear convert vibrations in the density of the air into electrical signals. Receptors on the skin (and also inside the body) convert pressure, stretch, temperature, and noxious chemicals into electrical signals. The nose converts drifting odor molecules, and the tongue converts taste molecules to electrical signals. In a city with visitors from all over the world, foreign money must be translated into a common currency before meaningful transactions can take place. And so it is with the brain. It’s fundamentally cosmopolitan, welcoming travelers from many different origins.
One of neuroscience’s unsolved puzzles is known as the “binding problem”: how is the brain able to produce a single, unified picture of the world, given that vision is processed in one region, hearing in another, touch in another, and so on? While the problem is still unsolved, the common currency among neurons – as well as their massive interconnectivity – promises to be at the heart of the solution.
Mike’s new corneas were receiving and focussing light just as they were supposed to. But his brain could not make sense of the information it was receiving. With the news cameras rolling, Mike looked at his children and smiled at them. But inside he was petrified, because he couldn’t tell what they looked like, or which was which. “I had no face recognition whatsoever,” he recalls.
In surgical terms, the transplant had been a total success. But from Mike’s point of view, what he was experiencing couldn’t be called vision. As he summarized it: “my brain was going ‘oh my gosh’”.
With the help of his doctors and family, he walked out of the exam room and down the hallway, casting his gaze toward the
