Here’s a fun experiment: Stick your finger in your eye. Well, not exactly. The experiment actually involves closing one eye, focusing the open eye on a target stimulus, like this letter O, and then gently pressing on the upper eyelid of that open eye. Give it a try.
It looks like the world moves a little, right? As you gently press back and forth on your eyelid, your field of view with that eye seems to move back and forth. Importantly, this is not because your eye is moving. In fact, the pattern of light landing on your retina is not shifting back and forth at all. The appearance of motion is an illusion. You can prove this to yourself by fixating that letter O and pressing very slowly. You’ll notice that the center of the O stays in the center of your vision.
What’s actually happening is that your eye muscles are fighting back against your finger’s pressure, thus keeping your fovea (the center of your retina) fixated on that letter O. And the neural signal for the flexing of those eye muscles tells your visual cortex that the eye is moving (because that’s usually what happens when those eye muscles flex). So your visual cortex accommodates for the assumed eye movement, even though the eye is not actually moving, and the result is an illusion that the visual scene is what’s moving.
It was physician and physicist Hermann von Helmholtz who first studied in depth the physiological mechanisms of this eye-press experiment in 1866. But during the turn of our recent century, it was psychophysicist Bruce Bridgeman who used it to discover how we perceive our visual environment. The late great Bruce Bridgeman was a professor of psychology at the University of California, Santa Cruz, from 1973 to 2016; and chief editor of the scientific journal Consciousness and Cognition for those last six years. Balding and sporting a grey goatee, and riding his bicycle to campus every day, he looked a bit like the German psychologists whose work he built upon.
Professor Bridgeman carried out controlled laboratory experiments with this eye-press method and determined that the neural signal about those flexing eye muscles was slightly inaccurate. That is, the information sent to the visual cortex about how much the eyes are moving (or trying to move) is rather noisy. So when your eyes naturally jump from object to object in a visual scene, the neural signal indicating the location they’ve moved to for each little snapshot of vision is not actually providing accurate information for the relative locations of those objects you looked at.
Now let’s do a second experiment. Look around your environment for a few seconds, then close your eyes, and generate a mental image of your environment. Your mental image looks pretty rich and detailed, right? Even though each snapshot of the scene that your eyes collect as they jump around is a tiny fragment of the full scene, it seems like your brain can assemble those tiny snapshots together like a jigsaw puzzle in your mind. It’s almost like you have a “mental model” of the entire visual scene, stored inside your brain. When you open your eyes and try to compare your mental model with the real world, it feels like the relative locations of objects in your mental image are pretty accurate. But how can that be if the eye-movement information about those relative locations was noisy and inaccurate to begin with?
While your eyes were closed, if someone moved one of those objects to a new location, or removed it completely, you would have noticed it when you opened your eyes again, right? Actually, there’s a pretty good chance that you wouldn’t notice the change. Bruce Bridgeman’s eye-press experiments helped set the stage for a cottage industry of research called “change blindness,” where people often seem to be blind to changes that are made in a visual scene. While it may seem like we have a detailed and accurate “mental model” of the visual world around us, much of this research suggests that this mental model is actually rather sparse and undetailed. It is as if the assembling of those snapshots was done with a noisy and inaccurate neural signal regarding what their relative locations should be. A jigsaw puzzle put together slightly wrong, and with lots of gaps, that we apparently don’t notice.
But if you don’t have a mental model that is detailed and accurate, and any place that your eyes go provides only a tiny snapshot of the scene, how is it that you can go about your daily life feeling like you are visually aware of the entire environment in a detailed and accurate manner? Where is that detail and accuracy coming from if not from your brain?
Vision researchers Kevin O’Regan (CNRS, Paris), Mary Hayhoe (U. Texas, Austin), and Dana Ballard (U. Texas, Austin), among others, have been zeroing in on an answer to that question. The detail and accuracy for your perceptual experience of the visual environment comes not from your brain but from the environment itself. You don’t actually have stored in your brain the color of that coffee cup in your peripheral vision. Color perception is notoriously bad in the periphery of your field of view. What you do have stored in your brain is the approximate location of that coffee mug. And when it occurs to you to wonder what color the cup is (perhaps because I just brought it up), a fraction of a second later your eyes are fixating the cup and perceptually accessing that color information from the environment – not cognitive accessing it from your “mental model.” The eyes gather that information so fast that it often feels like you knew it all along.
We use our environment as a memory database all the time. Not just for localizing objects and accessing their details, but also for remembering plans. We make “To-Do” lists for ourselves. We arrange appliances and drawers in the kitchen in a fashion that optimizes their use. Some of us hang our car keys by the front door so that the environment perceptually reminds us – at exactly the right time – to take those keys with us as we leave the house. We intelligently arrange our environments to do some of our thinking for us. The environment serves as a mental resource in a fashion similar to how various brain regions serve as a mental resource.
Bruce Bridgeman’s careful psychophysical experiments helped lead to the way to a theoretical framework that philosopher Andy Clark (Edinburgh U.) calls “extended cognition.” In this view, your mind is generated not just by your brain, but also by the continuous interaction between your brain, body, and environment. In the same way that the detail and accuracy in your visual experience comes substantially from outside of your body, many of your mental functions involve cognitive processes that happen outside of your body. Your environment really is part of your mind.