The grand delusion: What you see is not what you get
Your senses are your windows on the world, and you probably think they do a fair job at capturing an accurate depiction of reality. Don't kid yourself. Sensory perception - especially vision - is a figment of your imagination. "What you're experiencing is largely the product of what's inside your head," says psychologist Ron Rensink at the University of British Columbia in Vancouver, Canada. "It's informed by what comes in through your eyes, but it's not directly reflecting it."
Given the basic features of your visual system, it couldn't be any other way. For example, every 5 seconds or so, you blink. Yet unless you're thinking about it, as you probably are right now, you don't notice the blackouts because your brain edits them out.
Blinking is just the tip of the iceberg. Even when your eyes are open they're only taking in a fraction of the visual information that is available.
In the centre of your retina is a dense patch of photoreceptor cells about 1 millimetre across. This is the fovea, the visual system's sweet spot where perception of detail and colour is at its best. "When you move away from the fovea, visual acuity falls away really quickly, and colour vision disappears," says Rensink. About 10 degrees to the side of the fovea, visual acuity is only about 20 per cent of the maximum.
What that means is you can only capture a tiny percentage of the visual field in full colour and detail at any one time. Hold your hand at arm's length and look at your thumbnail. That is roughly the area covered by the fovea. Most of the rest is captured in fuzzy monochrome.
And yet vision doesn't actually feel like this: it feels like a movie. That, in part, is because your eyes are constantly flitting over the visual scene, fixing on one spot for a fraction of a second then moving on. These jerky eye movements are called saccades and they happen about 3 times a second and last up to 200 milliseconds. With each fixation your visual system grabs a bite of high-resolution detail which it somehow weaves together to create an illusion of completeness.
That's remarkable given that during saccades themselves, you are effectively blind. Your eyes don't stop transmitting information as they lurch from one fixation to the next, but for about 100 milliseconds your brain is not processing it.
Look in the mirror and deliberately flick your eyes from left to right and back again. You won't see your eyes move - not because the movement is too fast (other people's saccades are visible), but because your brain isn't processing the information.
Given that you perform approximately 150,000 saccades every day, that means your visual system is "offline" for a total of about 4 hours during each waking day even without blinking (Trends in Cognitive Sciences, vol 12, p 466). Yet you don't notice anything amiss.
Exactly how your brain weaves such fragmentary information into the smooth technicolour movie that we experience as reality remains a mystery. One leading idea is that it makes a prediction and then uses the foveal "spotlight" to verify it. "We create something internally and then we check, check, check," says Rensink. "Essentially we experience the brain's best guess about what is happening now."
In conjuring up this "now", the visual system has to do something even more remarkable: predict the future. Information striking the fovea cannot be relayed instantaneously to conscious perception: first it has to travel down the optic nerve and be processed by the brain. This takes several hundred milliseconds, by which time the world has moved on. And so the brain makes a prediction about what the world will look like about 200 milliseconds into the future, and that is what you see. Without this future projection you would be unable to catch a ball, dodge moving objects or walk around without crashing into things.
There's another huge hole in the visual system that can render you oblivious to things that should be unmissable. The jerky movements that shift your fovea around the visual scene don't happen at random - they are directed by your brain's attentional system. Sometimes you consciously decide what to attend to, such as when you read. At other times your attention is grabbed by a movement in your peripheral vision or an unexpected noise.
The problem with attention is that it is a limited resource. For reasons that remain unknown, most people are unable to keep track of more than four or five moving objects at once. That can lead your visual system to be oblivious to things that are staring you in the face.
The most famous demonstration of this "inattention blindness" is the invisible gorilla, a video-based experiment created by Daniel Simons and Christopher Chabris at the University of Illinois at Urbana-Champaign. Viewers are asked to pay close attention to a specific aspect of a basketball game, and around half completely fail to see a person in a gorilla suit walk slowly across the screen, beat their chest and walk off again.
