What the Brain Tells the Eye
Do we see what we want to see?
Posted August 30, 2012
Have you ever picked up a gallon milk bottle that you thought was full when it was empty instead? You realize your mistake as soon as you begin to lift the bottle because your hand and bottle fly over your head. Your brain assumed that the bottle was heavier than it was and thus instructed your muscles to exert more force than was necessary. Before we make any voluntary movement, a great deal of planning, which is largely unconscious, takes place in our brain.
The same is true for perception. Since our eyes sense what is around us, it’s easy to think that our visual system is quiescent unless stimulated by something from the outside. However, what we see is governed to a large extent by what we expect to see. As with our movements, our brain sets us up in advance for what we will see.
This idea came home to me one morning when I glanced out my kitchen window at the bird feeder outside. Small woodland birds, such as nuthatches, juncos , and chickadees, were the usual visitors to the feeder. But on this day, I happened to glance up from the kitchen sink and saw five enormous wild turkeys, one male and four females, looking in on me. The male was so tall, he practically looked me in the eye. Despite their large size and distinctive appearance, it took me a full second to figure out what I was seeing. Had I glanced outside and seen the usual juncos and chickadees, I would have recognized and distinguished these birds, despite their small size, in much less time.
So why did it take so long to see the big wild turkeys? Because I didn’t expect to see them. What we see depends to a large extent upon what we anticipate seeing. The first area of our visual cortex to receive input from our eyes is called the primary visual cortex. It was once thought that neurons in this area respond almost exclusively to stimuli coming from the eyes. But we now know that the activity of these neurons is affected by “higher” brain centers which are involved in prediction and planning. When the brain can predict what will be seen, it can prime the appropriate circuits in the primary visual cortex and other regions, allowing us to interpret visual stimuli more quickly. So, when I looked out the kitchen window that morning, my brain may have readied the circuits in my visual cortex for what I expected to see – the usual small birds at the feeder. The image of turkeys threw my visual system into a momentary state of confusion. Some circuits had to be suppressed and others activated in order for me to make sense of the surprising view outside my kitchen window.