Eyes on the Brain

A neurobiologist explores the amazing capacity of the brain to rewire itself at any age.

Susan R. Barry is a professor of neurobiology in the Department of Biological Sciences at Mount Holyoke College and the author of Fixing My Gaze (June, 2009). See full bio

June 12, 2009, Neuroscience

You See it but is it Real?

You see it but is it real?

Try this experiment. It’s easy to do: no special equipment is required although you do need to have good binocular vision.

Hold one index finger about six inches in front of your face. Look in the distance but be aware of the finger. How many fingers do you see?

If you see two images of your finger, don’t worry. This is perfectly normal, a phenomenon called physiological diplopia. As you look in the distance, your eyes turn out to fixate a distant target. As a result, the image of your finger, which is located close to you, falls on a different part of the retina of each eye. Since the finger is imaged on disparate parts of your two retinas, you see it in two places. You have a double image of the finger. Most of the time, we are not aware of these double images, but, in experiments like this one, we can bring the double images into conscious awareness.

So which of the finger images is the real one? Is it the image seen by the right or left eye? Try this: Repeat the finger experiment again but this time, take your free hand and try to grasp each image of your finger. KEEP LOOKING IN THE DISTANCE as you try to grab each finger image. Can you touch both images?

You probably could touch only one finger image. Although you weren’t consciously aware of this, your brain chose one of the images to be “real.” This was the image with a defined location in space, the image that you could reach for and touch.

Does that mean that an image is real only if we can locate it in space and act upon it?

(Note: I discuss this question in my book, Fixing My Gaze, and will write more about it in future posts.)