The pupil changes in size, but doesn't move within eye

"Why don't your pupils ever move?," asked Meghan Rolston, 11, a fifth-grader at Butterfield School in Libertyville.

In a split second, your brain makes millions of decisions about information gathered from the light that penetrates your eyes.

It's your pupil -- the black center of your eye -- that gathers light and directs it to the cells that rush the information to your brain. The pupil is surrounded by the colored part of the eye -- the iris. The iris contains muscles that expand and contract the pupil, permitting more or less light to penetrate.

When you're bathed in bright sunlight, the iris shuts down your pupil to allow just enough light inside so the brain can understand what's going on around you. In a dark room or at night, the pupil is forced to its widest opening so you can avoid jamming your toe on the corner of your bed.

"Pupils don't move within the eye because that's the way the eye is made," said Dr. Robert A. Levine, an optometrist with offices in Libertyville and Lake Zurich.

"The pupil will change with light. It acts like a camera aperture. With bright light it gets smaller; in a darker place, it needs more light, so it gets bigger."

Levine said the pupil has no vision; it's just an opening in the eye. The pupil can't detect and process images. "When you look at a pupil, what you see is the darkness that's in the back of the eye so the pupil appears black," Levine said.

Doctors use the pupil to peer inside the eye and examine the health of the eye tissue. They administer medication that paralyzes the pupil when it's open so it remains at its widest point while the examination takes place. "We can scan the inside of the eye to look for disease," Levine said.

Can people read your mind by examining your pupils? Pupils can become larger when a person sees something they like. Some salespeople believe they can use this to their advantage when bargaining.