In one famous experiment, psychologists asked people to count the number of times men playing basketball in a video passed the ball.

As the game went on in the film, a woman with an umbrella walked across the court. But the subjects, attending to the passed ball and not to the other events in the scene, did not note this strange occurrence.

Still, psychologists differ as to the identity of the units of visual information that the body pays attention to.

Historically, researchers have thought that the brain focuses on specific areas of space, tracking objects that move around the visual field by moving its spotlight of visual attention.

But in the mid-80s, scientists began to consider the possibility that objects themselves were in fact the units of visual attention.

In his classic 1984 experiment, John Duncan presented subjects with two pictures of squares with different kinds of holes in them and different dotted lines through them.

People responded faster when they were asked to give information about the dotted line and hole on a single square than when they were asked to give information about the dotted line on one square and the hole in the other square.