Rapid Attention Shifts Aid Thought Processing.

• Position: Brief Article

Finding a friend in a crowd, searching for a nickel in a pile of coins, and locating a jar of mustard in the refrigerator are examples of visual search tasks. Two University of Iowa, Iowa City, researchers have made an important step forward in understanding how these tasks are performed by the human visual system.

Geoffrey F. Woodman, a graduate student, and Steven J. Luck, associate professor of psychology, found that, when humans look at collections of objects, attention is focused on a single object at a time and moves from one to the next in about a tenth of a second. Research has shown that the more complex a visual image is, the longer it takes for the human brain to process the entire scene. Scientists have long debated the reason for this, with some arguing that the longer processing time indicates serial processing and others believing that it indicates parallel processing.

As an analogy, Luck notes that serial processing is like a construction company with 100 employees who work together to build one house at a time, whereas parallel processing is like a company that erects 10 houses simultaneously with 10 employees working on each house. Similarly, the human brain might devote all of its capacity to one object at a time and shift rapidly from one to the next, or it might process all of the objects in a visual scene nearly simultaneously, with just a fraction of the brain's capacity devoted to each.

Woodman and Luck are the first to show definitively that the human brain processes complex images serially--attending to one thing at a time, but shifting rapidly from object to object. "This may seem counterintuitive because it doesn't feel like we perceive only one object at a time," Luck indicates. Although it may seem that when you look at a scene you are seeing the whole picture, each object in the scene commands your attention in rapid succession.

The researchers demonstrated serial processing by having subjects search for a specific object in a display containing many other...