Grad Student's Work Helps Confirm New Look at Sight

William James, the founder of modern psychology, wrote in 1890 that "everyone knows what attention is."

But 110 years later, the debate over exactly how the brain decides where to focus its visual energies continues to flourish.

A new study published this month in the journal Nature by scientists from the Harvard Vision Sciences Laboratory and Rutgers University supports the idea that the brain breaks up the visual field into discrete objects and catalogues all the properties of each object together.

Alex O. Holcombe, the Harvard member of the research team, explains the findings by waving his ballpoint pen in the air.

The experiments show, he explains, that if an observer looks at the pen and tries to focus only on the "Papermate Fine Point" label and not the rest of the pen, he cannot. The brain perceives the pen as a distinct object and notes all of its properties together--whether people like it or not.

"You process all of the pen, not just the color of it...not just the writing part, even though you may not want to do that," Holcombe says.

The team's discovery is the latest development in decades of research on how just how people select certain bits of visual information for special attention.

When subjects in psychology experiments are asked to pay attention to one visual cue and ignore everything else, dramatic results can take place.

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.

Duncan interpreted the fast response time as suggesting that people perceive each square as a discrete, coherent unit and record various information about it together, allowing for quick recall.

But all the research that has fleshed out his work over the years has shared one major problem: the objects that the experiments contrasted were in different locations, so the assay could not completely discriminate between the location- and object- based theories of attention.

It took a combination of the expertise on visual attention of Rutgers Professor Zenon W. Pylyshyn and experimental methodology developed by Holcombe in the Harvard Vision Sciences Laboratory to resolve this problem.

For the research that led to his Ph.D. dissertation, Holcombe developed a technique in which two Gabor patches--spinning, ridged, colorful circles made out of sine waves--are superimposed on top of each other on a computer screen.

His technique is a modernization of the classic optical illustration developed by Edgar Rubin in which viewers see either a vase or two faces depending on how they direct their attention. The viewers perceive one of the two patches as being in the foreground and the other as being in the background by choosing where to direct their attention.

At Rutgers, the vision scientists exploited Holcombe's technology in experiments where a viewer was asked to pay attention to one of the two Gabor patches as the color, spin and stripe size of both were changed randomly.

Sometimes, the randomly changing attributes overlapped--both patches turning red at the same instant for example--which made following along a little tricky. But 90 percent of the time, subjects could identify which patch was which at the end of the experiment.

Because the features of the patches changed as the experiment went on, the researchers ruled out the possibility that observers kept track of them by a constant difference in their characteristics.

And since the superimposed objects shared the same location, the researchers were able to show that the subjects were not differentiating between patches based on where they were.

Instead, they concluded that the brain thinks of the patches as discrete, coherent objects, keeping tabs on all their properties together, and using that information to track them as they morph.

To confirm that this was the case, the researchers then tried to force people to pay attention to both of the patches at the same time.

In the previous experiment, the spin, color, and line spacing of the patches changed smoothly. But in this experiment, the researchers occasionally inserted simultaneous jumps in the gradients of all the characteristics and asked the subjects to keep track of these discontinuities.

The findings mirror those claimed by Duncan. When subjects were asked to note jumps in one characteristic of each patch, forcing them to pay attention to both patches, they were unable to do so accurately. But when asked to note two jumps for a single patch, the subjects were able to do so with great accuracy

Finding out lots of information about the same object is easy because objects are the units of visual information, the researchers say.

Holcombe, who will soon move to the University of California at San Diego to become a postdoctoral fellow, was amazed by the team's ability to experimentally disconnect object and location.

"It's a new human capability," he said. "We've discovered something humans can do that we didn't known they could do."