Brain scans track down attention systems.

To search for a friend in a crowded, noisy room, you could look for a single key attribute--say, your comrade's trademark red jacket--or you could scan the crowd for a person with several signature traits, such as moustache, bushy hair and red jacket. Either strategy may work, but different parts of your brain spring into action depending on how you allocate your attention, according to a new brain-imaging study.

The findings indicate that the brain's ability to concentrate on a small fraction of the information that bombards it depends not only on identifying specific stimuli, but also on whether attention focuses on one or several perceptual features of a stimulus, concludes a research team led by neurologist Maurizio Corbetta of Washington University School of Medicine in St. Louis.

"[This] suggests that similar perceptual judgments involve different neural systems, depending on attention strategies," the scientists assert in the just-released August JOURNAL OF NEURO-SCIENCE.

A series of trials with 11 healthy adults first demonstrated superior accuracy at noting a subtle alteration in the speed, color or shape of a visual stimulus when volunteers paid attention only to the feature that changed, rather than to all three attributes at once. Each participant watched a video minitor on which an image appeared for a fraction of a second. The image comtained 30 small squares or rectangles of identical shape and color, moving at a constant speed to the left or right. A second image then flashed on the monitor; in half the trials, the shape, hue or speed of these boxes differed from that of the first image.

During "selective-attention" trials, experimenters told volunteers to look only for changes in a single feature, such as a slight shift in hue. In "divided-attention" trials, participants tried to detect whether a change occurred in any of the three features.

In a second experiment, nine healthy adults completed a series of the same selective- and divided-attention tasks while the researchers monitored blood flow throughout their brains with positron emission tomography (PET) scans. Volunteers received injections of water labeled with minute amounts of a radioactive oxygen isotope. A scanning machine detected positively charged particles emitted by the isotope and translated the data into images of bloodflow activity in the brain.

The PET scans showed that both selective and divided attention activated the primary visual center at the back of the brain. But on selective trials, attention to speed, color or shape each activated different visual regions that assist the primary visual center. Outside the visual system, selective and divided attention also activated different sets of brain regions, Corbetta's group reports.

Overall, the new evidence supports the theory that selective attention does not aid perception by making better use of the same brain mechanism that handles divided attention, the researchers say. Instead, the brain treats the two types of attention in entirely different ways, with specific regions assigned to help process selected visual features.