WASHINGTON — Ever wonder why it's so much easier to remember people's faces than their names?
Neuroscientists have an explanation. They've identified a pea-sized region in the brain that reacts more strongly to faces than it does to cars, dogs, houses or body parts.
"The evidence is overwhelming that there is a specialized system dedicated to processing faces and not other objects," said Doris Tsao, a neuroscientist at Massachusetts General Hospital in Boston.
It's called the "fusiform face area," because it vaguely resembles a spindle — "fusus" in Latin. It's about halfway back in the head, near the bottom of the visual cortex, the part of the brain than handles vision.
(Actually, most people have two FFAs, one on each side of the head, but the one on the right is dominant, the other a backup.
The FFA system explains "why we are so good at recognizing and remembering faces," Tsao said. "A brief glance at a face conveys a wealth of information about identity, expression, gender, age, mood, intent, attractiveness, social states and even honesty."
Researchers say evolution may explain why humans and other primates developed a chunk of brain tissue dedicated to face recognition — it helped them quickly spot friends and foes. Our ancestors who were better able to distinguish faces tended to survive — and reproduce — better than others.
"The ability to extract this information within a fraction of a second of viewing a face is important for normal social interaction, and has probably played a critical role in the survival of our primate ancestors," said Nancy Kanwisher, an investigator at the McGovern Institute for Brain Research in Cambridge, Mass.
Understanding how face recognition works can have practical applications, Tsao said. Insights into these brain circuits may help prevent or treat depression, autism or social disorders. FFA images also may help scientists understand prosopagnosia, an impairment in the recognition of faces that's also known as face blindness.
People with prosopagnosia "are completely baffled why robbers cover their faces in movies," Tsao said.
Scientists identified the FFA a decade ago by scanning human and monkey brains with a then-new technology known as functional magnetic resonance imaging (fMRI). When a group of nerve cells, or neurons, are active — such as neurons responsible for vision — blood rushes to that area and it lights up on an fMRI scan.
"Blood flow mimics electrical activity," said Rebecca Saxe, a neuroscientist at the Massachusetts Institute of Technology. "We use blood flow as a proxy."
An fMRI, however, is a blunt instrument. It can observe a three-dimensional volume that contains about 200,000 neurons, but it can't measure individual neurons.
"We're looking at entire neighborhoods, not blocks or streets," Saxe said. "We want to look at individual rooms.''
Recently, researchers have found a way to use finer resolution fMRI to zoom in on smaller bits of the visual cortex. Now they can study a cube of brain tissue one millimeter on a side instead of three millimeters in a standard fMRI. (A millimeter is 0.04 inches.)
"It's like viewing grains of sugar rather than the whole cube," Kalanit Grill-Spector, a neuroscientist at Stanford University in Palo Alto, Calif., wrote in the journal Nature.
As a result, researchers are modifying their view of the face recognition area. When seen with higher precision, more neurons in the FFA recognize faces rather than any other objects, but scattered among them are little patches that react to different objects, such as arms or feet.
"It's no longer just a monolithic face-selective area," said David Ress, a neuroscientist at Brown University in Providence, R.I.
"A region could serve more than one function, said Scott Huettel, a professor of psychiatry at Duke University in Durham, N.C.
In addition to faces, scientists have also identified separate regions of the brain that respond to body parts or to buildings.
"Just by looking at an fMRI, we can tell if a subject is looking at faces or places," Kanwisher said.
FMRI research is valuable, she said, because "it tells us what kinds of minds we have, and that is one of the most fundamental questions I can imagine tackling."
For more information: http://web.mit.edu/bcs/nklab/media/pdfs/RSTB20061934p.pdf