Friday, July 06, 2007

Don't I Know You?

New research sheds light on how the brain recognizes faces.

By Emily Singer

Facial recognition: Scientists showed images of faces and other objects (pictured above) to monkeys and measured their brains' response using functional magnetic resonance imaging. The researchers identified several regions, highlighted in orange, that respond selectively to faces. Credit: Doris Tsao, University of Bremen, Germany

It only takes a millisecond to recognize celebrities on TV while flipping through the channels: Rachel Ray hawking coffee, David Hasselhoff judging a talent show, or Charles Gibson relaying the latest tragedy in Iraq. While it seems easy, recognizing those faces is a cognitively complex task. Your brain must identify the object you're seeing as a face, regardless of the size or angle; interpret the expression encoded by the particular arrangement of eyes and mouth; and access the memory part of the brain to determine if the face is familiar. By combining two of the most important tools in neuroscience--brain imaging and electrical recordings from single brain cells--scientists are poised to finally understand how the brain performs these complex computations.

"Shape recognition is one of the biggest unsolved questions in visual biology," says David Hubel, an emeritus neuroscientist at Harvard Medical School who won a Nobel Prize for his research on the visual system. "Combining these different techniques has tremendous power."

The visual system works like a series of relay stations. Visual information is fed into the brain via the retina and the optic nerve and is then shunted to different processing centers. This visual information, encoded as neural signals, is continually processed and rerouted--different areas analyze color, movement, and form--and is ultimately summed. This allows the brain to recognize objects, such as a moving truck, a steaming kettle, or a familiar face.

Facial recognition is an extremely important component of human social interaction, and our brains appear to have evolved a special processing center to carry out the complex task. Brain-imaging studies show that a particular region is active when people look at faces as opposed to other objects, such as houses or cars. And a stroke experienced by a particular part of the brain can knock out face-processing ability--a disorder known as prosopagnosia, or face blindness.

However, due to the relatively low resolution of brain-imaging technologies, scientists know little about how the brain actually processes faces. Doris Tsao, a neuroscientist at the University of Bremen, in Germany, aims to change that. To study facial processing step by step, she is combining magnetic resonance imaging (MRI), a brain-imaging technology only recently used in animals, and single-cell electrical recording.

In research published last year, Tsao and her colleagues identified several parts of the brain in monkeys that respond selectively to faces. She then used the detailed anatomical picture generated by MRI to guide an electrode precisely to one of those spots. By recording activity from a number of cells there, she found that different cells are active in response to different facial characteristics--the overall shape of the face or the size of the eyes, for example. This exquisite level of detail would have been impossible to generate using brain imaging alone, and it yields important clues into how our brains detect faces. "The combination of [brain imaging] and electrode recording allowed her to get really amazing insight into the behavior of these face cells," says Hubel.