Discriminating neurons pick the right face.

Discriminating neurons pick the right face

For most of us, a glance is enough to recognize a familiar face. And except in a poker game -- when important protective mechanisms come into play -- a person's facial expression tells us a lot about that person's mood.

Neuroscientists know that two parts of the brain -- the inferotemporal gyrus (ITG) and the superior temporal sulcus (STS) -- are important in recognizing faces and their expressions. Now they've begun pinpointing the brain neurons involved. By identifying and mapping the key neurons responsible for recognition of these complex patters, researchers hope to "teach" similar recognition skills to computerized neural networks. Scientists from California and Japan described their findings this week at the Society for Neuroscience annual meeting in Toronto.

Michael E. Hasselmo of California Institute of Technology in Pasadena and Gordon C. Baylis of the University of California, San Diego, monitored the activity of 45 individual neurons in the brains of two macaque monkeys while the monkeys were shown photographs of other monkeys' faces. One photo at a time, the researchers showed nine pictures of three different monkeys, including three different expressions for each monkey: calm, slightly threatening and strongly threatening. The identified nine neurons significantly associated with recognition of facial expression only, and 15 with recognition of facial identity only. The former were mostly located in the STS and the latter mostly in the ITG, strongly suggesting the two functions are encoded independently in the brain.

These findings may help elucidate the specific mechanisms behind two types of brain disease in humans: prosopagnosia -- in which the affected individual can identify emotions expressed on faces but cannot identify individuals by their faces -- and cerebral organic brain syndrome, where the opposite is true.

Kenji Kawano and his colleagues at the Electrotechnical laboratory in Ibaraki, Japan, monitored 446 neurons in the brains of monkeys trained to recognize three human faces in photos. The researchers measured 21 different indices for each face -- such as distances between nose, eyes and hairline -- then compared neuronal firing when the monkeys tried to recognize composite faces made from parts of three different faces. They found five neurons specifically attuned to particular facial indices.

Despite such neuronal specificities, Baylis and Kawano say, face recognition ultimately must be the result of a complex and still unexplained integration process.