Plenty of evidence indicates that the recognition of familiar faces depends largely on structures on the right side of the brain’s outer layer, or cortex. However, the brain appears to take a sharp left turn in fostering the ability to identify one’s own face.
That, at least, is the implication of experiments conducted with a so-called split-brain patient. To curb the spread of severe epileptic seizures at age 25, the now-48-year-old man had submitted to a surgical severing of nerve fibers connecting one side of his cortex to the other.
If confirmed in studies of people with intact brains, the new investigation indicates that left-brain networks assume primary responsibility for memories and knowledge about oneself, including the key visual distinction between “me” and “others,” says a team of neuroscientists led by David J. Turk of Dartmouth College in Hanover, N.H. Personal recognition allows for other types of complex thought, such as empathy and introspection, the scientists note.
“These findings suggest a possible [separation of] self-recognition and more generalized face processing within the human brain,” Turk and his colleagues contend in an upcoming Nature Neuroscience.
Glimmers of this neural division of labor have come from several prior studies of people with intact brains. Brain scans showed that recounting personal memories and seeing one’s own face sparked activity in left-hemisphere areas.
Split-brain patients afford a unique test of possible differences in what the hemispheres do (SN: 2/24/96, p. 124). For these individuals, the brain’s left side alone handles information presented in the right half of the person’s visual field, whereas items shown in the left visual field enter only the brain’s right side.
The new study focuses on a patient called JW. In a series of trials, JW saw 11 facial images briefly flashed one at a time and in random order. An equal number of presentations were made to each side of his brain.
One image was that of JW himself. Another showed Dartmouth’s Michael S. Gazzaniga, a study coauthor whom JW knows well. The remaining nine images, generated with computer-morphing software, exhibited varying mixes of JW’s and Gazzaniga’s facial features. For example, one face represented 90 percent JW and 10 percent Gazzaniga, one portrayed 80 percent JW and 20 percent Gazzaniga, and so on.
When asked whether a morphed image showed himself, JW reported much more self-recognition–even for faces containing as little as 30 percent of his own features–on left-hemisphere presentations. When asked whether morphed images showed Gazzaniga, JW recognized the researcher far more frequently in right-hemisphere trials, even when the patient’s own facial features predominated.
JW responded comparably to images of himself morphed with each of three other familiar persons: President George W. Bush, former President Bill Clinton, and one of JW’s close friends.
These findings tap into left-brain contributions to the conscious understanding of oneself, remarks neuroscientist Joseph LeDoux of New York University. However, LeDoux theorizes, a variety of brain systems are involved in self-understanding, most of which function unconsciously.