The left and right brain may be performing distinct functions in facial processing, says Professor Ming Meng of Dartmouth’s Department of Psychological and Brain Sciences. The two hemispheres of the cerebral cortex are known to divide language processing, a distinction that bears some correlation to right- and left-handedness. Visual processing, by contrast, has been thought to the product of parallel processing between hemispheres of the brain at the fusiform gyri of the temporal lobe, with little distinction in function. Meng, Tharian Cherian, Gaurav Singal and Pawan Sinha published a summary of their findings to the contrary, “Lateralization of face processing in the human brain,” in Proceedings of the Royal Society Biological Sciences (January 4, 2012; e. pub ahead of press).
MRI scans of test subjects were used to measure activity levels in distinct regions of the hemispheres while subjects were shown various photographs. The subjects of these photographs included faces, objects that did not look like faces and non-face objects that were deemed similar to faces by viewers. Changes in the activation pattern of the left fusiform gyrus were observed to correlate linearly with the scale of photograph face-similarity, suggesting that the left hemisphere is involved in determining degrees of face-similarity. By contrast, the right fusiform gyrus was unaffected by false alarms, but changed its activity significantly in response to true face images. Alongside the graded response of the left hemisphere to faces and face-like images, the right hemisphere appears to be capable of making categorical judgments about the presence of true faces in visual images.
The researchers experimentally verified this finding by taking portions of images judged to be face-like and providing context by revealing incrementally more of the image. In the left fusiform gyrus, the provision of true context did not alter response to non-face like images; the context also appeared to decrease pattern correlations to face-like false images and increase the response to true face images. Meanwhile, the patterns of activity in the right fusiform gyrus remained unchanged in the faces including additional information. Presumably, the images that previously appeared face-like to the left hemisphere were eliciting less of a response, while the right side of the brain continued its accurate processing unimpeded.
Given that the left and right fusiform gyri serve different roles, would they be expected to operate simultaneously? Temporal MRI data indicated that the right fusiform gyrus sustained its response 8 seconds after beginning exposure to the stimulus; by this time, the activity of the left fusiform gyrus had already reset. These results, according to Meng and his coauthors, suggest that higher level processing activity is present inside of the right hemisphere.
In their publication, Meng and his collaborators emphasize the use of multivariate analysis for teasing out functional asymmetries in the brain. In the case of visual processing, the change in the intensity of activity was found to be the same between the two halves of the brain. Only by pattern analysis could the two be distinguished. Moreover, the use of graduation in the face resemblance of images was instrumental in detecting the difference in processing between hemispheres, which otherwise have similar responses to non-face and true face images. The researchers suggest that further study concentrated on determining the role of cross-communication between the distinctive fusiform gyri. While face recognition is a good test for visual processing, much remains to be learned about the ways in which we view the world.
