The human brain is capable of locating imagines that resemble faces everywhere we look, whether it’s Jesus’ face on a tortilla or New Hampshire’s erstwhile granite “Old Man of the Mountain.” Although, according to a study by Pawan Sinha, professor of brain and cognitive sciences at MIT and his colleagues, the normal human brain almost never believes such objects are really human faces. The study was published January 4 in the Proceedings of the Royal Society B.
Sinha explains:
“You can tell that is has some “faceness” to it, but on the other hand, you’re not misled into believing that it is a genuine face.”
According to the investigation the left side of the brain, the fusiform gyrus – a region of the brain linked to face recognition – carefully determines how much an image resembles a face, while the right fusiform gyrus appears to use that data to quickly determine whether or not the face is genuine.
Even though hemispheric differences have been observed in other brain functions, such as language and spatial perception, this is one of the first known examples of the left and right sides of the brain taking on different roles in high-level visual-processing tasks, said Sinha.
Previous investigations have demonstrated that neurons located on the brain’s underside in the fusiform gyrus, respond preferentially to face.
In order to find out how this region of the brain determines whether faces are genuine or not, particularly when an object strikingly resembles a face, the team created a sequence of pictures that ranged from genuine faces to those that did not resemble faces. Images that greatly resembled human faces were found by analyzing photographs that machine vision systems had mistakenly tagged as faces.
These images were then shown to human observers who then rated how much each image resembled a face by doing a series of one-to-one comparisons. These results allowed the investigators to rank the images by how “facelike” they were.
Functional magnetic resonance imaging (fMRI) was then used to scan the participants brains as they categorized the images. The team discovered that on the right side of the brain, activation patterns within the fusiform gyrus remained quite consistent for all genuine face images, but changed significantly for all non-face images, regardless of how much they looked like a face, indicating that the right side of the brain is involved in determining whether a face is genuine or not.
In the analogous region on the left side of the brain, the team found that as images became more facelike, activity patterns changed gradually. In addition they discovered that there was no clear divide between non-faces and faces. The team concluded that the left side of the brain is responsible for determining how much an image resembles a face, but does not assign them to one category or the other.
Sinha explained:
“From the computational perspective, one speculation one can make is that the left does the initial heavy lifting. It tries to determine how facelike is a pattern, without making the final decision on whether I’m going to call it a face.”
Imaging-analysis technology allowed the researchers to examine activity patterns across the fusiform gyrus.
The team discovered that in the left side of the fusiform gyrus, activation was faster than the right side by a few seconds, backing the theory that the left side obtains information first and then transfers the data to the right side.
Sinha explains that given the sluggishness of fMRI signals (which rely on blood-flow changes), the timing does not yet constitute definitive evidence, “but it’s a very interesting possibility because it begins to tease apart this monolithic notion of face processing. It’s now beginning to get at what the constituents are of that overall face-processing system.”
The team’s goal is to get more evidence of temporal associations between the two sides with investigations using magnetocencephalography (MEG) or electroencephalography (EEG), two technologies that provide a much more accurate view of the timing of brain activity.
In addition, the researchers hope to find how and when the two hemispheres develop these independent functions by examining blind children who have their sight restored at a young age. Several children have received treatment by Project Prakash, an effort started by Sinha in order to find and treat blind children in India.
Ming Meng lead author of the report is a former postdoc in Sinha’s lab and currently assistant professor at Dartmouth College. Other authors are Tharian Cherian ’09 and Gaurav Singal, who recently earned an MD from the Harvard-MIT Division of Health Sciences and Technology and is currently a resident at Massachusetts General Hospital.
Written by Grace Rattue