Patterns in the world, like this rock formation in Ebihens, France, can sometimes fortuitously look like human faces. In a new study, Meng et al. have used this phenomenon of pareidolia to investigate how the neural processing of faces differs in the left and right halves of the brain. Image: Erwan Mirabeau
Objects that resemble faces are everywhere. Whether it's New Hampshire's erstwhile granite "Old Man of the Mountain" or Jesus' face on a tortilla, our brains are adept at locating images that look like faces. However, the normal human brain is seldom fooled into thinking such objects are human faces.
"You can tell that it has some 'faceness' to it, but on the other hand, you're not misled into believing that it is a genuine face," says professor of brain and cognitive sciences at MIT.
A new study from Sinha and his colleagues reveals the brain activity that underlies our ability to make that distinction. On the left side of the brain, the fusiform gyrus — an area long associated with face recognition — carefully calculates how "facelike" an image is. The right fusiform gyrus then appears to use that information to make a quick, categorical decision of whether the object is a face.
This distribution of labour 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, Sinha says. However, hemispheric differences have been seen in other brain functions, especially language and spatial perception.
The paper's lead author, published Jan. 4 in the Proceedings of the Royal Society B, is Ming Meng, a former postdoc in Sinha's lab and now an 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 now resides at Massachusetts General Hospital.
Face versus nonface
Many earlier studies have shown that neurons in the fusiform gyrus on the brain's underside respond preferentially to faces. Sinha and his students set out to investigate how that brain region decides what is and is not a face, particularly in cases where an object greatly resembles a look.
To help them do that, the researchers created a continuum of images ranging from those that look nothing like faces to real faces. They found images that closely resemble faces by examining photographs that machine vision systems falsely tagged as faces. Human observers then rated how facelike each image was by doing a series of one-to-one comparisons; those comparisons allowed the researchers to rank the ideas by how much they resembled a face.
The research team then used functional magnetic resonance imaging (fMRI) to scan the research subjects' brains as they categorized the images. Unexpectedly, the scientists found different activity patterns on each side of the brain: On the right side, activation patterns within the fusiform gyrus remained consistent for all genuine face images but changed dramatically for all nonface images, no matter how much they resembled face. This suggests that the right side of the brain is involved in making the categorical declaration of whether an image is a face or not.
Meanwhile, activity patterns changed gradually in the analogous region on the left side of the brain as images became more facelike, and there was no clear divide between faces and nonfaces. From this, the researchers concluded that the left side of the brain ranks images on a scale of how they look like they are, but not assigning them to one category or another.
"From the computational perspective, one speculation one can make is that the left does the initial heavy lifting," Sinha says. "It tries to determine how facelike is a pattern, without deciding whether I will call it a face."
Key to the research was imaging-analysis technology that allowed the scientists to look at activity patterns across the fusiform gyrus.
"This is a relatively recent innovation — looking at the pattern of activation as opposed to overall activation," says Thomas Busey, associate professor of psychological and brain sciences at Indiana University, who was not involved in this research. "Anytime you have a measure replicating and correlating with human behaviour, that seems to be a compelling story."
Timing is instructive
The researchers found that activation in the left side of the fusiform gyrus preceded the right side by a couple of seconds. This supports the hypothesis that the left side does its job first and then passes the information on to the right side.
Sinha says that given the sluggishness of fMRI signals (which rely on blood-flow changes), the timing does not yet constitute definitive evidence, "but it's an exciting possibility because it begins to tease apart this monolithic notion of face processing. It's now beginning to understand the constituents of that overall face-processing system."
The researchers hope to obtain more solid evidence of temporal relationships between the two hemispheres with electroencephalography (EEG) or magnetoencephalography (MEG) studies. These two technologies offer a much more detailed view of the timing of brain activity. They also hope to discover how and when the right and left sides of the fusiform gyrus develop these independent functions by studying blind children who have their sight restored at a young age. Many such children have been treated by Project Prakash, an effort initiated by Sinha to find and treat blind children in India.
Provided by Massachusetts Institute of Technology
This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/
), a popular site that covers news about MIT research, innovation and teaching.
"How does our brain know what a face is and what's not?." Jan. 9, 2012. http://medicalxpress.com/news/2012-01-brain.html
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Robert Karl Stonjek
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