LONDON: Two nerve clusters in the brain have been pinpointed as responsible for our ability to recognise faces, and which when damaged or impaired may be the cause of a condition known as ‘face blindness’ or prosopagnosia.
The researchers, from Stanford University in California, identified the clusters in the fusiform gyrus – an area on the left side of the brain long associated with face recognition. To do so, they used a painless procedure carried out on a patient with epilepsy who had electrodes temporarily implanted in his brain.
“Our research shows that there is a causal role of specific sites on the fusiform gyrus in the perception of faces. This suggests that impairment to these brain regions may affects people’s ability to perceive faces,” said senior investigator Kalanit Grill-Spector, who co-authored the research paper, published in The Journal of Neuroscience.
Who’s that in the mirror?
People with face blindness are unable to distinguish one face from another, even though they have normal vision and visual-information processing.
Some people are born with face blindness, known as developmental prosopagnosia, whereas others develop it as a result of brain injury or damage, known as acquired prosopagnosia, commonly after stroke.
Brain and behaviour expert Martin Eimer, from Birkbeck College in London, explained that people with this condition “frequently fail to recognise faces of friends and family; some even report an absence of familiarity when looking in the mirror”.
“Prosopagnosia is a significant social handicap. It has serious consequences and can lead to elevated rates of anxiety and chronic stress,” added Eimer, who was not involved in the research.
Researchers discovered that the fusiform gyrus is involved in face perception in the late 1990s.
“It’s well known that the fusiform gyrus is involved in face perception and thousands of research studies using functional magnetic resonance imaging (fMRI) have been conducted examining the functional properties of what is referred to as the fusiform face area (FFA),” explained study co-author Kevin Weiner.
In 2010, Grill-Spector and Weiner carried out a study using high-resolution fMRI, during which they discovered that the FFA is split up into at least two regions – called pFUS-faces and mFUS-faces – which responded more strongly to faces than to other visual stimuli, such as hands, legs, cars, guitars, flowers or buildings.
For the current study, Grill-Spector and Weiner collaborated with Stanford neurologist Josef Parvizi in a fortuitous experiment that enabled them to manipulate pFUS-faces and mFUS-faces directly.
A 47-year old patient called Ron Blackwell from Santa Clara in California was undergoing medical treatment from Parvizi and colleagues that involved temporary implantation of electrodes into his brain to try and determine the focus and potentially treat his epileptic seizures (see video below). By pure chance, two of the electrodes were implanted in his brain directly over the pFUS- and mFUS-faces nerve clusters.
“As these face-selective regions are typically 1cm apart and the electrodes implanted by the neurosurgeons are also 1cm apart, it was by sheer luck that the electrode placement overlapped both face-selective regions in the patient,” said Weiner.
Using a combination of fMRI and intracranial recording, along with electrical brain stimulation, the team demonstrated that the pFUS- and mFUS-faces nerve clusters are directly involved in face perception.
They found that stimulation of these clusters, but not surrounding brain areas, distorted Blackwell’s perception of Parvizi’s face so that his features appeared to melt. When the stimulation stopped, Blackwell’s view of Parvizi’s face returned to normal.
“This study provides perhaps the most compelling evidence to date that specific regions in the fusiform gyrus play a direct role in how we perceive faces,” said Eimer.
“Patients with acquired prosopagnosia often have brain damage in this specific area. Demonstrating that stimulation of this fusiform face area can distort face perception provides new evidence that damage to this area directly contributes to the problems that these patients have with face perception and recognition.”
An independent brain and emotion expert from the University of Leuven, Belgium, Jan Van den Stock, said that, while the findings provide interesting insight into the mechanics of face perception, they are unlikely to help scientists develop a treatment for prosopagnosia.
“For instance, they used electrical stimulation to disrupt the function of a brain area, while it should be increased in the case of a prosopagnosia remedy,” he said
The investigators hope to continue their work researching areas of the brain involved in face perception, including the broader function of the fusiform gyrus.
It is known that this area of the brain is also involved in object recognition, in the visual perception of words, and in the processing of whole-body expressions.
“There is ongoing debate about whether the fusiform gyrus is specialised in processing faces and bodies on the one hand, or whether it is more specialised in stimuli for which we have a great expertise,” explained Van den Stock.
“For instance, the fusiform gyrus of car/bird experts also responds when they are shown images of cars/birds,” he said.
Eimer remarked: “What the current study demonstrates is that different parts of the fusiform gyrus have separate functions, and that the area stimulated is specifically linked to the conscious visual perception of faces.”