A team of researchers from the Shaheed Beheshti University of Medical Sciences in Tehran, Iran, has found that artificial stimulation of a specific region of the brain of rhesus monkeys (Macaca mulatta) causes visual perception of faces. The findings are reported in the current issue of Nature.
Monkeys were trained to look to the left when presented with a face, and to look right if presented with another image. Their abiity to report perceiving a face was then tested using images containing varying degrees of visual noise to obscure the faces.
Fine electrodes were then used to microstimulate groups of neurons in the inferotemporal cortex (IT) in the monkeys’ brains. During trials in which the IT region was microstimulated, there was a far greater tendency for the monkeys to report seeing a face, in comparison to trials in which there was no microstimulation. Even when the visual stimulus they were presented with contained only noise, the monkeys reported seeing a face when the face-selcetive IT region was microstimulated.
The effect of stimulating face-specific neurons was shown to be highly specific, both in space and time. It was strongest when microstimulation was applied at the time when the cells would normally respond to visual stimuli. The face-selective neurons are found in large clusters in a part of the IT area, and are involved in higher-order visual processing; microstimulation of adjacent areas of the IT did not evoke the perception of faces.
We know that damage to the inferior surface of the temporal lobe in humans sometimes results in a condition called prosopagnosia (face blindness). Specifically, an area of the fusiform gyrus of the right hemisphere has been implicated in facial recognition. Neuroimaging studies show that prosopagnostics have lesions in this area.
Some prosopagnostics also have difficulties recognizing other objects, such as animals and abstract signs, and faces have some features that are common to other objects, and others that are unique. There is also some evidence that the region of the human brain involved in facial recognition is also involved in the recognition of novel objects. It can therefore be argued that the human brain does not have specific mechanisms for recognizing faces, and that face recognition is carried out by cells which are also involved in the recognition of other objects.
We only have a rudimentary knowledge of how the brain recognizes objects, and we know even less about how the brain recognizes objects. It is possible that facial recognition involves the successive recruitment of visual centres with increasingly specialized properties. Processing would therefore start in lower order visual centres, containing cells specialized for general object recognition tasks, and would continue, in a heirarchical manner, in higher order centres, before ending in a region containing face-selective cells.
The finding that there appear to be face-selective neurons in the brain of the rhesus monkey, which evoke the perception of a face when stimulated, would suggest that there are neurons specialized for facial recognition in the human brain too. The IT region in the rhesus brain is homologous to the inferior temporal cortex of the human brain, where the so-called fusiform face area (FFA) is located. Therefore, if the human brain does contain cells which are activated in response to faces, the FFA is where they are most likely to be found.