Scientists have identified two previously unknown areas of the brain that may explain how we recognise familiar faces.
Researchers have long known that the brain contains a network of areas that respond selectively to faces as opposed to other kinds of objects.
They also knew that humans process familiar and unfamiliar faces very differently.
For example, we excel at recognising pictures of familiar faces even when they are disguised by poor lighting or shot at odd angles.
Yet attempts at divining the neural basis for these differences between familiar and unfamiliar face perception in humans have proven inconclusive.
Researchers at The Rockefeller University in the US turned instead to macaques, close evolutionary cousins whose face processing networks are better understood and more easily studied than our own.
Using functional magnetic resonance imaging, they measured the animals' brain activity as they responded to pictures of other monkeys' faces.
Those faces fell into three categories: personally familiar ones belonging to monkeys that the macaques had lived with for years; visually familiar ones whose pictures they had seen hundreds of times; and totally unfamiliar ones.
The researchers expected the macaque face processing network to respond in much the same way to the first two types of faces.
However, the entire system showed more activity in response to the faces of long-time acquaintances. Faces that were only visually familiar, meanwhile, actually caused a reduction of activity in some areas.
"The whole network somehow distinguishes personally familiar faces from visually familiar faces," said Landi.
The faces of animals whom the macaques had known for years prompted the activation of two previously unknown face- selective areas.
One is located in a region of the brain associated with so-called declarative memory, which consists of facts and events that can be consciously recalled.
The other area is embedded in a region associated with social knowledge, such as information about individuals and their position within a social hierarchy - "a specific form of memory that is highly developed in primates, and certainly in humans, researchers said.
These findings will allow the researchers to further investigate the neural mechanisms that underlie face recognition and how the brain responds to different kinds of familiarity.
Since they reside in regions of the brain that are linked to different kinds of information, these novel areas should also provide an inroad to understanding cognitive and perceptual processes that go well beyond vision.
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