US researchers have discovered two previously unknown areas of the brain that help humans process familiar and unfamiliar faces differently.

Working with rhesus macaque monkeys — primates whose face-processing systems closely resemble our humans — Winrich Freiwald and Sofia Landi from the Rockefeller University in the US, identified brain areas that are capable of integrating visual perception with different kinds of memory.

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.

Unexpectedly, the macaque face processing network 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 researchers said.

Further, the faces of animals whom the macaques had known for years prompted the activation of two previously unknown face-selective areas — region of the brain associated with declarative memory, which consists of facts and events that can be consciously recalled, and area associated with social knowledge, such as information about individuals and their position within a social hierarchy.

This is “a specific form of memory, that is highly developed in primates, and certainly in humans”, Freiwald said.

When the researchers showed the macaques blurry images of personally familiar faces, which gradually became sharply defined over the course of half a minute or so, the activity of previously known face-processing areas increased steadily over time.

But the new areas first showed little or no initial increase in activity, followed by a sudden surge — an all-or-nothing response that evokes what Landi calls “the sudden ‘aha’ moment” we experience when we recognise a familiar face.

The new findings “opens a window to explore the interaction between face perception, memory, and social knowledge”, Landi said.