Doris Tsao / Caltech

Researchers believe they've cracked the code for how primates are able to recognize faces. And, it may be relatively simple: only 200 neurons or so in the brain appear to be responsible for the ability of macaque monkeys to identify the faces of humans that they know over those of complete strangers.

Why it matters: Humans and other primates are thought to use a similar face recognition system, so understanding how images are encoded by the neurons could inform the development of artificial vision.

The question: Researchers had previously identified six parts of the brain (in a place called the inferior temporal cortex) that were responsible for identifying faces. Specific nerve cells in these six regions of the brain respond more strongly when we see faces compared to objects. What scientists didn't know was the exact combination of these "face cells" required to identify those we know from strangers.

The study: Researchers inserted electrodes into the brains of macaque monkeys in the six regions. They then mapped various features of faces (like the distance between eyes) onto a grid and used them to create 2000 photos of manipulated human faces. When they were shown to the monkeys, just 205 neurons from two of the six regions where "face cells" exist activated. Using the pattern of neuron activation, the researchers were then able to reconstruct the faces the monkey was looking at.

Fun quote: "People always say a picture is worth a thousand words. But I like to say that a picture of a face is worth about 200 neurons." —Doris Tsao, professor of biology and biological engineering at the California Institute of Technology.

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