In both species, the face recognition system consists of face cells that are grouped into patches of at least 10,000 each. There are six of these patches on each side of the brain, situated on the cortex, or surface, just behind the ear.

When the image of a face hits the retina of the eye, it is converted into electric signals. These pass through five or six sets of neurons and are processed at each stage before they reach the face cells. As a result, these cells receive high-level information about the shape and features of a face.

One way in which the brain might identify faces is simply to dedicate a cell to each face. Indeed, there are cells in another part of the brain that do respond to images of specific people.

They are known to neuroscientists as Jennifer Aniston cells, after one such cell in an epilepsy patient undergoing surgery in 2005 responded when the patient was shown images of the actress. The cell ignored all other images, including one of her with Brad Pitt.

But this can’t be the way the brain identifies faces, because we can perceive a face we have never seen before. Instead, the Caltech team has found, the brain’s face cells respond to the dimensions and features of a face in an elegantly simple, though abstract, way.

In their experiments, the biologists first identified groups of face cells in a macaque monkey’s brain by magnetic resonance imaging, and then probed individual face cells with a fine electrode that records their signals.

The monkeys were shown photos of human faces that were systematically manipulated to show differences in the size and appearance of facial features.