There must be 50 ways to pin down a face John Lamb/Getty

Precision images of real faces have been recreated simply by monitoring the activity of certain cells in the brains of macaque monkeys as they looked at photographs of people.

The first step was to determine exactly how brain cells compute recognisable images in the higher visual cortex – the brain area that interprets signals from the eye. The study is the first to provide a full and simple explanation of how the brains of macaques – and by implication, humans – generate composite images of any face they see.

“We’ve cracked the brain’s code for facial identity,” says Doris Tsao at the California Institute of Technology.


50 dimensions

Tsao and her colleague, Steven Le Chang, have discovered that individual cells can generate an infinite range of facial images in the brain by pooling their activity, much in the same way that the combined activity of red, blue or green detectors in the eye allow the brain to “see” many more colours.

Earlier work has identified patches of these specialised face cells, which seem only to be active when a person is shown an image of a face.

In their study, Tsao and Le Chang tracked the activity of cells in monkey brains. Working together, the combined signals from these cells could encode 50 different aspects of a face – for example, face shape, eye distance, skin texture, and so on.

“The key is that even though there’s an infinite number of faces, you can describe all of them with just these 50 dimensions,” says Tsao. “It’s like computer-generated imagery, except it’s in our brains.”

They then inserted electrodes into three patches of face cells in the brains of macaques, enabling them to record the activity of 205 neurons. They then showed three of these macaques 2000 images of human faces.

They discovered that each of the face cells is tuned to view faces in slightly different ways – as if photographing a face from multiple angles at once. When all these disparate views are combined, they give a clear composite image, a bit like an identikit.

The researchers were able to develop algorithms from the face cell feedback, enabling them to recreate composite facial images just from monkey brain cell activity alone.

Recognise and memorise

While these patches of cells can be enough to recreate a person’s face, it’s likely that our memories of familiar faces go on to be stored in the hippocampus, the brain area vital for memory.

In the hippocampus, other work has found that individual cells are each responsible for registering specific people, such as the actress Jennifer Aniston.

“You could have the two systems in parallel or in tandem,” says Christof Koch of the Allen Institute for Brain Science in Seattle, Washington, whose team identified the so-called Jennifer Aniston cells in 2005. “One is a broad system to recognise any face, and the second is for recognising faces in a very abstract way,” he says.

“Tsao’s work provides the first specific hypothesis for how the response of face cells in the cortex can be utilised by cells in the hippocampus to form memories of individuals we’ve seen before,” says Ueli Rutishauser at the Cedars-Sinai Medical Center in Los Angeles, California.

Tsao plans next to study a different part of the brain, called the temporal lobe, to see if everyday objects or scenes are encoded in the brain using the same principles.

Journal reference: Cell, DOI: 10.1016/j.cell.2017.05.011