Why does that person look familiar to you?

The brain can identify a face in a few thousandths of a second, and retain the memory for a length of time, often for decades. The brain has its own facial recognition system, but why do certain people look familiar to you … and yet you can’t quite place them?

Two Caltech biologists, Le Chang and Doris Y. Tsao, reported in the May 2017 edition of Cell journal that they have deciphered the code to how the brain recognizes faces. Their experiments were based on electrical recordings from face cells, the name given to neurons that respond with a burst of electric signals when an image of a face is presented to the retina of the eye (i.e. when an image is seen).

By noting how face cells in macaque monkeys responded to manipulated photos of some 2,000 human faces, the Caltech research team learned exactly what aspects of the faces triggered the cells and how the features of the face were being encoded.

The researchers say that just 200 face cells are required to identify a face. After discovering how its features are encoded, the biologists were able to reconstruct the faces a monkey was looking at just by monitoring the pattern in which its face cells were ‘firing.’ The finding needs to be confirmed in other laboratories, but, if correct, it could help understand how the brain encodes what it sees.

Human and monkey brains have evolved dedicated systems for recognizing faces, presumably because, as social animals, survival depends on identifying members of one’s own social group and distinguishing them from strangers. 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. 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 found that the brain’s face cells respond to the dimensions and features of a face in a simple but abstract way.

In their experiments, the biologists first identified groups of face cells in a macaque monkey’s brain through magnetic resonance imaging. They probed individual face cells with a fine electrode that records their signals. The monkeys were shown photographs of human faces that were systematically manipulated to show differences in the size and appearance of facial features.

The Caltech team was able to create faces that showed exactly what each face cell was tuned to. The tuning of each face cell is to a combination of facial dimensions, a holistic system that explains why when someone shaves off his moustache, his friends may not notice for a while. Some 50 such dimensions are required to identify a face, the Caltech team reports. These dimensions create a mental “face space” in which an infinite number of faces can be recognized.

There is probably an average face, or something like it, at the origin, and the brain measures the deviation from this base.

Dr. Tsao said she was particularly impressed to find she could design a whole series of faces that a given face cell would not respond to, because they lacked its preferred combination of dimensions. This ruled out a possible alternative method of face identification: that the face cells were comparing incoming images with a set of standard reference faces and looking for differences. Dr. Tsao has been working on face cells for 15 years and views her new report, with Dr. Chang, as “the capstone of all these efforts.”

https://www.nytimes.com/2017/06/01/science/facial-recognition-brain-neurons.html?emc=edit_th_20170606&nl=todaysheadlines&nlid=72405193&_r=0

MARTINA NICOLLS is an international aid and development consultant, and the author of:- Similar But Different in the Animal Kingdom (2017), The Shortness of Life: A Mongolian Lament (2015), Liberia’s Deadest Ends (2012), Bardot’s Comet (2011), Kashmir on a Knife-Edge (2010) and The Sudan Curse (2009).