Scientists have reconstructed faces nearly perfectly [scientificamerican.com] by analyzing the activity of neurons in macaque brains:
[Using] a combination of brain imaging and single-neuron recording in macaques, biologist Doris Tsao and her colleagues at Caltech have finally cracked the neural code for face recognition. The researchers found the firing rate of each face cell corresponds to separate facial features along an axis. Like a set of dials, the cells are fine-tuned to bits of information, which they can then channel together in different combinations to create an image of every possible face. "This was mind-blowing," Tsao says. "The values of each dial are so predictable that we can re-create the face that a monkey sees, by simply tracking the electrical activity of its face cells."
Previous studies had hinted at the specificity of these brain areas [scientificamerican.com] for targeting faces. In the early 2000s, as a postdoc at Harvard Medical School, Tsao and her collaborator electrophysiologist Winrich Freiwald, obtained intracranial recordings from monkeys as they viewed a slide show of various objects and human faces. Every time a picture of a face flashed on the screen, neurons in the middle face patch would crackle with electrical activity. The response to other objects, such as images of vegetables, radios or even other bodily parts, was largely absent.
Further experiments indicated neurons in these regions could also distinguish between individual faces, and even between cartoon drawings of faces. In human subjects in the hippocampus, neuroscientist Rodrigo Quian Quiroga found that pictures of actress Jennifer Aniston elicited a response in a single neuron [scientificamerican.com]. And pictures of Halle Berry, members of The Beatles or characters from The Simpsons activated separate neurons. The prevailing theory among researchers was that each neuron in the face patches was sensitive to a few particular people, says Quiroga, who is now at the University of Leicester in the U.K. and not involved with the work. But Tsao's recent study suggests scientists may have been mistaken. "She has shown that neurons in face patches don't encode particular people at all, they just encode certain features," he says. "That completely changes our understanding of how we recognize faces."
Professor Rodrigo Quian Quiroga, a neuroscientist at the University of Leicester who was not involved in the work, described it as "quite a revolution in neuroscience". "It's solving a decades-long mystery," he added.
The puzzle of how the brain identifies a familiar face dates back to the 1960s, when the US neuroscientist, Jerry Lettvin, suggested that people have hyper-specific neurons that respond to specific objects, a notion that became known as "grandmother cells", based on the idea that you have a specific neuron that would fire on seeing your grandmother.
More recently scientists found "face patches", clusters of neurons that respond almost exclusively to faces [theguardian.com], but how recognition was achieved had remained a "black box" process. In the absence of proof otherwise, the grandmother model continued to appeal because it tallied with the subjective "ping" of recognition we experience on seeing a familiar face.
"This paper completely kills that," said Quian Quiroga.