A Rockefeller University research group discovered a new type of neuron that relates facial perception to long-term memory. The newly discovered neurons form a community of cells that collectively recall faces. The results are the first to explain how our brains imprint the faces of people we care about. (Photo credit: Rochester University/Sofia M. Landi)
Researchers from Rockefeller University have identified a new type of memory cell in the brain that specializes in identifying a familiar face, comparable to the hypothesized “grandmother neuron.”
A “grandmother neuron” is a single cell at the crossroads of sensory perception and memory that prioritizes an important face over others. Scientists have been looking for a class of brain cells that could explain the visceral reaction felt when someone sees a familiar face – such as that of a grandmother.
A recently published study has revealed a new type of neuron in the temporal pole region of the brain that relates facial perception to long-term memory. Rather than a single cell, it’s a community of cells that collectively recalls grandma’s face. The results, which were published in the journal Science, are the first to explain how our brains imprint the faces of people we care about.
“When I was coming up in neuroscience, if you wanted to ridicule someone’s argument you would dismiss it as ‘just another grandmother neuron’—a hypothetical that could not exist,” says Winrich Freiwald, professor of neurosciences and behavior at The Rockefeller University.
“Now, in an obscure and understudied corner of the brain, we have found the closest thing to a grandmother neuron: cells capable of linking face perception to memory.”
The grandmother neuron was initially proposed in the 1960s as a hypothetical brain cell capable of coding for a single, complicated thought on its own. The hypothesis suggested that one neuron was responsible for remembering one’s grandma, another for remembering one’s mother, and so on. The idea of a one-to-one relationship between brain cells and objects was born out of a desire to understand how the brain integrates what we perceive with our long-term memories.
Since then, scientists have found a slew of sensory neurons specialized to processing face information, as well as a slew of memory cells dedicated to retaining information from intimate interactions. However, no grandmother neuron or even a hybrid cell capable of connecting vision and memory has ever been discovered. “The expectation is that we would have had this down by now,” Freiwald says. “Far from it! We had no clear knowledge of where and how the brain processes familiar faces.”
Previous research by Feiwald and colleagues revealed that facial recognition may be aided by a tiny area in the temporal pole region of the brain. As a result, the researchers used functional magnetic resonance imaging to zoom in on the TP regions of two rhesus monkeys, recording the electrical signals of TP neurons while the monkeys watched images of familiar faces (which they’d seen in person) and unfamiliar faces (which they had only seen virtually) on a screen.
The researchers discovered that neurons in the TP area were extremely selective, reacting to known faces more strongly than unfamiliar ones. The neurons were very quick, distinguishing between recognized and unknown faces almost instantly after processing the image.
Fascinatingly, even though the individuals had seen the unknown faces numerous times digitally, these cells reacted three times more strongly to familiar faces than unfamiliar faces. “This may point to the importance of knowing someone in person,” says neuroscientist Sofia Landi, first author on the paper. “Given the tendency nowadays to go virtual, it is important to note that faces that we have seen on a screen may not evoke the same neuronal activity as faces that we meet in-person.”
The results of the research are the first to show that a hybrid brain cell, similar to the mythical grandmother neuron, exists. The cells in the TP area function like sensory cells, responding to visual inputs reliably and quickly. They also operate like memory cells, responding exclusively to stimuli that the brain has seen before—in this example, familiar people—reflecting a change in the brain as a result of previous experiences. “They’re these very visual, very sensory cells—but like memory cells,” Freiwald says. “We have discovered a connection between the sensory and memory domains.”
The cells, however, are not grandmother neurons in the literal sense. Instead of a single cell coding for a single familiar face, the TP region’s cells appear to operate together as a group. “It’s a ‘grandmother face area’ of the brain,” Freiwald says.
Researchers will soon be able to investigate how those cells encode familiar faces thanks to the identification of the TP region at the heart of facial recognition. “We can now ask how this region is connected to the other parts of the brain and what happens when a new face appears,” Freiwald asks. “And of course, we can begin exploring how it works in the human brain.”
The study was published in Science, on July 1st, 2021.
Abstract. The question of how the brain recognizes the faces of familiar individuals has been important throughout the history of neuroscience. Cells linking visual processing to person memory have been proposed, but not found. Here we report the discovery of such cells through recordings from an fMRI-identified area in the macaque temporal pole. These cells responded to faces when they were personally familiar. They responded non-linearly to step-wise changes in face visibility and detail, and holistically to face parts, reflecting key signatures of familiar face recognition. They discriminated between familiar identities, as fast as a general face identity area. The discovery of these cells establishes a new pathway for the fast recognition of familiar individuals.
Sofia M. Landi, Pooja Viswanathan, Stephen Serene, Winrich A. Freiwald. A fast link between face perception and memory in the temporal pole. Science, 2021; eabi6671 DOI: 10.1126/science.abi6671