A new study led by researchers at The Rockefeller University identified a class of neurons in the temporal pole region of the brain that are involved in the connection between face perception and long-term memory.
The findings, which were published in the journal ‘Science’, provide the first explanation for how our brains imprint the faces of those we cherish.
“When I was a student of neuroscience, if you wanted to mock someone’s argument, you would dismiss it as ‘just another grandmother neuron’ – a hypothetical that could not exist,” said Winrich Freiwald, a Rockefeller University professor of neurosciences and behaviour.
“Now, in a remote and understudied region of the brain, we’ve discovered the closest thing to a grandmother neuron: cells capable of connecting face perception to memory,” he added.
The grandmother neuron was first proposed in the 1960s as a hypothetical brain cell capable of independently coding for a specific, complex concept. One neuron is associated with the memory of one’s grandmother, another with the memory of one’s mother, and so forth. At its core, the concept of a one-to-one correspondence between brain cells and objects or concepts was an attempt to explain how the brain integrates what we see with our long-term memories.
Since then, scientists have discovered an abundance of sensory neurons dedicated to processing facial information, as well as an equal number of memory cells dedicated to storing data from personal encounters. 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 gotten this under control by now,” Freiwald explained. “Not at all! We lacked a comprehensive understanding of where and how the brain processes familiar faces.”
Freiwald and colleagues recently discovered that a small region in the temporal pole region of the brain may be involved in facial recognition. Thus, the team used functional magnetic resonance imaging to zoom in on the TP regions of two rhesus monkeys and recorded the electrical activity of TP neurons while the macaques watched images of familiar faces (which they had seen in person) and unfamiliar faces (which they had only seen virtually, on a screen).
The researchers discovered that neurons in the TP region were highly selective, responding more strongly to familiar faces than to unfamiliar ones. And the neurons were nimble, immediately discriminating between known and unknown faces upon image processing.
Interestingly, these cells responded threefold more strongly to familiar faces than to unfamiliar faces, despite the fact that the subjects had seen the unfamiliar faces virtually many times on screens. “This may emphasise the importance of meeting someone in person,” said Sofia Landi, the paper’s first author. “Given today’s trend toward virtuality, it’s critical to remember that faces we see on a screen may not elicit the same neuronal activity as faces we meet in person.”
The findings provide the first evidence for the existence of a hybrid brain cell similar to the fabled grandmother neuron. The cells in the TP region behave similarly to sensory cells, responding reliably and rapidly to visual stimuli. However, they behave similarly to memory cells, responding only to stimuli that the brain has previously encountered – in this case, familiar individuals – indicating that the brain has changed as a result of previous encounters.
“They’re these highly visual, highly sensory cells that behave similarly to memory cells,” Freiwald explained. “We have established a link between sensory and memory domains.”
However, the cells are not strictly grandmother neurons. Rather than each cell encoding a single familiar face, the cells of the TP region appear to work cooperatively.
“It’s the brain’s ‘grandmother face area,'” Freiwald explained. The identification of the TP region at the heart of facial recognition enables researchers to begin studying how those cells encode familiar faces.
“We can now investigate how this region of the brain is connected to the rest of the brain and what happens when a new face appears,” Freiwald speculated. “Of course, we can investigate how it functions in the human brain.”
The findings may also have clinical implications in the future for people who suffer from prosopagnosia, or face blindness, a socially isolating condition that affects about 1% of the population.
“Individuals who are face-blind frequently suffer from depression. It can be debilitating, as they may not even recognise close relatives in the worst-case scenario “According to Freiwald.
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