Scientists have spent years trying to locate the so-called ‘grandmother’s neuron’ in the brain
Instead, neuroscientists at Rockefeller University have found a whole tapestry of grandmother’s neurons.
The study, which has been published in the journal Science, could have important clinical implications for people with facial blindness.
Scientists have spent years researching and trying to locate what neurons are those that explain why humans have the ability to emotionally recognize a very familiar face, like for example that of our grandmothers. Scientists have searched for years in vain for the nicknamed ‘grandmother’s neuron’, a single cell that should be found somewhere in the brain between sensory perception and memory, and that would be able to prioritize and discriminate an important or familiar face over the rest.
Now, new research has found a class of neurons in the temporal lobe of the brain that links face perception with long-term memory. It is not exactly the neuron of the grandmother, since instead of a single cell, it is a population of neurons that collectively would remember Grandma’s face. The findings, which have been published in the journal Science, are the first to explain how our brains instill the faces of those we love.
“When I started in neuroscience, if you wanted to ridicule someone’s argument, you discarded it saying that it was ‘another neuron from the grandmother’, a hypothesis that was believed that it could not exist,” he explains Winrich Freiwald, professor of neuroscience and behavior at Rockefeller University. “Now, in a dark and little-studied corner of the brain, we have found the closest thing to a grandmother’s neuron: it is a set of cells capable of linking the perception of the face with memory.”
Where have I seen that face before?
The idea that a ‘grandmother’s neuron’ could exist as a theoretical brain cell that would encode a complex and specific concept of its own, first appeared in the 1960s. A neuron was sought for grandmother’s memory, another to remember the mother, etc. Deep down, the notion of a one-to-one relationship between brain cells and objects or concepts was an attempt to tackle the mystery of how the brain combines what we see with our long-term memories.
Since then, scientists have discovered many sensory neurons that specialize in processing facial information and as many memory cells dedicated to storing data about personal encounters. But a ‘grandmother’s neuron’, or even a hybrid cell capable of linking vision with memory, had never been found. “The hope is that by now we would have achieved it, says Freiwald. But nothing could be further from the truth! We did not have a clear understanding of where and how the brain processes familiar faces.”
What Freiwald and his colleagues recently discovered is that there is a small area in the temporal lobe of the brain that may be involved in facial recognition. The team used functional magnetic resonance imaging as a guide to zoom into the temporal regions of two monkeys and recorded the electrical signals from temporal neurons as the macaques looked at images of familiar faces (which they had seen in person) and faces of strangers who only they had seen virtually, on a screen.
The team found that neurons in that temporal region (TP) were highly selective, and responded more intensely to faces that subjects had seen before than to strangers. They saw how neurons rapidly discriminated between familiar and unfamiliar faces immediately after processing the image.
Curiously, these cells responded three times more strongly to familiar faces than to unfamiliar ones, even though the subjects had seen the unfamiliar faces many times virtually on the screens. “This may indicate the importance of meeting someone in person“says the neuroscientist Sofia Landi, first author of the article. “Given the current trend to go virtual, it is important to note that the faces we have seen on a screen may not evoke the same neural activity as the faces we meet in person.
A tapestry of grandmother’s cells
The findings constitute the first evidence for the existence of a hybrid brain cell, similar to the legendary ‘grandmother’s neuron’. The cells of this region of the temporal lobe (TP) behave like sensory cells, with reliable and rapid responses to visual stimuli, but also they act as memory cells that respond only to stimuli that the brain has seen before, in this case familiar ones, reflecting a change in the brain as a result of past encounters. “These neurons are very visual, very sensory, but like memory cells,” says Freiwald. “We have discovered a connection between the sensory and memory domains.”
Cells are not, strictly speaking, ‘grandmother’s neurons’. Instead of a cell encoding a single familiar face, cells in that region of the temporal lobe (TP) appear to function in concert, as a collective. “More than a ‘grandmother’s neuron’ it would be something like a ‘grandmother’s face area’ of the brain,” says Freiwald.
The discovery of the TP region located right in the center of the facial recognition area, means that researchers may soon begin to investigate how those cells encode familiar faces. “Now we can ask ourselves how this region is connected to the other parts of the brain and what happens when a new face appears,” explains Freiwald. “And of course we can begin to explore how it works in the human brain.”
In the future, the findings may also have clinical implications for people with prosopagnosia, a facial blindness that prevents them from recognizing familiar faces and affects about one percent of the population. “People with facial blindness often suffer from depression. It can be debilitating, because at worst they can’t even recognize their close relatives,” Freiwald says. “This discovery could one day help us devise strategies to help them.”