Give the humble neuron its due. Although neuroscientists often view single nerve cells as bit players in mental life, new evidence indicates that some star on their own in recognizing specific people or objects.
Far from working as simple switches or relays in a large neural ensemble, each of these critically situated cells assists in translating familiar sights into lasting memories, proposes a team led by neurosurgeon Itzhak Fried of the University of California, Los Angeles.
Fried refers to such brain cells as thinking neurons. “Thinking involves abstraction of reality, and the currency of these cells is abstract information about what already has been seen,” he says.
These neurons work with information from remarkably specific sources, the scientists report in the June 23 Nature. Depending on the person, particular cells in the medial temporal lobe—an area critical to forming long-term memories—get fired up only by, for example, images of actress Jennifer Aniston or of Australia’s Sydney Opera House.
Fried’s group studied three men and five women with severe epilepsy. Initial interviews guided the researchers’ choices of images that would be familiar to each volunteer. During the 7 to 10 days when the volunteers had electrodes implanted in their brains to identify the location of their seizures, the researchers tested the patients’ reactions to several series of images. Each participant viewed famous and nonfamous people, landmark buildings, animals, and foods.
After noting which images elicited pronounced electrical responses in at least one neuron, the scientists performed additional trials to identify neural reactions to views of the same subject from different angles.
In one person, for example, a single neuron fired strongly only in response to various images of Jennifer Aniston alone. All other images, including pictures of Aniston with actor Brad Pitt, drew no response or weak responses from the same cell.
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In another volunteer, a single neuron charged up only in response to pictures of the Sydney Opera House, the words Sydney Opera, and an image of a religious temple that the volunteer thought was the Sydney Opera House.
Overall, 132 neurons in the patients exhibited strong electrical responses to at least one image. Of that number, 51 reacted to only a specific person, building, animal, or food item.
Fried doubts that such neurons act as grandmother cells, a term for neurons that theoretically recognize only one person or object. He suspects that further testing of patients with epilepsy will show that individual neurons react to different classes of images.
More than 30 years ago, neuroscientist Robert Desimone of the Massachusetts Institute of Technology and psychologist Charles G. Gross of Princeton University reported that individual neurons in monkeys’ brains fire only in response to images of particular human and monkey faces. At the time, many other researchers asserted that these cells were responding to darker and lighter borders around images, not to facial identity.
The new findings add to evidence that “there’s amazing specificity in what some neurons do,” says Desimone.