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Scientific American/One face, one nerve cell

New research shows that to decode a given character our brain uses far fewer cells than previously thought. In fact, all it takes to decode and recognize a given character is a single neuron

Diane Martindale, Scientific American

When you see a celebrity on the cover of a magazine, your brain immediately recognizes the character, a process that probably happens thanks to a single nerve cell. New research shows that to decipher a given character our brain uses far fewer cells than previously thought. These findings can help neurobiologists understand how memories are formed and stored.

There are many disputes and hypotheses about how exactly the human brain perceives and remembers a figure. In the previous decades, two extreme approaches to this have developed. According to one approach, millions of neurons work together and combine different pieces of information into one coherent image, while the other approach claims that each object or person is recognized by a separate nerve cell in the brain. In the 60s, neurobiologist Jerome Letwin called the idea of ​​a separate neuron the "grandmother's cell" theory, meaning that the brain has a neuron dedicated exclusively to knowing each member of the family. If you lose this nerve cell, you won't be able to recognize grandma. For many years, experts dismissed this approach as too simplistic. But Rodrigo Kian Quiroga from the University of Leicester in England and his research partners decided to investigate just how picky single neurons can be. The group examined eight patients in the brain of each of whom had 64 tiny electrodes implanted before surgery to treat epilepsy (a procedure to locate the source of the epileptic seizures). Many electrodes were placed in the hippocampus, an area essential for storing long-term memory.

The electrodes recorded the activity of the brain cells while each patient looked at many pictures of celebrities, animals, objects and famous buildings. This filtering step determined which of the images caused strong neural activity of at least one neuron. The group then examined responses to three to eight variations of the images that remained after filtering.
In one patient, a single neuron responded to seven different images of actress Jennifer Aniston, but the same neuron completely ignored 80 other images of animals, buildings and non-famous people. "The first time we saw a nerve cell responding to seven pictures of Jennifer Aniston, and nothing else, we literally jumped out of our chairs," Kian Quiroga recalled.
Similar results were observed in another patient with a nerve cell unique to actress Halle Berry. The nerve cell responded not only to photographs but also to drawings and even to the sight of her name. Furthermore, even when Berry was presented disguised as Catwoman, the neuron still fired if the patient knew it was indeed her. "This neuron responds to the abstract concept Halli Berry and not to any specific visual cue. It's like saying, 'I won't remember every detail of the conversation, but I will remember what the topic of the conversation was.' This suggests that we store memories as abstract ideas," says Kian Kiruga. Apart from celebrities, also famous buildings, such as the Sydney Opera House and the Leaning Tower of Pisa, stimulated the activity of single nerve cells.
"Very few scientists would have predicted such clear activity of single neurons in response to certain people," says Charles Connor, a neurobiologist at Johns Hopkins University. "Now it will be possible to see exactly what information is represented by these cells, and this is a clear starting point for studies on memory encoding."
Although the nerve cells of "Jennifer" and "Hally" behave similarly to a grandmother's cell, the findings do not mean that a given brain cell will only respond to one person or object, says Christoph Koch, from the California Institute of Technology, who participated in the study. These cells respond, obviously, to a wide variety of items (some of the nerve cells responded to more than one person or one object). "We are not claiming that these are grandmother cells, but regarding familiar things, such as family members or celebrities, things that you see often, the neurons are wired and respond in a very specific way, much more specific than previously thought," explains Koch.

The findings, published in the journal Nature, could affect the study of diseases such as dementia, but Kian Kiruga identifies a more purposeful application, implanting prosthetic communication devices, or so-called brain readers. "We might be able to help patients communicate with the outside world, with their thoughts decoded by a computer," he predicts.

 

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