When you spot a celebrity on a magazine cover, your brain recognizes the image in an instant--an effect that seems to occur because of a single neuron. A recent study indicates that our brains employ far fewer cells to interpret a given image than previously believed, and the findings could help neuroscientists determine how memories are formed and stored.
Exactly how the human brain works to record and remember an image is the subject of much debate and speculation. In previous decades, two extreme views have emerged. One says that millions of neurons work in concert, piecing together various bits of information into one coherent picture, whereas the other states that the brain contains a separate neuron to recognize each individual object and person. In the 1960s neurobiologist Jerome Lettvin named the latter idea the "grandmother cell" theory, meaning that the brain has a neuron devoted just for recognizing each family member. Lose that neuron, and you no longer recognize grandma.
Experts long ago dismissed this latter view as overly simplistic. But Rodrigo Quian Quiroga of the University of Leicester in England and his colleagues decided to investigate just how selective single neurons might be. The team looked at eight patients who each had 64 tiny electrodes implanted in their brains before epilepsy surgery (a procedure to pinpoint the source of their seizures). Many of the electrodes were placed in the hippocampus, an area critical for the storage of long-term memories.
While each participant was shown a large number of images of celebrities, animals, objects and landmark buildings, electrodes recorded the brain cells' firings. This screening stage determined which images elicited a strong response in at least one neuron. The team then tested the responses to three to eight variations of those images from the narrowed list.
In one patient, a single neuron responded to seven different photographs of actor Jennifer Aniston, while it practically ignored the 80 other images of animals, buildings, famous or nonfamous people that were also presented. "The first time we saw a neuron firing to seven different pictures of Jennifer Aniston--and nothing else--we literally jumped out of our chairs," Quian Quiroga recalls.
Similar results occurred in another patient with a neuron specific for actor Halle Berry; the neuron responded not only to photographs but also to a drawing and an image of her name. What is more, even when Berry was costumed as the masked Catwoman, if the patient knew it was Berry, the neuron still fired. "This neuron is responding to the abstract concept of Halle Berry rather than to any particular visual feature. It's like, 'I won't recall every detail of a conversation, but I'll remember what it was about.' This suggests we store memories as abstract concepts," Quian Quiroga adds. Besides celebrities, famous buildings, such as the Sydney Opera House and the Tower of Pisa, elicited single-neuron firing.
"Not many scientists would have predicted such explicit single-neuron signals associated with individual people," says Charles Connor, a neuroscientist at Johns Hopkins University. "It should now be possible to look at precisely what information is represented by those cells--a clear starting point for studying how memories are encoded."
Although the "Jen" and "Halle" neurons behave much like a grandmother cell, the findings do not mean that a given brain cell will react to only one person or object, notes Christof Koch, one of the study's researchers at the California Institute of Technology. These cells probably respond to a wide range of items (some neurons responded to more than one person or object). "We are not saying that these are grand-mother cells, but for familiar things, like your family or celebrities, things you see frequently, the neurons are wired up and fire in a very specific way--much more so than previously thought," Koch explains.