A lone neuron in one of the brain's key memory centers may be able to distinguish a specific person or place, negating a long-standing tenet that a group of neurons is needed to encode any memory.

The single-neuron hypothesis comes from a recent study of epilepsy patients. A team led by researchers at the California Institute of Technology and the University of California at Los Angeles implanted small electrodes in the epileptics’ brains to monitor seizure activity. The patients volunteered to watch a rapid slide show of random images, including photographs of famous landmarks, politicians and celebrities. The researchers found that single cells responded to single images, by sorting neural activity based on the cells’ unique timing and response characteristics.

One patient had a neuron that responded to a photograph of actress Halle Berry but not to images of other actresses. The same neuron fired when the patient was shown line drawings of the actress or her name typed on a screen but not other drawings or names. A single neuron in another patient responded to photographs and words denoting the Sydney Opera House but not other landmarks.

The experiments advance work the group started two years ago that prompted media discussion of a Bill Clinton gene. Although the single-concept neuron theory dates back to the 1960s, it had been dismissed by scientists. Itzhak Fried, one of the current investigators, suggests that one-to-one correlations may be key to efficient memory storage. It is possible, however, that neurons not monitored during the procedure were responding to the people or places presented. Conversely, some neurons fired when two different images were presented. Co-investigator Rodrigo Quian Quiroga suggests this might occur if we “associate one particular person with one particular object and we want to store this association in long-term memory.”