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Monitoring the Brain’s Memory-Making Cells

The brain cells that fire when a person watches a brief film clip are triggered again when the person thinks back on that imagery a few minutes later, a new study shows. The research offers insights into how the brain summons up past experiences and may also provide clues to brain disorders, like Alzheimer's disease, that harm short-term memory.

Scientists have known for decades that memories are often processed and retrieved by the hippocampus, a curved structure deep in the brain. But exactly how memories are recorded and recalled remains a mystery.

Dr. Itzhak Fried, a professor of neurosurgery at the University of California, Los Angeles, has been exploring the mechanisms of memory at the single-neuron level in the human brain. For the past 10 years, his research has focused on patients with severe epilepsy who have many tiny electrodes implanted in their brains. The electrodes are used to pinpoint seizure-causing brain regions for surgical removal, but they can also provide information about how individual brain cells process memories.

In his latest study, published in the online edition of Science on September 4, 2008, Dr. Fried and colleagues at the Weizmann Institute of Science in Israel monitored electrode recordings of hundreds of neurons while patients watched a series of film clips and then later recalled them. Thirteen patients participated in a total of 43 viewing and recall sessions. The study was funded in part by NIH's National Institute of Neurological Disorders and Stroke (NINDS).

The patients were shown about a dozen brief clips, each lasting about 5 to 10 seconds, of famous people, characters, landmarks or activities. The clips included segments of TV shows like The Simpsons and Seinfeld, portions of a speech by Martin Luther King, the Golden Gate Bridge and the famous Hollywood sign. Each clip was replayed a few times during each viewing session. After viewing sessions, the patients engaged in a different task for a few minutes. They were then asked to think about the video clips they'd seen and say aloud which clips came to mind.

During the videos, more than half of the monitored neurons had a significant response to one or more clips, and these neurons became reactivated each time those clips were replayed. Later, as the clips were remembered, the same neurons fired again. In general, there was a 2-second lag between neuron firing and verbal reporting of remembered clips.

For example, a neuron located near the hippocampus in one patient showed a powerful response each time a clip from The Simpsons appeared. It fired with less intensity when Seinfeld was shown and remained essentially silent during 46 other clips. When the patient later thought about the Simpsons clip, the neuron fired for several seconds but had little or no response to other memories. In all the patients, several neurons in and around the hippocampus showed similar, selective responses to different film clips.

It's not clear exactly what aspects of the clips triggered the cells’ responses. Nevertheless, this study adds to a growing body of evidence that neurons in the brain's memory centers can play a dual role, responding first to sensory input and then re-activating when that experience is later remembered.

—by Vicki Contie