Researchers have discovered that the same nerve cells involved in forming memories also are involved in replaying them. The finding, published today in the online edition of Science, provides new insight into how complex memories are laid down in a single neuron (nerve cell) and how neural firing, or communication, patterns created during memory formation are maintained during recall.

Scientists at the University of California, Los Angeles, (U.C.L.A.) showed 13 volunteers—epilepsy patients with therapeutic electrodes implanted in their brains—several five- to 10-second clips from videos such as The Simpsons.

The researchers found that a small sample comprising some 50 neurons in the hippocampus and entorhinal cortex (memory centers in the brain) fired in distinctive repeatable patterns that differed for each clip.

"The results were quite astounding," says senior study author Itzhak Fried, director of the U.C.L.A. Health System's Epilepsy Surgery Program. The same neuron that activated during the original viewing of a specific snippet also fired during recall, and the action began a second or so before the patient reported seeing the clip.

That means, Fried says, that "the very neuron that was selectively active during the encoding, during the original viewing, suddenly came to life. It essentially replayed that memory by firing."

Larry Squire, a neuroscientist at the VA San Diego Medical Center and the University of California, San Diego, says this is the first time such a process has been demonstrated in the human brain. He adds that this is also the first report of so-called free recall extending into the hippocampus, formerly believed to manage spatial navigation and simply record memories rather than play them back.

Free recall is when the brain, without any external stimulus, spontaneously generates a pattern and its description from a past event it has experienced. The phenomenon cannot be studied in animals because they cannot report spontaneous memories. For ethical reasons, in humans free recall can only be studied in epilepsy patients who already have electrode implants for therapeutic purposes. In contrast, during what's known as "recognition," the brain, given a pattern from an image (for example, a familiar face) is able to generate a description of the pattern (the person's name) from a set of features it has previously stored.

Another Science paper this week describes a related rat study. Rats were trained using rewards to run alternately through two arms of a maze and on a wheel between each maze run. Researchers saw the same neuron-firing patterns (on fMRIs) during both the maze arm– and wheel-running. This suggests that while in the wheel, the rats recalled their previous arm run so they would be sure to switch to the other arm on the next go-around to get their rewards.

"We would reliably predict from reading the sequences during steady running in the wheel which direction the rat would choose 10 to 20 seconds prior to the actually observed choice," lead study author György Buzsáki, a neuroscientist at Rutgers University's Center for Molecular and Behavioral Neuroscience in Newark, N.J., wrote in a e-mail.

Both research teams hypothesize that humans and rats employ the same brain mechanisms for memory, but use them somewhat differently. "We suggest that the mechanisms in the hippocampus which evolved first to allow 'simple' animals to navigate in space," Buzsáki wrote, "are the same as the mechanism that allows us to navigate in 'cognitive space', that is, from one thought to the next."