From Scientific American:
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.”