Ripples race in the brain as memories are recalled

Fast waves of activity ripple in the brain a half second before a person calls up a memory. The finding, published in the March 1 Science, hint that these brain waves might be a key part of a person’s ability to remember.

The results come from a study of 14 people with epilepsy who had electrodes placed on their brains as part of their treatment. Those electrodes also allowed scientists to monitor neural activity while the people learned pairs of words.

One to three minutes after learning the pairs, people were given one word and asked to name its partner. As participants remembered the missing word, neuroscientist and neurosurgeon Kareem Zaghloul and his colleagues caught glimpses of fast brain waves rippling across parts of the brain at a rate of around 100 per second.

These ripples appeared nearly simultaneously in two brain regions — the medial temporal lobe, which is known to be important for memory, and the temporal association cortex, which has a role in language. When a person got the answer wrong, or didn’t answer at all, these coordinated ripples were less likely to be present, the researchers found. “We see this happening, and then we see people remember,” says Zaghloul, of the National Institutes of Health in Bethesda, Md.

While recalling a memory, “you mentally jump back in time and re-experience it,” Zaghloul says. Just after the ripples, the researchers saw telltale signs of that mental time travel — an echo of brain activity similar to the brain activity when the memory of the word pair was first formed.

The finding that ripples precede that neural echo is “the most beautiful part of the story,” says neuroscientist György Buzsáki of New York University, because it lends heft to the idea that the brain waves are involved in retrieving memories.

It’s not yet possible to say whether the ripples actually cause a person to remember, perhaps by sharing information among brain areas required for the memory. “Do they contain information, or are they a consequence of something else?” Zaghloul says. “We just don’t know.”