In a new study, scientists successfully observed the neural signatures of recalled memories in real time.

For the study, researchers at the National Institutes of Health monitored the neural activity of epilepsy patients while they learned word pairings.

Scientists spotted the electrical signature of a forming memory as patients first learned a word pairing. Then, during follow up tests, researchers were able to recognize the same neural patterns just before patients recalled what they had learned earlier. The electrodes and word tests allowed scientists to watch the electrical signal sparked by a replayed memory in a real time.

The study, published this week in the journal Science, was part of a broader effort to develop new therapies for patients with drug-resistant epilepsy.

"Memory plays a crucial role in our lives," senior study author Dr. Kareem Zaghloul, a neurosurgeon and researcher at the National Institute of Neurological Disorders and Stroke, said in a news release. "Just as musical notes are recorded as grooves on a record, it appears that our brains store memories in neural firing patterns that can be replayed over and over again."

For the broader study, Dr. Zaghloul and his colleagues used surgically implanted electrodes to try to locate the origins of seizures in patients with drug-resistant epilepsy. The study offered researchers the chance to study the neural patterns responsible for memory formation.

Previous studies involving rodent models have suggested episodic memories are stored in unique neuronal firing sequences. For the latest memory study, scientists wanted see if they could find similar neuronal sequences in humans.

"We thought that if we looked carefully at the data we had been collecting from patients we might be able to find a link between memory and neuronal firing patterns in humans that is similar to that seen in rodents," said study leader Alex Vaz, a bioengineer and doctoral student at Duke University.

During the word-pairing tests, researchers focused on the electrical signals produced by neurons in the anterior temporal lobe, which is key to the formation of verbal memories and language learning. Scientists watched as unique neuronal sequences fired during the knowledge acquisition process, as patients learned word pairings, like "cake" and "fox." When patients were later prompted with one of the words, scientists watched the same neuronal sequence fire just before patients successfully recalled the associated word.

"These results suggest that our brains may use distinct sequences of neural spiking activity to store memories and then replay them when we remember a past experience," said Dr. Zaghloul.

In a previous study, Dr. Zaghloul and his colleagues found unique electrical waves, or ripples, propagated through the brain just prior to memory being recalled. The latest study revealed a connection between ripples and neural spikes. Just before the neural spikes registered in the anterior temporal lobe, electrodes recorded ripples in the medial temporal lobe.

"Our results support the idea that memories involve coordinated replay of neuronal firing patterns throughout the brain," said Dr. Zaghloul. "Studying how we form and retrieve memories may not only help us understand ourselves but also how neuronal circuits break down in memory disorders."

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