Epilepsy study helps researchers replay memories in real time

A study of epilepsy patients let researchers monitor the firing patterns of cells when patients learned a word.

Researchers at the National Institutes of Health monitored the electrical activity of thousands of individual brain cells, called neurons, as patients took memory tests.

They found that the firing patterns of the cells that occurred when patients learned a word pair were replayed fractions of a second before they successfully remembered the pair.

“Memory plays a crucial role in our lives. 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,” said Kareem Zaghloul, M.D., Ph.D., a neurosurgeon-researcher at the NIH’s National Institute of Neurological Disorders and Stroke (NINDS) and senior author of the study published in Science.

The study was part of an NIH Clinical Center trial for patients with drug-resistant epilepsy whose seizures cannot be controlled with drugs.

Dr. Zaghloul’s team recorded electrical currents of patients temporarily living with surgically implanted electrodes designed to monitor brain activity. The hope was to identify the source of a patient’s seizures, but also provided the opportunity to study neural activity during memory. In this study, his team examined the activity used to store memories of our past experiences, which scientists call episodic memories.

In 1957, an epilepsy patient could not remember new experiences after part of his brain was surgically removed to stop his seizures. The breakthrough in memory research pointed to the idea that episodic memories are stored and replayed when triggered by things like a familiar scent or the riff of a catchy tune.

Over the past two decades, rodent studies have suggested that the brain may store memories in unique neuronal firing sequences.

“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 Alex Vaz, a bioengineer who specializes in deciphering the meaning of electrical signals generated by the body who joined Dr. Zaghloul’s lab.

The lab analyzed firing patterns of individual neurons located in the anterior temporal lobe, a brain language center. Patients sat in front of a screen and were asked to learn word pairs such as “cake” and “fox.” The researchers discovered unique firing patterns of individual neurons associated with learning each new word pattern.

“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.

Later, when a patient was shown one of the words, such as “cake,” a very similar firing pattern was replayed just milliseconds before the patient correctly recalled the paired word “fox.”

“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.”