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Memories involve replay of neural firing patterns

Memories and their recall involve coordinated activity throughout the human brain. metamorworks / iStock / Getty Images Plus

A familiar scent or the chorus of an old song can trigger a memory. Scientists are working to understand how the brain stores and retrieves these memories of our past experiences. Studies in rodents have suggested that memories are stored as unique firing patterns of brain cells, or neurons. These patterns are replayed when the memory is retrieved.

Researchers led by Dr. Kareem Zaghloul of NIH’s National Institute of Neurological Disorders and Stroke (NINDS) set out to determine whether similar brain activity takes place in people when recalling memories. The study was supported by NINDS, along with NIH’s National Institute of General Medical Sciences (NIGMS). Results appeared in Science on March 6, 2020.

The team was conducting a study of patients with epilepsy whose seizures couldn’t be controlled with drugs. As part of the trial, the patients were temporarily living with electrodes surgically implanted in their brains to monitor neural activity. This allowed the scientists to also investigate how memories are stored and retrieved.

Six participants were given memory tests while researchers monitored the electrical activity of thousands of individual neurons. Participants sat in front of a screen and were asked to learn unrelated word pairs such as “cake” and “fox.”

Our brains may store memories in specific neuronal firing patterns, visualized here, that are replayed fractions of a second before remembering. Individual neurons are represented by different colors.Zaghloul lab, NINDS

The scientists focused on activity in the anterior temporal lobe, a brain language center, and the medial temporal lobe, an area linked to memory recall in rodents. They also analyzed “ripples”—high-frequency brain activity when many neurons are activated at once. In a recent study, the team found that ripples emerge in the brain fractions of a second before recalling a memory.

The researchers observed unique firing patterns of individual neurons when learning each new word pairing.

Later, the participants were shown one of the words, such as “cake,” and asked to recall the paired word, “fox.” A very similar firing pattern replayed in their brains just milliseconds before they correctly recalled the paired word.

The team also discovered a link between the ripples and the neural firing patterns seen during learning and recall. They found that ripples recorded in the medial temporal lobe occurred slightly before the replay of firing patterns observed in the anterior temporal lobe during learning. 

These findings suggest that memories involve coordinated replay of neuronal firing patterns throughout the human brain.

“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,” says 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,” he adds.