Memory can be a fickle thing. The sequence of events that one person remembers may be completely different than another’s, but why that changes, where memories are stored, and how a person recalls them have long been the subject of intense scientific study.
As part of a clinical trial of patients with drug-resistant epilepsy, researchers at the National Institutes of Health (NIH) monitored and recorded the electrical activity of the brain in real-time. As memories were spurred in the patients, the brain exhibited unique neural patterns, which may be used to store memories and replay them later.
Previous research indicates there may be a link between epilepsy and the part of the brain that holds memories. In 1957, 27-year-old American Henry Molaison underwent a bilateral medial temporal lobectomy to surgically remove a part of his brain to stop his seizures but was unable to form new memories after the procedure.
In this new study, electrodes were surgically implanted in the epilepsy patients in order to try and identify where seizures were sourced in the brain. These electrodes recorded currents as patients sat in front of a screen and were asked to learn word pairs like “cake” and “fox”. Each new connection was shown to be associated with a unique neural firing pattern in the language center of the brain known as the anterior temporal lobe. When a patient was later shown one of the words, a similar pattern was replayed milliseconds before the patient recalled its 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 study author and Kareem Zaghloul, MD, PhD, a neurosurgeon-researcher at the NIH's National Institute of Neurological Disorders and Stroke, in a statement.
It appears that this activity may be used to store memories of past experiences, known as episodic memories. Retrieval of episodic memory is thought to rely on replaying past experiences triggered by things like a familiar scent or a particular song.
"Our results support the idea that memories involve coordinated replay of neuronal firing patterns throughout the brain," said 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."
The findings, which are published in Science, suggest that the brain may use these distinct sequences to store memories and replay them later using a distinct electrical pattern in the neurons.