March 9, 2020Neurology
A study of epilepsy patients, published in Science (6 March 2020), monitored the electrical activity of thousands of individual brain cells, called neurons, as patients took memory tests. In this study, the authors examined the activity used to store memories of our past experiences, which is called episodic memories. Results showed 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.
The authors have been recording electrical currents of drug-resistant epilepsy patients temporarily living with surgically implanted electrodes designed to monitor brain activity in the hopes of identifying the source of a patient's seizures. This period also provided an opportunity to study neural activity during memory.
In 1957, the case of an epilepsy patient H.M. provided a breakthrough in memory research. H.M could not remember new experiences after part of his brain was surgically removed to stop his seizures. Since then, research has pointed to the idea that episodic memories are stored, or encoded, as neural activity patterns that our brains replay when triggered by such things as the whiff of a familiar scent or the riff of a catchy tune. But exactly how this happens was unknown.
Over the past two decades, rodent studies have suggested that the brain may store memories in unique neuronal firing sequences. Recently, the authors decided to test this idea in humans. To do this they analyzed the firing patterns of individual neurons located in the anterior temporal lobe, a brain language center. Currents were recorded as patients sat in front of a screen and were asked to learn word pairs such as cake and fox. The authors discovered that unique firing patterns of individual neurons were associated with learning each new word pattern. 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. According to the authors, 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.
Last year, the authors showed that electrical waves, called ripples, may emerge in the brain just split seconds before we remember something correctly. In this study, the team discovered a link between the ripples recorded in the anterior temporal lobe and the spiking patterns seen during learning and memory. They also showed that ripples recorded in another area called the medial temporal lobe slightly preceded the replay of firing patterns seen in the anterior temporal lobe during learning. According to the authors, the results support the idea that memories involve coordinated replay of neuronal firing patterns throughout the brain, and that studying how we form and retrieve memories may not only help us understand ourselves but also how neuronal circuits break down in memory disorders.