On Mar. 5, 2020, in a study of epilepsy patients, researchers at the National Institutes of Health (NIH) monitored the electrical activity of thousands of individual brain cells, called neurons, as patients took memory tests.

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