Electrodes Show Memories Emerging in a Brain

Seconds before a memory pops up, certain nerve cells jolt into collective action. The discovery of this signal sheds light on the mysterious brain processes that store and recall information. Electrodes implanted in the brains of epilepsy patients picked up neural signals in the hippocampus, a key memory center, while the patients were shown images of familiar people and places, including former President Barack Obama and the Eiffel Tower in Paris. As the participants took in this new information, electrodes detected a kind of brain activity called sharp-wave ripples, created by the coordinated activity of many nerve cells in the hippocampus.

Later blindfolded, the patients were asked to remember the pictures. One to two seconds before the participants began describing each picture, researchers noticed an uptick in sharp-wave ripples, echoing the ripples detected when the subjects had first seen the images.

A second or two before a person remembers a face or a monument, electrodes (yellow) caught ripples of brain activity in the hippocampus (colored areas). That echo suggests that these ripples are important for learning new information and for recalling it later, Yitzhak Norman of the Weizmann Institute of Science in Rehovot, Israel, and colleagues write in the study. Earlier studies suggested that these ripples in the hippocampus were important for forming memories. But it wasn’t clear if the ripples also had a role in bringing memories to mind. In another recent study, scientists also linked synchronized ripples in two parts of the brain to better memories of word pairs.

CITATIONS Y. Norman et al. Hippocampal sharp-wave ripples linked to visual episodic recollection in humans. Science. Vol. 365, August 16, 2019, p. 65.