Brain activity ripples linked to creation of long-term memories

An anatomical map with raw sample traces of neocortical ripples, left, color coded to show the brain regions in which they occurred. György Buzsáki, M.D., Ph.D., NYU School of Medicine

While we’re asleep, the brain is working to store new information as long-term memories. Storing a memory likely involves interactions between the brain’s hippocampus and parts of the cortex. Scientists have been trying to determine the precise connections by examining electrical activity within these regions of the brain.

When hundreds or thousands of nerve cells, or neurons, in the brain become activated at the same moment, the high-frequency electrical activity shows up on recordings as ripples. Previous work by Dr. György Buzsáki at New York University revealed ripples of high-frequency activity in the rat hippocampus during sleep and suggested that ripples play a role in memory storage.

In the current study, Buzsáki’s team set out to record electrical activity in multiple regions of the brain for evidence of cross-talk during sleep. The work was funded in part by NIH’s Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative, which is managed and funded by several NIH components including NIH’s National Institute of Neurological Disorders and Stroke (NINDS) and National Institute of Mental Health (NIMH). Results were published on October 20, 2017, in Science.

The researchers created a thin, flexible array of tightly packed, tiny electrodes that can be placed on the surface of the brain. This array can record the electrical activity of single neurons. Using the device, called NeuroGrid, on top of the rat brain along with recording electrodes placed deeper into the brain, the team recorded activity in several brain regions during non-rapid eye movement (NREM) sleep. They noticed activity ripples in the association neocortex, an area on the brain’s surface involved in processing complex information. At the same time, ripples occurred in the hippocampus. The simultaneous ripples suggested that the two regions were communicating.

Six rats were given a memory training session to find water in a maze. The researchers then recorded the rats’ brain activity during NREM sleep. In these trained rats, the learning task increased the cross-talk between the association neocortex and the hippocampus. A second training session boosted the cross-talk even more. Four rats who didn’t undergo memory training were allowed to roam freely through a maze without reward. These untrained rats didn’t have synchronized ripples in the association neocortex and the hippocampus during sleep. The findings suggest that communication between these brain regions is important for the creation and storage of memories.

“Identifying the specific neural patterns that go along with memory formation provides a way to better understand memory and potentially even address disorders of memory,” says co-first author Dr. Jennifer Gelinas of New York University and Columbia University.

The researchers plan to use the NeuroGrid to study whether disrupting the ripples has an effect on memory formation in lab animals. They also plan to use the NeuroGrid to find out whether ripples occur in the same brain regions of people.