How Brain Ripples Keep Memory Online

Research by scientists at University of California San Diego and colleagues has shed new light on how the human brain coordinates working memory, the process in which the brain encodes information for immediate or short-term use.

The team found that high-frequency brain waves called ripples may help distant brain regions synchronize with one another during working memory tasks. Scientists had previously linked ripple activity to long-term memory, but it had remained unclear whether ripples help coordinate neural activity across long distances in the human brain during active cognition.

Working memory is essential for daily tasks such as following instructions, navigating unfamiliar locations and remembering information from conversations. The new results shed light on how the brain coordinates this process and may also help scientists better understand disorders in which brain connectivity is affected, such as Alzheimer’s disease or attention-deficit/hyperactivity disorder (ADHD).

The new study, led by Ilya Verzhbinsky, a third-year medical student in the Medical Scientist Training Program at UC San Diego School of Medicine, and Eric Halgren, PhD, professor of radiology and neurosciences at UC San Diego School of Medicine and affiliate faculty at the UC San Diego Qualcomm Institute, examined brain recordings from 35 patients who had previously had electrodes implanted into the brain for epilepsy monitoring. Participants were asked to look at one or three images, hold them in their mind over a short period of time, and then decide whether a test image matched one from the original set. The researchers analyzed activity in multiple brain regions involved in memory and cognition to look for coordinated patterns of neural activity.

As described in Nature Neuroscience on August 12, the researchers found:

• Ripple oscillations increased during all stages of working memory.
• When ripples occurred at the same time in different brain regions, neurons in those regions were about 30% more likely to fire together, even across distances of up to 220 millimeters.
• This coordination increased as the memory load increased during the task.
• During retrieval — the stage of working memory in which the brain accesses stored information — co-ripples helped the brain recreate patterns of neuron firing used to form the initial memory.

Together, the findings suggest that ripple oscillations may serve as a mechanism for long-range neural communication, helping the brain integrate information. The findings may also help researchers better understand how memory-related brain networks function in health and disease, including distinguishing healthy cognitive brain signals from abnormal high-frequency activity associated with neurological disease.

Halgren and team are continuing their studies of ripples in early Alzheimer’s Disease patients using QI’s Magnetoencephalography (MEG) Center.

This article first appeared in UC San Diego Today and was edited for the QI website.