Neuroscientists from NYU Grossman School of Medicine have uncovered a fascinating mechanism that determines which memories are deemed important enough to be permanently etched into the brain. This mechanism, detailed in a study published in the prestigious journal Science on March 28, sheds light on the intricate processes at play when the brain decides which experiences to retain and which to discard.
The study, led by senior author György Buzsáki, the Biggs Professor of Neuroscience in the Department of Neuroscience and Physiology at NYU Langone Health, delves into the intricate workings of brain cells known as neurons. These neurons are responsible for encoding memories by transmitting electrical signals through rhythmic cycles of firing, creating complex sequences of signals within the brain's hippocampus region.
At the heart of this mechanism are "sharp wave-ripples," which are essentially the simultaneous firing of 15 percent of hippocampal neurons. These sharp wave-ripples, aptly named for the distinctive shape they take when captured by electrodes and recorded on a graph, have been linked to the formation of memories during sleep. The new study, however, unveils a crucial finding: daytime events immediately followed by five to 20 sharp wave-ripples are more likely to be replayed during sleep and thus consolidated into permanent memories, while events followed by very few or no sharp wave-ripples tend to fade away.
Dr. Buzsáki says, "Our study finds that sharp wave-ripples are the physiological mechanism used by the brain to 'decide' what to keep and what to discard." This discovery represents a significant step forward in understanding the intricate processes governing memory consolidation and retention.
The study is rooted in the observation that mammals, including humans, experience the world in brief intervals followed by pauses, a pattern that forms the basis for the brain's computation and assessment of experiences. The research team, building on their prior work, observed that sharp wave-ripples occur during these idle pauses after waking experiences, with the tagged neuronal patterns reactivated during post-task sleep.
Importantly, sharp wave-ripples are constituted by the firing of hippocampal "place cells" in a specific order that encodes the layout of environments, such as rooms or maze arms. These cells play a pivotal role in the reactivation of memories during sleep, effectively "playing back the recorded event thousands of times per night," which serves to strengthen the connections between the cells involved.
The study's findings were underpinned by meticulous tracking of successive maze runs by study mice, using electrodes to monitor the firing patterns of hippocampal cells during waking pauses and subsequent reactivation during sleep. Notably, sharp wave-ripples were frequently recorded when a mouse paused to indulge in a sugary treat after a maze run, highlighting the brain's shift from exploratory to idle patterns conducive to memory consolidation.
In their quest to unravel the complexities of memory formation, the research team encountered the challenge of recording up to 500 neurons simultaneously in the hippocampus of animals during maze runs. However, by reducing the dimensions in the data, they were able to gain a deeper understanding of neuronal activity and form hypotheses without compromising the integrity of the data.
