by Jonathan Learmont
When the discovery of Rapid Eye Movement (REM) sleep was published in 1953, it threw off the perception that sleep was exclusively associated with lower levels of brain activity. This is because when REM sleep occurs, the observed brain waves in people sleeping are similar to when they are awake. This state is associated with vivid dreaming, and despite a lack of consensus on its function, may give credence to a theory about why we sleep the first place; replaying memories in our minds in this way was an evolutionary advantage.
Subsequent research has indeed shown that sufficient sleep is associated with a greater ability to recall what was memorised the previous day relative to those who did not sleep enough. But the reasons why certain information is able to be remembered after sleep but some not at all was not well understood.
A study conducted by researchers at Ruhr University Bochum and the University of Bonn in Germany has sought to unravel this mystery. By using a sample of epilepsy patients, who already had electrodes implanted into their skulls, they were able to gather specific information on their brain wave patterns to reveal how activities were remembered.
Participants were given a set of pictures to memorise and had their high frequency brain wave fluctuations, referred to as gamma band activity, examined that were associated with each image when looked at. They recorded information on both the superficial and deep processing of the image in the brain, with superficial activity recorded in the first half a second after looking at the image, and deep processing the remainder of the time.
Using these recorded fluctuations, the scientists could spot the gamma band patterns recurring when subjects slept. Most interestingly, these patterns appeared even when the individual could not recall the memory after napping, suggesting simply replaying the memory won’t ensure it is remembered. Dr Hui Zhang, from the Department of Neuropsychology in Bochum, said “The forgotten images do not simply disappear from the brain”.
Researchers found that being able to recall after sleep was contingent on the involvement of the hippocampus. A region of the brain known to be important for memory, it displayed extremely rapid fluctuations in activity called ‘ripples’ concurrently when deeply processed memories were replayed during certain sleep phases. This relationship was not found when participants were awake.
Perhaps these revelations into how our brain memorises during sleep can bring us closer to understanding the origins of sleep. But more likely, it will have serious implications for subsequent therapies and treatments designed to remedy all kinds of memory disorders.