“I think sleep has always intrigued a lot of people because not everyone even today believes sleep contributes to useful active processing in the brain,” Karen Debas, PhD, neuropsychologist, University of Montreal, Quebec, Canada, told Medscape Medical News.
“So the fact that we found sleep contributes to the consolidation of memory is important and to prove that it is doing so is to understand the mechanisms that are taking place during sleep.”
The study is published in the October issue of NeuroImage.
The aim of the study was to directly compare changes in functional connectivity related to the consolidation process of a motor memory in 2 groups of young adults. Some 24 participants between 19 and 30 years of age who had no experience playing a musical instrument executed a 5-element version of the finger sequence learning task. Essentially, participants were taught a new sequence of piano-type finger movements on a box.
However, one group performed the task in the evening and was retested the following morning (night/sleep group), while the other group performed the same task during the morning and repeated the task again that evening (day/awake group). All participants performed the test and repeated test while being observed using functional MRI.
Changes in connectivity within the cortico-striatal network were significantly different depending on whether participants had slept or not, investigators report. Specifically, results showed a 21% decrease in the level of integration in the cortico-striatal region in the day/awake group compared with a 7% increase in integration levels following a night of sleep (night/sleep group) (P = .03).
Investigators also noted that only the cortico-striatal system revealed a strong tendency toward an increase in integration following sleep and that none of the other networks demonstrated significant sleep-related changes in integration.
These findings suggest that there is greater synchrony of activity between regions forming the cortico-striatal network and that communication between the various regions of the network is optimized after a period of sleep. As Dr. Debas explained, the network the investigators evaluated consists of a lot of subcortical and cortical areas, including the putamen, that are important for motor skill consolidation.
“After a night of sleep, we found that this network was more integrated than the others, that is, interaction among these regions was greater when consolidation had occurred,” Dr. Debas said in a press release. “So a night of sleep seems to provide active protection of this network, which the passage of daytime does not provide.”
Dr. Debas emphasized that findings from this study are essentially a step forward toward a better understanding of the mechanisms involved in memory consolidation.
“Ultimately, however, we believe that we will be better able to explain and act on memory difficulties presented by certain clinical populations who have sleeping problems and help patients who are relearning motor sequences in rehabilitation centers,” she suggested.
Stephan Swinnen, PhD, Francqui Research Professor, Motor Control Laboratory, Movement Control and Neuroplasticity Research Group, Leuven, Belgium, told Medscape Medical News the study’s results are very interesting and thought-provoking because the data show that when someone practices a motor task during the day, it does not mean that the task-related activity is abolished as soon as practice has stopped.
“There may be exciting brain activity going on during sleep that results from the activity in particular brain areas that were recruited while you were involved in practice,” he elaborated. “It is as if the brain replays this activity in one way or another and this may help in laying down this experience in your memory in a more durable or long-lasting manner.”
Dr. Swinnen added that as a result of sleep following practice, a “privileged interaction” is established between the brain areas that were strongly active during practice — that is, the cortico-striatal pathway — as part of a broader brain network.
“Again, it is as if these brain areas resonate better with each other as a consequence of sleep following practice of the motor task,” he explained. “It’s like improving traffic on the freeway between 2 important cities such that the 2 towns interact more intensively with each other and function better as a result.”
Mark Mahowald, MD, professor of neurology (retired) University of Minnesota, Minneapolis, and a member of the American Academy of Neurology, told Medscape Medical News that the study adds further evidence to the concept that at least one of the functions of sleep is memory consolidation.
“In our society today, we equate sleep and sleepiness with laziness and depression and other defects of character and where sleep deprivation is held out as a badge of honor,” Dr. Mahowald said. “When in fact, any degree of sleep deprivation will impair performance and will impair learning. This is good evidence that the brain is not resting during sleep — it’s consolidating memories and performing extremely important memory-related functions. Sleep is important.”
The study was funded by the Canadian Institutes of Health Research and the Fonds de recherché due Quebec en santé. The authors, Dr. Swinnen, and Dr. Mahowald have disclosed no relevant financial relationships.
NeuroImage. 2014:99:50-58. Abstract