Electric stimulation boosts beneficial effect of sleep on memory

Having a nap is a great way to consolidate your memory for what you’ve just learned. Now it appears researchers have found a way to boost this beneficial effect.

A control condition confirmed the benefits of sleep: 13 participants remembered 37.42 words in a memory task before sleep, compared with 39.5 on waking. On another occasion with a different set of words, Jan Born and colleagues applied an oscillating electrical current through the participants’ skulls just as they were entering a period of slumber known as slow wave sleep. In this case the participants recalled an average of 36.5 words before sleep, compared with 41.27 words when they were tested on waking – a larger benefit than in the control condition.

"This improvement in retention following stimulation is striking considering that most subjects were medical students, who were highly trained in memorising facts and already performed well in the sham [control] condition", the researchers said.

During slow wave sleep, populations of neurons oscillate between activity and rest, and the application of an oscillating electric current at this time seemed to accentuate the process. The stimulation also caused more sleep spindles – these are bursts of activity that the researchers said could have led to a strengthening of the synaptic connections involved in memory.

Crucially, the stimulation didn’t boost the participants’ memory when it was given at a different frequency or at a different time (just before waking). It also didn’t help participants to learn patterns of finger movements. Such a task depends on procedural memory as opposed to the declarative memory tested by the word task. "Our results indicate that slow oscillations have a causal role in consolidating hippocampus-dependent memories during sleep", the researchers said.
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Marshall, L., Helgadottir, H., Molle, M. & Born, J. (2006). Boosting slow oscillations during sleep potentiates memory. Nature, DOI: 10.1038/nature05278.

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