Sleep and memory: can learning be enhanced?

The history of sleep and memory research began with Ebbinghaus in 1885. Research remained dormant for some time, but by the late 1980s, studies involving trampolining (Percept. Mot. Skills 1988;6:635-45), intensive study of a foreign language (Int. J. Psychophysiol. 1989;8:43-7), and learning Morse code (Physiol. Behav. 1989;46:639-42) all demonstrated increases in REM sleep after successful learning.

The modification or pruning of memories may also occur during REM sleep (Nature 1983;304:111-4). Poe has demonstrated that neural firing during the peak volume of hippocampal neurons induces long-term potentiation, and, while on the trough, longterm depression (Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 2007;75:011912). This provides a clue as to how episodic memories may slowly be transformed into semantic memory. It seems that through time, the less important aspects of a memory are pruned while the core of what we really need to know is consolidated.

In addition, other sleep stages are important in memory. In motor procedural tasks, an increase in the total number of stage II sleep spindles, especially those in the last quarter of the night, may be seen for the groups that do well on posttraining retesting (Neuron 2002;35:205-11).

The 1-2 Hz, large amplitude, predominantly frontal lobe synchronous firing during slow wave sleep (SWS) is also involved (J. Neurosci. 1999;19:9497-507), and there is a link in the timing of these slow waves and the firing patterns of cells in the hippocampus (J. Neurophysiol. 2006;96:62-70).

The body of research as a whole is pointing toward the conclusion that REM sleep, the sleep spindles of stage II sleep, and SWS are all important in memory, for different but complimentary reasons.

Is there a way to enhance the learning process to improve upon our ability to learn? Learning may be enhanced when auditory clicks are presented first while learning and then later during bursts of rapid eye movements in REM sleep (Psychiatr. J. Univ. Ott. 1990;15:85-90). Neuronal excitation may occur when auditory clicks are presented during the sleep spindles of stage II sleep during the ascent from SWS to REM sleep (Sleep Res. 1977;6:24). Born has demonstrated that transcranial electrical stimulation provided to the frontal lobes during SWS seems to enhance learning (J. Neurosci. 2004;24:9985-92), and that the olfactory scent of a rose provided during learning and then again during SWS enhanced learning as retested the following day (Science 2007;315:1426-9).

Theoretically, a device could be devised to provide a stimulus during conscious learning, and then again during the occurrence of specific neurophysiologic waveforms of sleep.

For instance, repertitive transcranial magnetic stimulation (rTMS) could be provided over the precentral gyrus of the left hemisphere during wakefulness while the subject performs a task involving dexterity of the right hand. During sleep, rTMS stimulation could again be provided during the slow waves of SWS, during the sleep spindles of stage II sleep (especially for the memory processing of motor procedural tasks), or during the sawtooth waves of REM sleep that typically occur just prior to the phasic bursts of rapid eye movements. Poe's research may indicate that the timing of stimulation on the peak versus the trough of REM sleep EEG waveforms in the theta frequency (presumably sawtooth waves) may make the difference between consolidation and pruning.

As TMS may be expensive, potentially epileptogenic, and of limited availability, other tools may potentially be used to enhance memory consolidation. Short auditory snippets that are similar to cell phone ring tones, or short clips of well known songs--each unique and each lasting several seconds in duration--could be played during the learning of a specific task, fact, or relationship that is to be remembered by the subject, and then played again during the EEG waveforms of sleep, as outlined for rTMS.

It is also conceivable that visual or somatosensory information could be provided during conscious learning and later during sleep in a similar fashion as that described for auditory snippets.

Is the enhancement of the normal learning process really possible? Only time will tell. It isn't clear if we will all soon be wearing electrode grids or hearing auditory snippets while we learn and while we sleep so that we can all learn at a superhuman rate. In the meantime, watching how the research pans out will surely be interesting.

DR. WALTER is the author of "REM Illumination, Memory Consolidation" (Grove City, Ohio: Lotus Magnus LLC, 2007), and is a neurologist and codirector of a sleep diagnostic practice in Grove City, Ohio.

BY TIMOTHY J. WALTER. M.D.