Slow Wave Stimulation May Help Treat Brain Disorders

by Warren Grill, senior technical editor

We are taught that long hours of study or practice are the best means to learn new material. However, recent work indicates that sleep is essential for consolidating new memories. The underlying mechanisms are not well understood but two competing theories posit that it is the effects of chemical modulators or the widespread oscillations in brain electrical activity that contribute to memory consolidation during sleep.

A recent study demonstrating that enhancing brain oscillations with applied electric fields improves memory formation appears to support the latter view. Lisa Marshall and colleagues from Jan Born’s group at the University of Lübeck, Germany, measured the effects of applied currents, meant to produce biological like-oscillations in brain activity, on memory performance in a group of medical students. Their findings, published in Nature, indicated that generation of slow cortical oscillations enhanced the recall of word lists studied the previous night. There was an almost 13 percent increase in word retention in the group that received stimulation, whereas the control group’s retention increased just over 5 percent.

Stimulation was applied with surface electrodes at frontolateral locations. The applied currents were estimated to generate electric fields within the brain that were comparable to those calculated from local field potential recordings during slow oscillatory activity. Subjects received five 5-minute periods of stimulation, separated by 1-minute intervals of no stimulation.

The improvement in word recall occurred only when slow wave stimulation (0.75 Hz) was applied early in sleep during a period of nascent slow wave sleep, and not when stimulation was applied shortly before waking. Further, the effect was not observed with more rapid theta frequency (5 Hz) stimulation.

The effect appeared to be restricted to certain types of declarative memory, as stimulation had no impact on a sequenced finger tapping task. Performance on this non-declarative procedural motor skill did improve following sleep, but the gain was not enhanced by stimulation.
The applied stimulation increased slow oscillations and sleep spindles in the EEG and increased the length of time spent in slow wave sleep. The authors suggest that the increase in sleep spindles, which accompany robust firing in cortical neurons, may lead to long-term potentiation of cortical synapses, and thereby enhanced memory formation.

This finding may herald an era where potentially simple neurotechnology devices can be used to treat devastating neurodegenerative diseases. Methods to enhance neuronal plasticity might help compensate for neurons lost to aging or disease. Interestingly, aging is associated with a reduction in slow wave sleep, and this study suggests that this might be reversed with applied electric fields.

A commentary by Jorge Palop and colleagues from UCSF, also appearing in Nature, suggested that the dramatic daily fluctuations in function observed in persons with neurodegenerative disease are the result of alterations in neuronal network function. Imaging studies have revealed that the distribution of changes in neuronal activity in diseased brains far exceeds the regions where neurons have degenerated. The abnormal patterns of neuronal activity and network interactions, which are susceptible to pharmacological manipulation, may also be susceptible to modulation with applied electric fields and open up new opportunities to treat neurodegenerative diseases.