Closed-loop auditory stimulation can be used to modulate alpha and theta oscillations during REM sleep

The aim of your study was to see if alpha and theta activity can be modified during REM sleep. Can you first tell us why it would be interesting to be able to modify alpha and theta activity in REM sleep? (What roles are alpha and theta activity thought to have? If you were able to modulate alpha and theta activity what affects would you expect this to have?)

During wakefulness, alpha and theta activity have been linked to many cognitive functions, such as attention, perception, and memory. During REM sleep, we also see alpha and theta activity, but the function of this oscillatory activity is still not very clear. It has been hypothesized that these oscillations may be involved in memory consolidation and dreaming. By having a tool that can modify these oscillations we can more directly test the role of these oscillations in cognitive functions.

Your study also aimed to see if any effects you found are dependent of the type of REM sleep – tonic or phasic – can you explain the difference between tonic and phasic REM sleep and why you might expect different effects?

Phasic REM is characterised by the occurrence of rapid eye movements whereas tonic REM sleep represents the periods in-between, in which rapid eye movements are absent. Previous research has shown that the brain is less responsive to sounds in phasic compared to tonic REM sleep and it is therefore more difficult to wake someone up from phasic REM sleep. Since our stimulation was based on playing sounds during sleep we wanted to see if the effects may differ between the two substages.

You used closed-loop auditory stimulation to modulate alpha and theta activity in REM sleep. What is closed-loop auditory stimulation and how does it work?

In closed-loop auditory stimulation, brain activity is recorded continuously, and sounds are precisely timed to a specific part of the ongoing oscillation. The sounds are delivered at a low volume to avoid waking up the sleeping person. Depending on which part of the oscillation is targeted the oscillations can be modified differently.

So what did you find in your study?

We found that when we targeted the trough of the alpha or theta oscillation, we could speed up these oscillations, whereas when we targeted the rising part of the oscillation (for alpha) or the peak (for theta), we could slow them down. This modulation seemed to work equally well in phasic and in tonic REM sleep.

What are the implications of these findings? What could the modulation of alpha and theta activity in REM sleep be used for?

These findings show that indeed closed-loop auditory stimulation can be used to modulate alpha and theta oscillations during REM sleep. Such a tool is useful to further study the function of these oscillations. In addition, since these oscillations slow down in ageing and dementia which is associated with cognitive impairments, such a tool may be used as a therapeutic intervention to speed up these oscillations. However, further research is needed to evaluate the usefulness of this approach as a clinical intervention, and whether beneficial effects on cognition can be achieved.