During REM sleep, dreams are the epitome of living in the moment, of harnessing the power of now. During these dreams, consciousness is concerned solely with the immediate present. Once we awaken, humans—"and supposedly only humans," according to a German study out in Nature Neuroscience—inhabit a different mode of consciousness that allows higher-order cognitive functions like abstract thinking, volition, and self-reflective awareness.
But sometimes, we humans can have lucid dreams. These dreams take place during REM sleep, but we are aware we are in a dream. We can even control the dream's plotline.
EEG and fMRI studies have shown that there is elevated activity in what’s called the lower gamma frequency band, notably in frontal and temporal regions of the brain, during lucid dreams. Gamma band activity is modulated by sensory input and internal processes like working memory and attention. It is most often viewed as a byproduct of neural activity; it is not clear if it contributes to brain function.
Activity in the frontotemporal region is related to executive ego functions; it is characteristic of being awake, and it's not usually seen during REM sleep. So the question obviously is: does lucid dreaming trigger this atypical electrical activity, or vice versa? And either way, is it required for lucid dreaming?
To find out, these researchers subjected people to frontotemporal transcranial alternating current stimulation (tACS) at various frequencies ranging from 2-100Hz. Controls got sham conditions—simulated stimulation without current flow. Unlike an older method of brain stimulation, transcranial magnetic stimulation, tACS has no side effects like noise or tactile sensations that might disturb the subjects' sleep. Brain activity was then monitored by continuous electroencephalography (EEG), electrooculography (EOG), and electromyography (EMG). The subjects were woken up and asked to rate their dream consciousness.
Stimulation with 25 and 40Hz increased activity in the lower gamma band, which is not generally seen during REM sleep. More significantly, it increased five out of eight factors associated with lucid dreaming: insight into the fact that one is dreaming; control over the dream plot; sense of realism; access to waking memory; and dissociation akin to taking on a third-person perspective.
Rapid eye movements were maintained throughout the stimulation and dream sequence, so subjects were definitely still in REM sleep. The researchers conclude that their induced gamma band oscillations altered awareness during sleep.
An increase in lower gamma band power was significantly stronger in the presence of lucid dreaming, but it was also detectable in its absence. Lower gamma-band stimulation thus might act to enhance neuronal synchronization, which then "sets the stage for lucidity in dreams." So if you're planning to incept Alec Baldwin, just make sure to avoid first stimulating frontotemporal gamma activity—you definitely do not want him to be lucid.
Nature Neuroscience, 2014. DOI: 10.1038/nn.3719 (About DOIs).
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