What happens in the brain when conscious awareness is changed during dreaming onset?

Question

I've long been interested in dreaming and there's a peculiar phenomenon of the very start of the dreaming process. Different people experience it differently, but even for the same person there's anecdotal evidence of different experiences during dreaming onset.

One of the most dramatic examples is the "wake induced lucid dreaming" transition, where the subject experiences intense hallucinations. These could be sight, sound, feeling of presence, falling, buzzing, etc. As these hallucinations subside, they are replaced by dream imagery.

There are other kinds of transitions as well.

What interests me is if these phenomena have been studied using modern neuroimaging techniques. Does science know what happens in the brain when the persons perception shifts from the real world to dreaming mentations?

Answer 1

Here is the first study with fMRI, EEG and eye-video simultaneously recorded while subjects were performing a task (Poudel et al., 2014).

"fMRI analysis revealed a transient decrease in thalamic, posterior cingulate, and occipital cortex activity and an increase in frontal, posterior parietal, and parahippocampal activity during microsleeps. The transient activity was modulated by the duration of the microsleep. In subjects with frequent microsleeps, power in the post-central EEG theta was positively correlated with the BOLD signal in the thalamus, basal forebrain, and visual, posterior parietal, and prefrontal cortices. These results provide evidence for distinct neural changes associated with microsleeps and with EEG theta activity during drowsiness in a continuous task."

Poudel, G. R., Innes, C. R., Bones, P. J., Watts, R., & Jones, R. D. (2014). Losing the struggle to stay awake: divergent thalamic and cortical activity during microsleeps. Human brain mapping, 35(1), 257-269. doi: 10.1002/hbm.22178.

Answer 2

You're describing Hypnagogia. There has been some research on the EEG states during hypnagogia:

To identify more precisely the nature of the EEG state which accompanies imagery in the transition from wakefulness to sleep, Hori et al. proposed a scheme of 9 EEG stages defined by varying proportions of alpha (stages 1–3), suppressed waves of less than 20μV (stage 4), theta ripples (stage 5), proportions of sawtooth waves (stages 6–7), and presence of spindles (stages 8–9). Germaine and Nielsen found spontaneous hypnagogic imagery to occur mainly during Hori sleep onset stages 4 (EEG flattening) and 5 (theta ripples).