Not long after the development of the electoencephalogram (EEG) it was found that the EEG during sleep shows more synchronized and slower activity most of the time compared to our waking EEG. This was termed "slow wave sleep." These predominating slow wave sleep periods are interrupted by briefer periods of desynchronized, faster EEG activity, during which there are rapid movements of the eyes: this is termed REM sleep.
Most, but not all, dreams occur during REM sleep. This makes sense if faster and desynchronous brain activity is present when the brain is actively processing experiences, since dreams seem to be a kind of experience the brain can process. The rapid eye movements are similar to those we use when looking around us when awake, and it is likely that visual imagery during REM sleep is accompanied by the dreamer moving the eyes in a similar way they would if the dream images were actually in their waking vision. Other muscles of the body are quite relaxed during dreams: if they are not, a sleep disturbance ("REM behavior disorder") in which dream actions are accompanied by body movement may occur.
What of the dreams during slow wave (non-REM, or NREM) sleep? The study below suggests that even though much of the brain during such dreams shows slow activity, there is still a small area of the posterior brain which shows faster, desynchronized activity during dreams that occur during non-REM sleep. The area of the brain which has such faster activity during NREM dreaming is the same area activated by visual imagery during wakefulness, suggesting that, philosophically speaking, visual qualia are being generated during such dreams.
Studies such as this are difficult, time consuming, and expensive to do well, since one must, for each data point, have subjects sleep in the lab with an investigator or well trained tech able to wake the subjects at the right moment to ask if the subject was indeed dreaming when awakened. The investigators are to be complimented for their work.
One quibble: the investigators say at the end of their paper that "our findings indicate that the neural correlate of consciousness, rather than involving the broad lateral fronto-parietal network, is restricted to a posterior hot zone." This would be true if having visual imagery means we must be conscious. But are we really conscious while dreaming? Certainly we are not conscious in the same way we are when awake.
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ABSTRACT
The neural correlates of dreaming
Francesca Siclari, Benjamin Baird, Lampros Perogamvros, Giulio Bernardi, Joshua J LaRocque, Brady Riedner, Melanie Boly, Bradley R Postle, Giulio Tononi
doi: http://dx.doi.org/10.1101/012443
Consciousness never fades during wake. However, if awakened from sleep, sometimes we report dreams and sometimes no experiences. Traditionally, dreaming has been identified with REM sleep, characterized by a wake-like, globally ‘activated’, high-frequency EEG. However, dreaming also occurs in NREM sleep, characterized by prominent low-frequency activity. This challenges our understanding of the neural correlates of conscious experiences in sleep. Using high-density EEG, we contrasted the presence and absence of dreaming within NREM and REM sleep. In both NREM and REM sleep, the presence of dreaming was associated with a local decrease in low-frequency activity in posterior cortical regions. High-frequency activity within these regions correlated with specific dream contents. Monitoring this posterior ‘hot zone’ predicted the presence/absence of dreaming during NREM sleep in real time, suggesting that it may constitute a core correlate of conscious experiences in sleep
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