Typically a person is said to be dreaming when they are in rapid-eye-movement (REM) sleep, indicated in the brain by high-frequency electrical activity. However, dreaming outside of REM sleep is also possible but the mechanisms behind this have not been well understood… until now.
‘It seemed a mystery that you can have both dreaming and the absence of dreaming in these two different types of stages,’ said Francesca Siclari, co-author of the research from the University of Wisconsin-Madison in the US. However, the research now seems to have solved the mystery.
Published in ‘Nature Neuroscience’, and including not only the US partners but also scientists from Italy and Switzerland, the study shows that when dreaming was reported in both REM and non-REM sleep, a decrease in low-frequency activity occurred in the posterior cortical region. This is an area at the back of the brain associated with spatial reasoning and attention. The research team say they could correctly predict whether a volunteer was dreaming 92 % of the time, simply by monitoring activity in this ‘hot zone’.
Additionally, the team discovered that dreaming about faces was linked to increased high-frequency activity in the region of the brain involved in facial recognition. Dreams involving spatial perception, movement and thinking were similarly linked to the corresponding parts of the brain that handle those tasks and process in waking life.
To reach these findings, the researchers carried out a series of experiments involving 46 participants. All of them had their brain activity monitored using an electroencephalogram (EEG) net worn on the head, covered in 256 electrodes. The participants were woken periodically, then asked whether or not they had been dreaming. They first looked specifically at REM and non-REM sleep, noting that volunteers reported they had dreamt when the ‘hot zone’ was activated, regardless of what state of sleep they were in. ‘Overall in the whole experiment, we did over a thousand awakenings,’ commented Siclari.
In a second experiment, the participants were asked to describe the content of their dreams based on key themes the neuroscientists could identify in the posterior cortex: the aforementioned faces, spatial setting, movement and speech. If a volunteer reported hearing speech in their dream, it would correlate with the region of the brain responsible for language and understanding; if they dreamt about people, the region responsible for facial recognition was ignited. ‘This means that we probably use the same areas of the brain during dreaming, as we do when awake, explaining the sense of reality a dream often portrays for an individual,’ explained Siclari.
What was most exciting for the team was the fact that they could use these findings to predict whether participants had dreamt or not when asleep. In an experiment with 7 participants, they managed to correctly predict instances of dreaming an impressive 87 % of the time.
Moving forward, greater understanding of changes in the ‘hot zone’ and what causes them may reveal whether dreaming has a specific purpose, for example in memory processing. Casting the net wider, the study authors also claim that their results could help shed light on the very nature of consciousness itself, revealing what happens in the brain during sleep when we switch from being unconscious to having conscious experiences.
Co-author Giulio Tononi commented that the experiments will help to ‘zoom in on the brain regions that truly matter for consciousness.’
Source: Based on information from CORDIS.