Why we dream is still a mystery to science. But differences in brain activity may give a clue as to why some people frequently remember their dreams while others rarely do, according to neuroscientists in France.
Perrine Ruby, an Inserm Research Fellow at the Lyon Neuroscience Research Center, and colleagues report their findings in the journal Neuropsychopharmacology.
In previous work, they had established that high dream recallers (people who remember their dreams nearly every day) wake up at night twice as often as low dream recallers (people who rarely remember their dreams).
They also found that the brains of high dream recallers are more reactive to sounds during sleep and wakefulness.
For instance, they found that high dream recallers responded more strongly to hearing their name.
More periods of wakefulness may give brain chance to memorize dreams
The researchers suggested that having a brain that reacts more to sounds may trigger the wakeful periods at night, and it is during these periods that the brain memorizes any dreams. As Ruby states, "the sleeping brain is not capable of memorizing new information; it needs to awaken to be able to do that."
In this new study, they sought to identify the brain regions involved and how they differed between the high dream recallers and the low dream recallers.
They recruited 41 volunteers - 21 high dream recallers who recalled their dreams on average five mornings a week, and 20 low dream recallers who recalled their dreams on average only twice a month.
Using Positron Emission Tomography (PET scans) they monitored the brain activity of these two groups during sleep and wakefulness.
They found that the temporo-parietal junction (TPJ) and the medial prefrontal cortex showed more spontaneous activity in the high dream recallers, both during sleep and when awake.
Previous studies by the South African neuropsychologist Mark Solms had also shown that lesions in these two brain areas stop people being able to recall dreams altogether.
This new study is important because it highlights that both during sleep and wakefulness, high and low dream recallers have different levels of brain activity in these regions.
More active TPJ could explain higher levels of wakefulness
The researchers suggest if the TPJ - a part of the brain that acts like an information processing hub - is more active in high dream recallers, then this could be making them more alert to external stimuli, which in turn causes them to be awake more at night, giving their brain more opportunities to encode dreams into memory.
However, they do not exclude the possibility that dream production, as opposed to dream memorization, could also be different between the two groups, and they conclude:
"Our results suggest that high and low dream recallers differ in dream memorization, but do not exclude that they also differ in dream production. Indeed, it is possible that high dream recallers produce a larger amount of dreaming than low dream recallers."