To sleep, to dream—but why exactly? Having gone the "dream no more" route, Hamlet never really got around to pondering that specific question. Scientists, however, have been wondering what purpose sleep and dreaming serve for about as long as the field has been in existence. Now, a group of researchers think they have a partial answer: Dream-stage sleep may help young brains solidify their daily experiences into long-lasting neural circuits essential to a properly functioning brain.
Dream or rapid eye movement (REM) sleep likely has something to do with processing the day's events or consolidating memories, but the story's a little more complicated than that. Notably, babies throughout the mammal class get a lot more REM sleep than adults do—about eight hours a day total, compared to around two hours in adults. There's evidence that all that extra dreaming has something to do with the proper development of the brain's neural circuitry, but exactly how that happens remains unclear.
Something about REM sleep solidified kittens' visual experiences into more permanent brain circuits.
To investigate, a team of neuroscientists led by Michelle Dumoulin Bridi and Marcos Frank decided to study the development of the neural pathways responsible for vision in kittens. The team first implanted electroencephalograms (EEG) in 33 kittens to track electrical activity in their brains. Later on, each kitten wore an eye patch over one eye for six hours—ordinarily, that makes the uncovered eye dominant, since the brain never learns to see out of the covered eye. (That can also leave an infant with congenital cataracts permanently blind in the obstructed eye, even if doctors eventually remove the obstruction.) Next, each kitten took a nice little nap, except that the researchers woke a third of the kittens right after they started dreaming, usually five or 10 minutes after they drifted off to sleep. The researchers woke up another third during non-REM sleep, while the final third were allowed to sleep for the entirety of the experiment.
As expected, EEG measurements showed that the eye patches led kittens to develop a dominant eye—just not in the kittens who'd had their dreams interrupted. Apparently, something about REM sleep solidified kittens' visual experiences into more permanent brain circuits. More to the point, without REM sleep, their brains just weren't developing at all. That is, kittens' experiences should have solidified into something, even if it wasn't quite the right thing, but interrupting kitten dreams prevented that from happening.
So how does REM sleep solidify experiences? Additional studies indicate that disrupting REM sleep prevented the activation of a protein called extracellular signal-regulated kinase (ERK), which the team suggests may play a role in strengthening connections between individual neurons.
"These findings support a long-standing hypothesis that REM sleep in early life promotes circuit formation," the team writes. "Our findings suggest that REM sleep achieves this function by promoting molecular and network events that reinforce neural patterns present" in daily experiences. Now, it's time to take a nap with some kittens.
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