The researchers next exposed the rats to four kinds of novel objects in the dark, since largely nocturnal rodents depend on the sense of touch via their whiskers to investigate their environment. The four objects were a golf ball mounted on a spring, a fingernail brush, a stick of wood with pins attached and a tube that dispensed cereal treats.
The researchers recorded and analyzed brain signals from the rats before, during and after their exploration, for several days across natural sleep-wake cycles. Analyses of those signals revealed "reverberations" of distinctive brain wave patterns across all the areas being monitored for up to 48 hours after the novel experience.
According to Ribeiro, "We found that the activity of the brain when the animal is in a familiar environment does not 'stick' -- that is, the brain keeps moving from one state to another. In contrast, when the animal is exploring a novel environment, that novelty imposes a certain pattern of activity, which lingers in all the areas we studied. Also, we found that this pattern was much more prevalent in slow-wave sleep than in REM sleep."
Conversely, previous studies by Ribeiro and his colleagues demonstrated that the activation of genes able to effect memory consolidation occurs during REM sleep, not slow-wave sleep.
"Based on all these results, we're proposing that the two stages play separate and complementary roles in memory consolidation," he said. "Periods of slow-wave sleep are very long and produce a recall and probably amplification of memory traces. Ensuing episodes of REM sleep, which are very short, trigger the expression of genes to store what was processed during slow-wave sleep." In principle, this model explains studies such as those by Robert Stickgold and his colleagues at Harvard University, showing that both slow-wave and REM sleep have beneficial effects on memory consolidation, he said. According
Contact: Dennis Meredith
Duke University Medical Center