Sleep-induced changes in associative memory

The notion that dreaming might alter the strength of associative links in m emory was first proposed almost 200 years ago. But no strong evidence of su ch altered associative links has been obtained. Semantic priming can be use d to quantify the strength of associative links between pairs of words; it is thought to measure the automatic spread of activation from a "node" repr esenting one word to nodes representing semantically related words. Semanti c priming could thus be used to test for global alterations in the strength s of associative links across the wake-sleep cycle. Awakenings from REM and nonREM (NREM) sleep produce a period of state carry -over during which performance is altered as a result of the brain's slow t ransition to full wakefulness, and cognitive testing in this period can pro vide information about the functioning of the brain during the prior sleep period. When subjects were tested across the night-before and after a night 's sleep as well as immediately following forced awakenings from REM and NR EM sleep-weak printing (e.g.,thief-wrong) was found to be state dependent ( p = 0.016), whereas strong priming (e.g., hot-cold) was not (p = 0.89). Wea k primes were most effective in the presleep and REM sleep conditions and l east effective in NREM and postsleep conditions. Most striking are analyses comparing weak and strong priming within each wa ke-sleep state. Contrary to the normal pattern of priming, subjects awakene d from REM sleep showed greater priming by weak primes than by strong prime s (p = 0.01). This result was seen in each of three protocols, in contrast, strong priming exceeded weak priming in NREM sleep. The shift in weak priming seen after REM sleep awakenings suggests that cog nition during REM sleep is qualitative lp different from that of waiting an d NREM sleep and may reflect a shift in associative memory systems, a shift that we hypothesize underlies the bizarre and hyperassociative character o f REM-sleep dreaming. Known changes in brainstem activity that control the transition into and maintenance of REM sleep provide a possible explanation of this shift.