Learning while asleep is a dream of mankind, but is often deemed impossible because sleep lacks the conscious awareness and neurochemical milieu thought to be necessary for learning. Current evi- dence for sleep learning in humans is inconclusive.
To explore conditions under which verbal learning might occur, we hypothesized that peaks of slow waves would be conducive to verbal learning because the peaks define periods of neural excit- ability.
While in slow-wave sleep during a nap, a se- ries of word pairs comprising pseudowords, e.g., "tofer,"" and actual German words, e.g., "Haus" (house), were played to young German-speaking women and men. When the presentation of the sec- ond word of a pair (e.g., "Haus" of "tofer-house") coincided with an ongoing slow-wave peak, the chances increased that a new semantic association between the pair had been formed and retained.
Sleep-formed associations translated into awake ones, where they guided forced choices on an im- plicit memory test. Reactivations of sleep-formed associations were mirrored by brain activation in- creases measured with fMRI in cortical language areas and the hippocampus, a brain structure critical for relational binding.
We infer that implicit relational binding had occurred during peaks of slow oscilla- tions, recruiting a hippocampal-neocortical network comparable to vocabulary learning in the waking state.
Humans can encode verbal associative information (word pairs) during slow-wave sleep
Memory was best for pairs that were encoded in sync with ongoing slow-wave peaks
Slow-wave sleep appears to entail windows of opportunity for synaptic potentiation
The hippocampus mediated retrieval, suggesting episodic memory formation during sleep
While we slumber deeply, important memory processes take place in the brain. Certain signals during the night can improve memory and even help us learn new things.
Brain rhythms regulate information processing in different states to enable learning and memory formation. The < 1 Hz sleep slow oscillation hallmarks slow-wave sleep and is critical to memory consolidation.
Here we show in sleeping humans that auditory stimulation in phase with the ongoing rhythmic occurrence of slow oscillation up states profoundly enhances the slow oscillation rhythm, phase-coupled spindle activity, and, consequently, the consolidation of declarative memory.
Stimulation out of phase with the ongoing slow oscillation rhythm remained ineffective. Closed-loop in-phase stimulation provides a straight-forward tool to enhance sleep rhythms and their functional efficacy.