Neural signatures of classical conditioning during human sleep

Scientific efforts to teach humans novel verbal information during sleep have been largely unsuccessful. However, recent findings demonstrate that sleeping humans can learn entirely new non-verbal information in a stage-dependent manner. The brain activity supporting the learning of novel information presented during sleep is still unknown. Here, we aimed to elucidate the brain processes enabling sleep-stage-dependent novel associative learning during sleep. We presented auditory-olfactory conditioning during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep while measuring electrophysiological and behavioural responses during sleep and the subsequent morning. We found that associative learning during sleep modulated learning-related delta and sigma power in NREM sleep. Furthermore, learning-related delta power was associated with learning-related behaviour both during sleep and the subsequent morning, but notably, this delta-behaviour relationship changed between learning during sleep and during retrieval the following morning. Together, these findings suggest that slow waves have a functional role in sleep stage-dependent associative learning in sleep.