Sleep, and specifically deep or slow-wave sleep [non–rapid eye movement (REM) sleep], benefits memory formation by reactivating neuronal traces that were formed during the preceding period of wakefulness. This reactivation of specific memories leads to their strengthening and transformation. Such reactivation can be experimentally induced during slow-wave sleep by presenting cues that were present during the prior period of memory acquisition. Initial studies showed that an odor present during learning of object locations enhances these memories when the participant is reexposed to the odor during slow-wave sleep after learning. These findings have been confirmed in numerous studies investigating different memory systems and also when auditory instead of olfactory cues are used. This basic research has firmly established the possibility of influencing sleep to enhance specific newly learned memories by targeted memory reactivation.
The findings by Hu et al. now suggest that this method can also be used to influence implicit attitudes that are known to typically manifest themselves early during childhood and remain very stable into adulthood. Before a 90-min nap, participants underwent training aimed at countering typical implicit gender and racial biases by learning to associate genders and races with opposing attributes; that is, to associate female faces with science-related words and black faces with “good” words. Critically, presentation of the to-be-learned counterassociations was combined with a sound, which served as a cue to promote the reactivation of the newly learned associations during a subsequent nap while the participant was deep in slow-wave sleep. Only when this sound was re-presented during slow-wave sleep did the posttraining reduction in implicit social bias survive and was even evident 1 week later. These findings are all the more convincing as the authors conducted the reactivation step during a 90-min daytime nap. During normal sleep at night, the effects are expected to be even stronger, owing to the generally deeper and longer periods of slow-wave sleep and REM sleep. Additionally, the accompanying neuroendocrine milieu makes nocturnal sleep even more efficient for memory reinforcement.
Previous studies have shown that such targeted reactivation of memory during sleep can effectively extinguish unwanted behavior such as experimentally induced fear in humans. The present study is the first to demonstrate that this method can be used to break long-lived, highly pervasive response habits deeply rooted in memory and thereby influence behavior at an entirely unconscious level.A caution:
However, Aldous Huxley's description of a dystopian “brave new world” where young children are conditioned to certain values during sleep reminds us that this research also needs to be guided by ethical considerations. Sleep is a state in which the individual is without willful consciousness and therefore vulnerable to suggestion. Beyond that, Hu et al.'s findings highlight the breadth of possible applications to permanently modify any unwanted behavior by targeted memory reactivation during sleep.