Implicit memory for words heard during sleep

Abstract

When we fall asleep, our awareness of the surrounding world fades. Yet, the sleeping brain is far from being dormant and recent research unraveled the preservation of complex sensory processing during sleep. In wakefulness, such processes usually lead to the formation of long-term memory traces, being it implicit or explicit. We examined here the consequences upon awakening of the processing of sensory information at a high level of representation during sleep. Participants were instructed to classify auditory stimuli as words or pseudo-words, through left and right hand responses, while transitioning toward sleep. An analysis of the electroencephalographic (EEG) signal revealed the preservation of lateralized motor activations in response to sounds, suggesting that stimuli were correctly categorized during sleep. Upon awakening, participants did not explicitly remember words processed during sleep and failed to distinguish them from new words (old/new recognition test). However, both behavioral and EEG data indicate the presence of an implicit memory trace for words presented during sleep. In addition, the underlying neural signature of such implicit memories markedly differed from the explicit memories formed during wakefulness, in line with dual-process accounts arguing for two independent systems for explicit and implicit memory. Thus, our results reveal that implicit learning mechanisms can be triggered during sleep and provide a novel approach to explore the neural implementation of memory without awareness.

Implicit Vocabulary Learning during Sleep Is Bound to Slow-Wave Peaks

“…It is well known that learning new information acquired while awake depends upon consolidation during sleep [1]; however, whether or not complex new information may be learned while asleep is not yet well established. A study using mice [2] provided evidence that sleep learning is possible by the mammalian brain; studies in humans suggest that simple auditory learning may be possible, but complex verbal learning remains to be demonstrated [1,[3][4][5][6][7]. Current sleep theories posit that teaching new vocabulary during slow-wave sleep is impractical, because this sleep stage does not provide the conditions necessary for learning.…”

Daytime Exposure to Blue-Enriched Light Counters the Effects of Sleep Restriction on Cortisol, Testosterone, Alpha-Amylase and Executive Processes

“…In the recall session, participants were presented with words from the exposure list mixed with new words taken from a new list of masculine and feminine words never presented to participants. Participants were instructed to indicate, for each word, if it belonged or not to the list presented in the morning (classical old/new memory paradigm) (Ebbinghaus, 1885;Eichenbaum et al, 2007;Andrillon and Kouider, 2016). During the recall test, the proportions of novel and old words were identical (N = 24, chance level = 50%).…”

Formation and suppression of acoustic memories during human sleep

“…However, little is known about how sleep and its associated rhythms eventually modulate the ability to learn. Past studies revealed the surprising ability of the sleeping brain to form new memories 6 , 7 , 23 , 24 and to process sensory information in a complex and flexible fashion 25 – 28 ; these results advocate for a more detailed investigation of whether and how memories can be formed during sleep. In particular, the question of how sleep stages modulate environmental learning remains unanswered.…”