![]() To investigate the brain’s ability to form memory traces during sleep (here, the memory of having heard a specific item), we relied on the classical old/new paradigm. Yet, recent studies have shown that even hippocampal-dependent forms of learning are possible during sleep ( Arzi et al., 2012 de Lavilléon et al., 2015), putting into question the opposition between memory consolidation and memory formation. Alternatively, changes in neuromodulation occurring during sleep could impair synaptic plasticity itself and therefore the encoding of new memories ( Hennevin et al., 2007 Tononi and Cirelli, 2014). For example, memory systems (such as hippocampal structures) could get disconnected from sensory circuits so as to prevent external input from interfering with the consolidation process ( Rasch and Born, 2013). ![]() To explain this discrepancy, the main theories on the role of sleep in memory have proposed that consolidation mechanisms directly or indirectly prevent the formation of new memories ( Diekelmann and Born, 2010 Hasselmo, 1999 Tononi and Cirelli, 2014). This contrasts with the abundance of results showing the crucial role of sleep in promoting memory consolidation ( Rasch and Born, 2013) as well as studies showing how external stimulations can improve this consolidation ( Oudiette and Paller, 2013). Until recently, only some forms of learning independent from hippocampal structures had been evidenced. ![]() From pure noise ( Andrillon et al., 2015) to more complex types of sensory inputs such as words ( Pulvermüller et al., 2001), processing a given piece of information, even passively, leads to the formation of a new memory trace or the strengthening of an existing one ( Kolb and Whishaw, 1998).Įxploring the sleeping brain’s ability to learn has been a long-lasting scientific quest ( Emmons and Simon, 1956) but positive results are scarce. But can such processes trigger long-term memory? Indeed, when we are awake, experience constantly imprints on the brain. We recently showed that the sleeping brain could even build upon sensory processes and use semantic information to prepare task-relevant responses ( Kouider et al., 2014). Sleepers can also process sensory information at a high level of representation such as the semantic level ( Bastuji et al., 2002 Brualla et al., 1998 Ibanez et al., 2006). On the contrary, the sleeping brain can encode sensory information ( Issa and Wang, 2008 Nir et al., 2013), recognize salient or familiar sounds such as a person’s own name ( Perrin et al., 1999), process sounds in their context and detect the violation of simple rules. Sleepers are not disconnected from their environment during sleep. 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. 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. However, both behavioral and EEG data indicate the presence of an implicit memory trace for words presented 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). ![]() 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. Participants were instructed to classify auditory stimuli as words or pseudo-words, through left and right hand responses, while transitioning toward sleep. We examined here the consequences upon awakening of the processing of sensory information at a high level of representation during sleep. In wakefulness, such processes usually lead to the formation of long-term memory traces, being it implicit or explicit. Yet, the sleeping brain is far from being dormant and recent research unraveled the preservation of complex sensory processing during sleep. When we fall asleep, our awareness of the surrounding world fades. ![]()
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