Falling asleep leads to a loss of sensory awareness and to the inability to interact with the environment . While this was traditionally thought as a consequence of the brain shutting down to external inputs, it is now acknowledged that incoming stimuli can still be processed, at least to some extent, during sleep . For instance, sleeping participants can create novel sensory associations between tones and odors  or reactivate existing semantic associations, as evidenced by event-related potentials [4; 5; 6 ; 7]. Yet, the extent to which the brain continues to process external stimuli remains largely unknown. In particular, it remains unclear whether sensory information can be processed in a flexible and task-dependent manner by the sleeping brain, all the way up to the preparation of relevant actions. Here, using semantic categorization and lexical decision tasks, we studied task-relevant responses triggered by spoken stimuli in the sleeping brain. Awake participants classified words as either animals or objects (experiment 1) or as either words or pseudowords (experiment 2) by pressing a button with their right or left hand, while transitioning toward sleep. The lateralized readiness potential (LRP), an electrophysiological index of response preparation, revealed that task-specific preparatory responses are preserved during sleep. These findings demonstrate that despite the absence of awareness and behavioral responsiveness, sleepers can still extract task-relevant information from external stimuli and covertly prepare for appropriate motor responses.
Sid Kouider, Thomas Andrillon, Leonardo S. Barbosa, Louise Goupil, Tristan A. Bekinschtein, Inducing Task-Relevant Responses to Speech in the Sleeping Brain, Current Biology, Volume 24, Issue 18, 22 September 2014, Pages 2208-2214, ISSN 0960-9822, http://doi.org/10.1016/j.cub.2014.08.016.