Processing of semantic incongruency at the onset of sleep: An auditory N400 evoked potential study.

The ability to organize self-generated thought into coherent, meaningful semantic representations is a central aspect of human cognition and undergoes regular alterations throughout the day. To investigate whether changes in semantic processing might explain the loss of coherence, logic, and voluntary control over thinking typically accompanying the transition to sleep, we recorded N400 evoked potentials from 44 healthy subjects. Auditory word pairs with varying semantic distance were presented while they were allowed to fall asleep. Using semantic distance and wakefulness level as regressors, we found that semantic distance reliably elicited an N400, and lower wakefulness levels were associated with increased frontal negativity within a similar time range. Additionally, and contrary to our initial hypothesis, the results showed an interaction of semantic distance and wakefulness that is best interpreted as an increased N400 effect with decreasing wakefulness. While these results do not rule out a possible role of semantic processes in the generation of diminished logic and thought control during the transition to sleep, we discuss the possibility of additional brain mechanisms that usually constrain the inner stream of consciousness during wakefulness.