During the transition to sleep, reflective awareness—the ability to recognize experiences as internally generated—breaks down before phenomenal awareness, the basic experience itself, fades. Dream-like experiences, characterized by uncontrolled thinking and perceptual images mistaken as real, are associated with an inverse relationship between cognitive effects and physiological activation in the brain. In 45 healthy young subjects, EEG microstate analysis showed that dream-like experiences correlated with increased presence of a microstate sourced in the superior and middle frontal gyrus and precuneus, and decreased presence of a microstate linked to higher-order visual areas. This suggests disengagement of cognitive control systems mediated by specific inhibitory microstates.
Monitoring whether an experience is real or not is crucial for everyday functioning, but this reality monitoring can go awry during dreams or hallucinations. To investigate brain processes underlying perceptual reality monitoring—the online assessment of reality during an experience—healthy young participants were immersed in a highly naturalistic virtual reality environment containing either realistic or dream-like bizarre elements. Dream-like bizarreness altered subjective experiences of reality and bizarreness and increased the contribution of a specific EEG microstate, labeled C'. This microstate is linked to suspending disbelief, or suppressing mismatches that seem bizarre. The findings suggest that prefrontal meta-conscious control processes are important for perceptual reality monitoring.
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, N400 evoked potentials were recorded from 44 healthy subjects. Auditory word pairs with varying semantic distance were presented while subjects were allowed to fall asleep. Semantic distance reliably elicited an N400, and lower wakefulness levels were associated with increased frontal negativity. Contrary to the initial hypothesis, the results showed an increased N400 effect with decreasing wakefulness, suggesting that semantic processes alone may not explain diminished thought control during sleep onset.