Sensory processing reallocation from auditory to cardiac signals in REM sleep.

The brain continuously integrates information from the external environment (exteroception) and the internal bodily milieu (interoception). However, how the balance between these two processing streams shifts across vigilance states with differing levels of environmental responsiveness remains poorly understood. Here, we examined neural responses to external auditory and internal cardiac signals across wakefulness and rapid eye movement (REM) sleep microstates-tonic and phasic REM-which are characterized by progressively reduced responsiveness to external stimulation. High-density electroencephalography (EEG) was recorded in healthy participants (n = 25). Auditory evoked potentials (AEPs) and heartbeat evoked potentials (HEPs) served as indices of exteroception and interoception, respectively, and were compared across vigilance states. AEPs progressively decreased from wakefulness to tonic REM and were most attenuated during phasic REM. In contrast, HEPs were preserved across REM microstates and were enhanced relative to wakefulness, indicating sustained-and even amplified-processing of cardiac signals during REM sleep. To quantify the relative weighting of external and internal signals, we introduce an audio-cardio index, defined as the ratio of auditory to cardiac neural responses. This index decreased systematically across vigilance states, revealing a graded shift from auditory-oriented processing during wakefulness to cardiac-oriented processing during phasic REM, with tonic REM occupying an intermediate position. Together, these findings demonstrate that while responsiveness to auditory stimuli diminishes during phasic REM, the brain continues to prioritize physiologically relevant cardiac signals. The exteroceptive-interoceptive balance may thus provide a novel, mechanistically grounded marker of altered consciousness, which is particularly informative in contexts where behavioral responsiveness cannot be assessed.