When alertness fades: Drowsiness-induced visual dominance and oscillatory recalibration in audiovisual integration

Multisensory integration allows the brain to align inputs from different sensory modalities, enhancing perception and behavior. However, transitioning into drowsiness, a state marked by decreased attentional control and altered cortical dynamics, offers a unique opportunity to examine adaptations in these multisensory processes. In this study, we investigated how drowsiness influences reaction times (RTs) and neural oscillations during audiovisual multisensory integration. Participants performed a task where auditory and visual stimuli were presented either in a coordinated manner or with temporal misalignment (visual-first or auditory-first uncoordinated conditions). Behavioral results showed that drowsiness slowed RTs overall but revealed a clear sensory dominance effect: visual-first uncoordination facilitated RTs compared to auditory-first uncoordination, reflecting vision's dominant role in recalibrating sensory conflicts. In contrast, RTs in coordinated conditions remained stable across alert and drowsy states, suggesting that multisensory redundancy compensates for reduced cortical integration during drowsiness. At the neural level, distinct patterns of oscillatory activity emerged. Alpha oscillations supported attentional realignment and temporal alignment in visual-first conditions, while Gamma oscillations were recruited during auditory-first uncoordination, reflecting heightened sensory-specific processing demands. These effects were state-dependent, becoming more pronounced during drowsiness. Our findings demonstrate that drowsiness fundamentally reshapes multisensory integration by amplifying sensory dominance mechanisms, particularly vision. Compensatory neural mechanisms involving Alpha and Gamma oscillations maintain perceptual coherence under conditions of reduced cortical interaction. These results provide critical insights into how the brain adapts to sensory conflicts during states of diminished awareness, with broader implications for performance and decision-making in real-world drowsy states.