Effective Connectivity Reveals a Shift to Left-Hemisphere Dominance for Spatial Attention with Diminishing Arousal

Spatial attention becomes biased toward the right side of space when alertness wanes, but the neural mechanisms underlying this shift remain unresolved. Competing theories propose that spatial attention depends either on a dominant right hemisphere, interhemispheric competition, or interactions between dorsal and ventral attention networks. Here, we used drowsiness as a model to investigate how diminishing alertness reshapes the cortical dynamics supporting auditory spatial attention. Thirty healthy participants completed a lateralised auditory localisation task while high-density EEG was recorded during wakefulness and drowsiness. Behaviourally, drowsiness selectively impaired localisation accuracy for left-sided stimuli, replicating the typical neglect-like rightward attentional bias observed with lower arousal in healthy people, and pathological attention deficits in brain injury patients. Using Dynamic Causal Modelling and Parametric Empirical Bayes, we identified a striking reconfiguration of effective cortico-cortical connectivity across arousal states. During wakefulness, spatial attention was characterised by reciprocal interhemispheric signalling between inferior frontal regions, consistent with balanced bilateral coordination. During drowsiness, this pattern shifted toward reduced right frontoparietal connectivity together with increased left-hemispheric parietofrontal connectivity. These findings suggest that waning alertness destabilises balanced interhemispheric control and allows left-hemispheric processing to dominate attentional processing. Rather than fully supporting existing theories of spatial attention, our results point toward a hybrid mechanism in which neglect-like spatial bias emerges from the interaction between right-hemispheric dysconnection and left-hemispheric dominance under low arousal. Diminishing alertness reshapes interhemispheric coordination rather than simply reducing right-hemisphere function.