Hemispheric Asymmetry in Cortical Auditory Processing: The Interactive Effects of Attention and Background Noise

Background/Objectives: Speech processing engages both hemispheres of the brain but exhibits a degree of hemispheric asymmetry. This asymmetry, however, is not fixed and can be shaped by stimulus-related and listener-related factors. The present study examined how background noise and attention influence hemispheric differences in speech processing using high-density cortical auditory evoked potentials (CAEPs). Methods: Twenty-five young adults with clinically normal hearing listened to meaningful bisyllabic Kannada words under two background conditions (quiet, speech-shaped noise) and two attentional conditions (active, passive). N1 peak amplitudes were compared between the left and right hemispheres across conditions using linear mixed-effects modeling. Results: Results revealed significantly larger N1 amplitudes in the left hemisphere and during active compared to passive listening, confirming left-hemisphere dominance for speech processing and robust attentional modulation. In contrast, background noise did not significantly modulate N1 amplitude or hemispheric asymmetry. Importantly, a significant Hemisphere × Attention interaction indicated that hemispheric asymmetry depended on attentional state, with clear left-hemisphere dominance being observed during active listening in both quiet and noise conditions, whereas hemispheric differences were reduced or absent during passive listening, irrespective of background. Conclusions: Together, these findings demonstrate that attentional engagement, rather than background noise, plays a critical role in modulating hemispheric specialization during early cortical speech processing, highlighting the adaptive nature of auditory cortical mechanisms in challenging listening environments.