Authors: Srikanth Nayak¹; Arivudai Nambi²; Jayashree Bhat³
¹Yenepoya Medical College, Yenepoya University; ²All India Institute of Speech and Hearing; ³Yenepoya University
Background: Understanding speech in noise requires efficient neural encoding across the auditory pathway. While the roles of the brainstem and cortex are documented, their interaction under attention and during the processing of degraded stimuli is less explored. This study investigated subcortical and cortical neural coding and interactions for intact and degraded stimuli.
Method: Simultaneous brainstem and cortical responses were recorded in 43 young adults with normal hearing using nonsense CVVC stimuli. Stimuli were pitch-shifted (0, 1, 2, 4 semitones) and degraded by removing temporal fine structure. Responses were recorded during passive and active listening. Brainstem processing was assessed via frequency-following response (FFR), while cortical processing was measured using ORN and P400 components.
Results: FFR amplitude showed significant effects for Stimulus, Task, and Semitone, with intact stimuli eliciting higher amplitude and greater pitch sensitivity than degraded stimuli. ORN amplitude was significantly more negative for intact stimuli, with no reliable ORN observed for degraded signals. P400 amplitude varied across pitch differences for intact stimuli but showed no significant differentiation for degraded stimuli, indicating a failure at late cortical stages to process degraded pitch cues.
Conclusion: Stimulus degradation disrupts pitch processing throughout the auditory pathway. Reduced brainstem encoding and the absence of reliable cortical responses to degraded signals underscore the critical importance of spectral cues for pitch differentiation and speech perception.
