Subcortical auditory system in tinnitus with normal hearing: insights from electrophysiological perspective

Purpose: The mechanism of tinnitus remains poorly understood; however, studies have underscored the significance of the subcortical auditory system in tinnitus perception. In this study, our aim was to investigate the subcortical auditory system using electrophysiological measurements in individuals with tinnitus and normal hearing. Additionally, we aimed to assess speech-in-noise (SiN) perception to determine whether individuals with tinnitus exhibit SiN deficits despite having normal-hearing thresholds. Methods: A total 42 normal-hearing participants, including 22 individuals with chronic subjective tinnitus and 20 normal individuals, participated in the study. We recorded auditory brainstem response (ABR) and speech-evoked frequency following response (sFFR) from the participants. SiN perception was also assessed using the Matrix test. Results: Our results revealed a significant prolongation of the O peak, which encodes sound offset in sFFR, for the tinnitus group (p < 0.01). The greater non-stimulus-evoked activity was also found in individuals with tinnitus (p < 0.01). In ABR, the tinnitus group showed reduced wave I amplitude and prolonged absolute wave I, III, and V latencies (p ≤ 0.02). Our findings suggested that individuals with tinnitus had poorer SiN perception compared to normal participants (p < 0.05). Conclusion: The deficit in encoding sound offset may indicate an impaired inhibitory mechanism in tinnitus. The greater non-stimulus-evoked activity observed in the tinnitus group suggests increased neural noise at the subcortical level. Additionally, individuals with tinnitus may experience speech-in-noise deficits despite having a normal audiogram. Taken together, these findings suggest that the lack of inhibition and increased neural noise may be associated with tinnitus perception.