Influence of Tinnitus on Auditory Spectral and Temporal Resolution and Speech Perception in Tinnitus Patients.

Although cochlear damage is believed to trigger the perception of tinnitus in the central auditory system, its pathophysiological mechanism is still unclear. We aimed to investigate the pathophysiology of tinnitus using psychoacoustic assessments of auditory spectral and temporal resolution and speech perception in noise. Human subjects with tinnitus and symmetric hearing thresholds were divided into three groups: nine unilateral tinnitus subjects with normal hearing thresholds (Group 1), 12 unilateral tinnitus subjects with hearing loss (HL; Group 2), and nine bilateral tinnitus subjects with HL. Fifteen normal-hearing subjects without tinnitus were also tested as a control group. Four different tests were administered: (1) the spectral-ripple discrimination (SRD) test, (2) the temporal modulation detection (TMD) test, (3) the Schroeder-phase discrimination (SPD) test, and (4) the speech recognition threshold (SRT) in noise test. There were no significant differences in SRD, TMD, and SPD between the tinnitus-affected ears (TEs) and non-tinnitus ears (NTEs) in Groups 1 and 2 (p > 0.05). In contrast, the TEs showed poorer SRTs than the NTEs in these two subject groups (p = 0.022 and 0.049). No inferiority of spectral/temporal resolution in TEs compared with NTEs suggests that there may be no more outer hair cell (OHC) damage on the tinnitus side given that damaged OHCs are associated with broadening the auditory filters. The decoupling of the SRT results from the spectral/temporal resolution data could imply that the occurrence of tinnitus does not depend upon the degree of damage to the OHCs, but rather upon different plastic changes in the central auditory system after cochlear damage. SIGNIFICANCE STATEMENT: We can easily find unilateral tinnitus patients who have symmetric hearing thresholds. Our research question was what kind of difference would be responsible for the tinnitus in the tinnitus-affected ears but not in the non-tinnitus ears of subjects with symmetric hearing thresholds. The answer to this fundamental question could help us to understand the pathophysiology of tinnitus. We evaluated the potential influence of tinnitus upon the subjects' auditory spectral and temporal resolution and speech perception in noise by comparing these psychoacoustic performances between tinnitus-affected ears and non-tinnitus ears in the same subjects.