Date of Award

Summer 7-16-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Communication Sciences and Disorders

Advisor(s)

Vander Werff, Kathy

Keywords

auditory brainstem response, cortical auditory evoked potential, inhibition, noise exposure, sensory gating, tinnitus

Subject Categories

Communication Sciences and Disorders | Medicine and Health Sciences | Speech Pathology and Audiology

Abstract

Background: The perception of tinnitus may be triggered by a reduction in inhibitory function in the central auditory nervous system. Evidence, primarily from invasive studies of animal models of tinnitus, indicates that these changes occur at both the subcortical and cortical level. Auditory evoked potential (AEP) indices of subcortical inhibition [auditory brainstem response (ABR) 〖"V/I" 〗_"amp ratio" ] and cortical inhibition [cortical auditory evoked potential (CAEP) sensory gating ratios] may provide an objective index of whether reduced subcortical and/or cortical inhibition is associated with tinnitus perception in humans. The aims of this study were to assess whether ABR and/or CAEP indices of subcortical and cortical inhibition distinguish between a group with constant tinnitus and matched non-tinnitus controls, and whether tinnitus presence and/or other factors [age, noise exposure history, hearing loss, speech perception in noise (SPIN)] predicted ABR and/or CAEP outcomes related to inhibition.

Methods: Individuals with tinnitus and control counterparts matched for sex, age, and hearing thresholds completed the study (n = 18 per group). ABRs were recorded with a tiptrode in response to high intensity click ABRs to determine the 〖"V/I" 〗_"amp ratio" . CAEPs were recorded in response to two successive high intensity 10 ms clicks. A ratio of the amplitude or area of the first (conditioning CAEP) and second (test CAEP) click response was determined ("test CAEP" /"conditioning CAEP" ) as the primary measure of sensory gating. The latency ratio was also determined as a secondary outcome which may relate to sensory gating. For both the ABR 〖"V/I" 〗_"amp ratio" and CAEP sensory gating ratios, a larger value indicated reduced inhibition. Ratios were compared between the two groups using independent t-tests. The relative predictive value (proportional reduction in error, PRE) of tinnitus, age, noise exposure history, hearing loss, and SPIN on ABR and CAEP outcome variables related to inhibition was analyzed using regression.

Results: Individuals with tinnitus, relative to controls, exhibited similar ABR 〖"V/I" 〗_"amp ratio" , and significantly larger sensory gating 〖"P1" 〗_"lat ratio" . None of the variables assessed significantly predicted the ABR 〖"V/I" 〗_"amp ratio" . Tinnitus significantly predicted 〖"P1-N1" 〗_"amp ratio" , but not when taking into account age, noise exposure history, hearing loss, and SPIN. The 〖"P1" 〗_"lat ratio" was significantly predicted by both tinnitus and age, however, best predicted by age.

Conclusions: Tinnitus-related reduced inhibition was not evident at the subcortical level based on the ABR 〖"V/I" 〗_"amp ratio" . At the cortical level, the predictive influence of tinnitus on the 〖"P1-N1" 〗_"amp ratio" supports the association between reduced sensory gating with tinnitus presence in humans. The significantly larger 〖"P1" 〗_"lat ratio" in the tinnitus group may also support reduced sensory gating and/or a change in the recovery time, or refractoriness, of auditory evoked responses in individuals with tinnitus. The strong predictive influence of age on the 〖"P1" 〗_"lat ratio" indicates that increasing age reduced sensory gating above and beyond the effects of tinnitus. Potential limitations to the current study, including the non-normally distributed participant characteristics and AEP methodologies, as well as considerations for future research aiming to improve the reliability and validity of tinnitus AEP assessments are discussed.

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