Auditory Brainstem Responses with Simple and Complex Stimuli in Tinnitus.

Tinnitus can cause functional or even structural changes at cortical regions. Given the possible role of the inferior colliculus (IC) in tinnitus due to the distribution of lateral inhibition and receiving input from top-down pathways, we investigated whether tinnitus can also affect subcortical regions, mainly the IC. Auditory brainstem responses to click (cABR) and speech stimuli (sABR) were recorded in 18 individuals with tinnitus and 22 controls without tinnitus matched based on their ages and genders. All subjects had normal hearing sensitivity. Latencies of cABR in waves V and Vn, as well as interpeak latencies (IPLs) of III-V and I-V were significantly longer in individuals with tinnitus compared to the controls. Individuals with tinnitus presented significantly longer latencies of all sABR waves than the control group. The tinnitus patients also presented significant decrease in the amplitude and slope of the V-A complex and declined encoding of the first and higher formants. A combination of bottom-up and top-down processes were involved in the impaired timing in the individuals with tinnitus. The deficits in the auditory processing at the subcortical level (presumably the mi dBrain) could be caused by abnormal bottom-up inputs (i.e. those from more peripheral auditory structures) or abnormal top-down inputs (i.e. abnormal modulations from more central structures through efferent pathways. As a main possible generator of late waves of cABR and all waves of sABR is the IC, these findings indicate that the subcortical regions, particularly the IC, may undergo maladaptive plasticity following tinnitus. The consistency between the results of cABR and sABR indicated that tinnitus might affect the processing of simple and complex stimuli in the same manner. [ABSTRACT FROM AUTHOR]