Neuroscience
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The acoustic middle-ear-muscle reflex (MEMR) has been suggested as a sensitive non-invasive measure of cochlear synaptopathy, the loss of synapses between inner hair cells and auditory nerve fibers. In the present study, clinical MEMR thresholds were measured for 1-, 2-, and 4-kHz tonal elicitors, using a procedure shown to produce thresholds with excellent reliability. MEMR thresholds of 19 participants with tinnitus and normal audiograms were compared to those of 19 age- and sex-matched controls. ⋯ MEMR thresholds were unrelated to either SPiN or noise exposure, despite a wide range in both measures. It is possible that thresholds measured using a clinical paradigm are less sensitive to synaptopathy than those obtained using more sophisticated measurement techniques; however, we had good sensitivity at the group level, and even trends in the hypothesized direction were not observed. To the extent that MEMR thresholds are sensitive to cochlear synaptopathy, the present results provide no evidence that tinnitus, SPiN, or noise exposure are related to synaptopathy in the population studied.
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Following noise overexposure and tinnitus-induction, fusiform cells of the dorsal cochlear nucleus (DCN) show increased spontaneous firing rates (SFR), increased spontaneous synchrony and altered stimulus-timing-dependent plasticity (StDP), which correlate with behavioral measures of tinnitus. Sodium salicylate, the active ingredient in aspirin, which is commonly used to induce tinnitus, increases SFR and activates NMDA receptors in the ascending auditory pathway. NMDA receptor activation is required for StDP in many brain regions, including the DCN. ⋯ First, we show that animals administered salicylate show evidence of tinnitus using both behavioral paradigms, cross-validating the tests. Second, fusiform cells in animals with tinnitus showed increased SFR, synchrony and altered StDP timing rules, like animals with noise-induced tinnitus. These findings suggest that alterations to fusiform-cell plasticity are an essential component of tinnitus, regardless of induction technique.
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Review
Age-related Changes in Neural Coding of Envelope Cues: Peripheral Declines and Central Compensation.
Aging listeners often experience difficulties in perceiving temporally complex acoustic cues in noisy environments. These difficulties likely have neurophysiological contributors from various levels of auditory processing. Cochlear synapses between inner hair cells and auditory nerve fibers exhibit a progressive decline with age which is not reflected in the threshold audiogram. ⋯ This results in a modulation frequency selective increase in the representation of envelope cues at the level of the auditory midbrain and cortex. These changes may be shaped by mechanisms such as decreased inhibitory neurotransmission occurring with age across various central auditory nuclei. Altered representations of the differing temporal components of speech due to these interactions between multiple levels of the auditory pathway may contribute to the age-related difficulties hearing speech in noisy environments.
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Attention may be an important factor in tinnitus. Individuals most disturbed by their tinnitus differ from those who are not in terms of attention allocation. This study used an operant-conditioning animal model to examine the interaction between tinnitus and auditory vigilant attention as well as auditory selective attention. ⋯ A brief free-field sound cue, consisting of either a short train of identical noise pulses (standard stimulus), or a noise train with one substituted tone pulse (oddball stimulus), cued a left or right nose poke for food. On this selective attention task, Tinnitus animals performed consistently worse than Non-tinnitus or Unexposed control animals regardless of stimulus features. As predicted, animals with behavioral evidence of tinnitus showed tinnitus-related attentional changes, including impaired selective attention but increased vigilance to sounds approximating their tinnitus.
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The effects of traumatic noise-exposure and deafening on auditory system function have received a great deal of attention. However, lower levels of noise as well as temporary conductive hearing loss also have consequences on auditory physiology and hearing. Here we review how abnormal acoustic experience at early ages affects the ascending and descending auditory pathways, as well as hearing behavior.