In long-term passive noise exposure, diffusion tensor imaging (DTI), voxel-based statistics (VBS) revealed greater fractional anisotropy (FA) in the pyramidal tract and decreased FA in the tectospinal tract and trigeminothalamic tract ( Abdoli et al., 2016). In rats, we have demonstrated acute and chronic noise exposure result in structural and functional changes in auditory structures from the midbrain to the cortex ( Abdoli et al., 2016 Yang et al., 2018 Wong et al., 2020). In humans, we found that hearing loss not only affects auditory structures, but is brain-wide, multi-focal, and impacts regions and tracts differently depending on auditory input and compensatory mechanisms. We have recently assessed the structural foundations of hearing loss across the lifespan in humans using a meta-analysis and meta-regression ( Manno et al., 2021). The auditory system is greatly impacted due to CHL, yet investigations into structural-functional correlations in hearing loss are lacking, with studies concentrating either on structural or functional approaches.Īnimal Models of Hearing Loss Using Magnetic Resonance Imaging We recognize the variety of studies in CHL complicate any general conclusions, as the majority of studies were conducted during development. Here it was shown that the density of olivocochlear terminals in the cochlear epithelium is reduced ( Liberman et al., 2015). Nevertheless, efferent pathways are also altered by CHL ( Liberman et al., 2015). The perceptual deficits found in CHL are likely due to aberrant processing in the auditory cortex (AC i.e., decrease of inhibitory synaptic strength Yao and Sanes, 2018, argue not in the auditory brainstem). Additionally, developmental CHL animals display slower rates of acquisition for AM discrimination tasks due to an impaired ability to generalize newly introduced stimuli ( von Trapp et al., 2017). Early CHL induced by malleus removal results in task-specific behavioral threshold increases of ≈35 decibel (dB) for sinusoidally amplitude modulated (AM) stimuli and increases of ≈40 dB in neural auditory brainstem responses to 100 μs clicks or 4 ms pure tones ( Rosen et al., 2012). Interestingly, the emergence of the inhibitory synaptic strength results in a critical period where deficits can be induced in juveniles, but not adults ( Takesian et al., 2012). The loss of peripheral sound input (i.e., afferent) results in a dramatic decrease to inhibitory synaptic strength (GABA is decreased Kotak et al., 2008, 2013) and increase in excitatory strength (Glutamate is increased – Kotak and Sanes, 1997 Kotak et al., 2005) in the lateral superior olive auditory pathway. Bilateral CHL produces air-conducted sound attenuation but does not raise bone-conducted thresholds (indicating no cochlear damage: Tucci et al., 1999). The results and implications for humans consist of structural-functional brain alterations following short term hearing loss in adults.Ĭonductive hearing loss (CHL) results in the attenuation of air conducted sound ( Willcox and Artz, 2007). The results present an exploratory neuroimaging assessment of structural alterations coupled to a change in functional connectivity after conductive hearing loss. Short term conductive hearing loss altered functional connectivity in the AC and IC, but not the somatosensory cortex. To determine functional connectivity changes due to hearing loss, seed-based analysis (SBA) and independent component analysis (ICA) were performed. For baseline comparison, DTI and tractography alterations were not found for the somatosensory cortex. Tractography found the lateral lemniscus tract leading to the bilateral inferior colliculus (IC) was reduced. Diffusion tensor imaging (DTI) revealed fractional anisotropy (FA) and axial diffusivity alterations after hearing loss that circumscribed the auditory cortex (AC). Here, we have assessed structural and functional magnetic resonance imaging (MRI) in an adult (P60) rat model of short-term conductive hearing loss (1 week). How a change in air conducted sound alters the auditory system resulting in cortical alterations is not well understood. Conductive hearing loss (CHL) results in attenuation of air conducted sound reaching the inner ear.
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