Infoscience

Thesis

Neuroimaging of human auditory cortex: data analysis and applications

Modern neuroimaging technologies provide us with non-invasive methods for study- ing structure and function, and plasticity of the human auditory cortex. Here we have acquired high spatial resolution functional magnetic resonance imaging (fMRI) data of the auditory cortex in patients with unilateral hearing loss and tinnitus. Compared to healthy controls, we found evidence of over-representation and hyperactivity in parts of cortical tonotopic map that correspond to low frequencies sounds, irrespective of the hearing loss and tinnitus range, which in most cases affected higher frequencies. These findings suggest that hearing loss has a destabilizing effect on the tonotopic organization in primary auditory cortex that is not restricted to the corresponding frequency range of hearing loss and tinnitus. One of the challenges in tinnitus studies is the high relevance of the hyperacusis con- dition in these patients. The similarity and co-occurrence of these two disorders con- tributes to the debate about the specificity of findings associated with tinnitus alone. For classifying tinnitus patients with and without hyperacusis, we thus implemented partial least square correlation (PLSC) analysis that builds upon the cross-covariance information between audiogram and fMRI data. This method provides us with a multi modal measure to successfully detect the hyperacusis condition in tinnitus patients with 80% cross-validation accuracy, for the first time. The PLSC driven biomarker, also yields the brain area playing a major role in the condition. There is a vast heterogeneity in the anatomy of auditory cortex across subjects, therefore individualized delineation the exact border between primary and secondary auditory cortex is a demanding research topic. Here we established a model of ex- trinsic connectivity of auditory cortex with the two other regions of the brain that have discriminative connectivity with primary and secondary auditory cortices. The resting state fMRI connectivity features are used for segmentation and are performed exploiting two methods as dynamical causal modeling (DCM) of effective connectivity with 74% accuracy and functional connectivity with 71% accuracy. This data-driven segmentation method, not only facilitate the definition of auditory cortex subregions for fundamental research in the normal brain, but also yields a useful tool for following the modification in the functional anatomy of this region as a result of hearing disorders like tinnitus.

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