Kok, Tori Elyssa; (2023) Non-invasive brain stimulation and auditory connectivity in tinnitus. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Background: This PhD thesis is an evaluation of transcranial direct current stimulation (tDCS) and high-definition tDCS (HD-tDCS) for the management of tinnitus, as well as an investigation of auditory network connectivity in tinnitus. Tinnitus is the perception of a sound without an external source, and it affects around 10% of the adult population. It can be severely debilitating, and therefore effective therapies are needed, but currently there is no cure. Tinnitus mechanisms are not fully understood. TDCS is a non-invasive brain stimulation tool which uses electrodes to administer a small current to the cortex, thereby increasing or decreasing cortical excitability. HD- tDCS is a type of tDCS which uses a cap of micro-electrodes for more focal stimulation. Both have been explored as management options for tinnitus. Methods: This PhD project aimed to evaluate the efficacy and acceptability of (HD- )tDCS for tinnitus management by conducting a scoping review on the current evidence (Chapter 3) and surveying tinnitus patients’ opinions on the technique (n = 272) (Chapter 4). This project also aimed to examine tinnitus auditory connectivity by way of a scoping review (Chapter 5) and by conducting a resting-state functional magnetic resonance imaging study (rs-fMRI) (Chapter 6). Results and discussion: This thesis found that the evidence-base for the efficacy of tDCS in tinnitus is not robust, and current tDCS trial outcomes are far from delivering the effect sizes required by tinnitus sufferers to render tDCS an acceptable management option. It was also found that tinnitus is associated with alterations to functional connectivity patterns, although exact results are rarely replicated. This thesis found increased functional connectivity between bilateral thalamus and right visual association cortex in tinnitus patients compared to controls, suggesting a role for thalamic hyperactivity in tinnitus. Future research should investigate the role of the thalamus in tinnitus further.

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