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1. Introduction
Subjective tinnitus, a phantom sound perception in the absence of an identifiable objective, external sound source [1], afflicts 5%–21% of adults at some point in their lifetime and increases in people exposed to work-related [2] or leisure-related [2, 3] noise exposure. This high prevalence has been attributed to the free energy principle, in which the reduced auditory input results in Bayesian frequency specific updating in an attempt to reduce environmental auditory uncertainty associated with this auditory deafferentation [4]. Although often not fully appreciated by the general public, tinnitus is one of the most debilitating audiological disorders and affects almost all aspects of daily life [5, 6], lowering the quality of life in 1% of the total population [7–9]. Cognitive impairments, sleep disturbances, negative emotions, and other psychiatric comorbidities such as depression associated with tinnitus are especially bothersome for patients and their families [10, 11].
Although numerous management disciplines including pharmacological and nonpharmacological treatments have been introduced, evidence for a uniformly successful treatment that can eliminate tinnitus is lacking [12]. Because the initial diagnosis and evaluation of treatment effects cannot be objectified easily, the treatment goals are aimed at symptomatic relief relying on patients’ subjective symptom reports in the majority of cases. The absence of standardized single gold-standard treatment for tinnitus thus necessitates combinations of treatment strategies or developments of novel treatment modalities.
With the development of the idea that the unified tinnitus percept is an emergent network property resulting from activity in multiple, parallel, partially overlapping but separable networks [13] encompassing both auditory and nonauditory brain areas [14, 15], new treatments are being developed, including both pharmacological [16] and neuromodulatory approaches [17].
Over the last decade, noninvasive neuromodulations such as transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), transcutaneous electrical nerve stimulation, and neurofeedback have been used, as well as invasive neuromodulation techniques. These include implantable cortical electrodes on the auditory and the dorsolateral prefrontal cortex (DLPFC), as well as subcutaneous occipital nerve stimulation, and deep brain stimulation [18], especially for cases of intractable tinnitus.
Of these neuromodulation methods, tDCS might become...