Tinnitus is characterised by the perception of sounds in the absence of any objective physical sound source (Weisz et al., 2005). While some individuals who experience tinnitus are not very troubled by it, others find their tinnitus causes a considerable amount of distress. Various strategies are used in the management of tinnitus including sound therapy, counselling, hearing aid use and cognitive behavioural therapy (Langguth, 2015; Langguth et al., 2019). However, these treatments are primarily aimed at reducing the impact of tinnitus, rather than as a causal intervention for the tinnitus percept (McFerran et al., 2019). This is a treatment approach that is often at odds with the goals of those with tinnitus. This was evident in a study carried out in the United States by Husain et al. (2018), as when asked to define tinnitus treatment success, audiologists defined this as decreased awareness and anxiety relief for their patients. Those with tinnitus on the other hand, defined treatment success as a reduction of tinnitus loudness and elimination of tinnitus. Dissatisfaction with current tinnitus treatment has been reported in several countries, with patients desiring a pharmacological solution over other modalities (McFerran et al., 2018; McFerran et al., 2019; Sanchez & Stephens, 2000; Zarenoe & Ledin, 2014).

A comprehensive understanding of the neurophysiological mechanism of tinnitus remains lacking, which has hampered progress towards a causal intervention such as via pharmacology, auditory training or via an auditory prosthesis. Recurring research challenges have contributed to the absence of a thorough mechanistic understanding and consequently, a potential cure. For instance, control over tinnitus onset is not usually possible; those with tinnitus only present themselves once tinnitus has already set in, which necessitates comparison against non-tinnitus controls. Therefore, there is no within-subjects baseline to compare anatomical or physiological features in the tinnitus-affected auditory system, without introducing unwanted inter-subject variation.

Heterogeneity amongst tinnitus study participants has also made it difficult to pinpoint the underlying mechanism of tinnitus. Hearing loss, which can be a trigger for tinnitus, is often a further uncontrolled and confounding variable. It is a confound because tinnitus is expected to alter the activity of the auditory system but so is hearing loss, therefore assigning causation of the altered activity can be problematic. One way to address these methodological challenges is via experimentally induced tinnitus. Pre-onset baseline measures are therefore available, enabling a within-subjects comparison and thus eliminating a large source of variability, while hearing status is also controlled across conditions. Therefore, an improved mechanistic understanding of tinnitus may be possible via studying individuals with experimentally induced tinnitus.

The following review will provide introductory information relating to tinnitus. This will first include details on prevalence of the symptom and current methods of assessment. The review will then focus on research which has sought to determine the mechanisms underlying the tinnitus percept, with an analysis of where the gaps in knowledge lie. A rationale regarding the need for an objective assessment of tinnitus will be presented as well as the relevance of investigating a tinnitus biomarker. Finally, the rationale behind the present research will be put forward before outlining the specific aims and hypotheses of this study.