Emerging Network Connectomics in Phantom Sound Perception

The brain is hypothesized to be a hierarchical prediction system. It constantly compares the incoming information from the sensory pathways with a pre-existing model of the environment. Damage at different locations in these pathways leads to the loss of sensory input, resulting in the brain receiving different input from what it would expect. This may lead to an uncertainty that may be minimized by the production of a phantom percept. A phantom percept is the perception of a sensation in the absence of an external source. In this dissertation, we specifically focus on the auditory phantom percept: Tinnitus.

Tinnitus is accompanied by a “loudness” component and a variable emotional component or “distress.” The loudness is unique to the disorder (domain-specific) and distress is common to other pathologies as well (domain-general). A wealth of information is available about the relationship between these behavioral correlates and changes in static functional connectivity. However, their relationship with dynamic changes in network connectivity is yet to be explored. Thus, the aim of this study is to investigate changes in the flexibility and stability of time-varying functional connectivity (temporal variability) in tinnitus and its relation to loudness and distress.

In the current study, we observed a spatio-temporal change of temporal variability in tinnitus. We also observed that the relationship between temporal variability and behavioral symptoms depended on the amount of distress experienced. In participants with low distress, there was no linear relationship between temporal variability and loudness or distress. However, in participants with high distress, we observed that as loudness increases, temporal variability increases; and as distress increases temporal variability decreases. This trend is observed in the parahippocampus in the alpha2 frequency band.

The increasing temporal variability with increasing loudness may relate to a Bayesian search for updating deafferentation-based missing information. On the other hand, the decreasing temporal variability with increasing distress possibly reflects less adaptive, contextual processing. Therefore, the findings may reveal a way of understanding the changes in network dynamics not just in tinnitus, but in other brain disorders as well.

This dissertation is divided into 5 Chapters. Chapter 1 systematically reviews the phantom percepts in various sensory domains and the different mechanisms of compensation to sensory uncertainty. Chapter 2 focuses specifically on tinnitus. It reviews the current literature on tinnitus, their drawbacks and outlines the purpose of the current study. It also serves as an introduction to temporal variability, the measure of dynamic functional connectivity used in this dissertation. Chapter 3 describes the tinnitus and control groups, materials, and experimental methods used in the current study. Chapter 4 details the results of the current study. Finally, Chapter 5 discusses these results, limitations and future applications of the study.