Cortical oscillations in tinnitus

Tinnitus, described as a phantom perception of sound in its absence is a condition that affects millions of people worldwide. In its chronic form it can be associated with increased levels of anxiety and depression and can be highly debilitating in extreme cases. It is commonly held that it is related to deficits in hearing, either as a result of ageing or due to noise exposure. However beyond this there is little else known about the precise mechanisms involved in its generation or maintenance. In particular, little is known about the neural properties of tinnitus. Neural oscillations of various frequencies have been linked to tinnitus, although the evidence for which specific frequencies are involved is somewhat mixed. The main aim of the present thesis was to investigate these neural oscillations further, both in how they relate to the perception of tinnitus and how they can potentially be modulated. The first study presented is an investigation into the subjective characteristics of tinnitus. The main aim was to investigate the relationship between hearing loss and tinnitus pitch, it was hypothesised that the primary tinnitus pitch would fall within the region of hearing loss. There were also two exploratory aims of the study, the first was to investigate the levels of agreement between two measures of tinnitus loudness one that was measured by a questionnaire (Tinnitus Case History Questionnaire) and the other that was measured as a rating of similarity to an externally presented sound. The second exploratory aim was to investigate the potential relationship between perceived tinnitus loudness and hyperacusis, or auditory hypersensitivity. As these secondary aims were exploratory there were no specific hypotheses made. To investigate the primary aim, the characteristics of each participant’s tinnitus were gathered alongside their audiometric profiles up to 12 kHz. A series of regression models were applied to the audiometric profile in order to determine the edge of hearing loss, slope of hearing loss and the frequency of worst hearing. These variables were then correlated with the dominant tinnitus pitch. The secondary aims were assessed by performing Bland-Altman Limits of Agreement analysis on the two measures of tinnitus loudness, and then performing a Pearson’s R correlation between those two measures of tinnitus loudness and participants score on the Hyperacusis Questionnaire. Our results indicated that, as hypothesised, the dominant tinnitus pitch did appear to fall within the region of hearing loss, however this was not confirmed statistically. We also found that the two measures of tinnitus loudness we collected did agree with one another. However, only the questionnaire reported measure of tinnitus loudness correlated with the hyperacusis score, potentially indicating an emotional component inherent in the questionnaire based approach to measuring tinnitus loudness. The second study presented aimed to investigate the role of delta (1-4 Hz), and alpha (8-12 Hz) oscillations in tinnitus using EEG. The phenomenon of residual inhibition (the temporary quietening of the tinnitus percept following presentation of a masking noise) was utilised to provide a within-subjects design. Oscillatory power in each of the four frequency bands was assessed in a baseline period, before residual inhibition was induced and compared with power after residual inhibition had been induced across two groups, one group that experienced residual inhibition and one group that did not experience any change in their tinnitus loudness. It was hypothesised that we would see significant increases in the alpha band and significant decreases in the delta band in the group that experienced residual inhibition. Exploratory analyses in the theta (5-7 Hz) and the beta (13-20 Hz) bands were also conducted, though no specific hypotheses were made regarding these. An ANOVA showed significantly higher alpha and beta power in the group that experienced residual inhibition versus the group that did not. There were no significant changes observed between baseline and the residual inhibition period in any of the frequency bands, although significantly higher alpha was observed in the first 5 seconds of the residual inhibition period compared to the last 5 seconds. The final experiment presented aimed to investigate the effects of transcranial direct stimulation (tDCS) on alpha and delta band oscillations. Both of these oscillatory frequencies have been implicated in the generation and maintenance of tinnitus and being able to modulate them may have effects on the tinnitus perception. A forward masking paradigm was implemented, while normal hearing control subjects underwent anodal tDCS at the same time as having their neural activity recorded via MEG. Cluster based permutation statistics revealed significant decreases in delta band activity, as well as increases in alpha band activity as a result of anodal stimulation. This indicates that tDCS is capable of modulating oscillatory frequency bands associated with tinnitus