High Definition Transcranial Direct Current Stimulation and Auditory Residual Inhibition for Tinnitus Relief: a Combination Treatment Approach

Abstract

Background: This thesis examines a novel approach to the treatment of tinnitus; the combination of High Definition Transcranial Direct Current Stimulation (HD-tDCS) of the left temporoparietal area (LTA) with auditory residual inhibition (RI). HD-tDCS is a neuromodulation strategy capable of altering the excitability of cortical neurons. Its action is highly focal; the device can specifically modulate activity within the region confined by its 4 x 1 ring of electrodes, such that the targeted region becomes more amenable to neuroplastic change. HD-tDCS of the LTA has previously been shown effective at reducing tinnitus loudness and its associated annoyance. Conversely, auditory RI is the transient suppression of tinnitus following the presentation of a loud masking sound; a phenomenon which is thought to be neurologically driven. Methods: This study employed a within-subjects (n=13), counterbalanced and sham-controlled design to assess whether HD-tDCS of the LTA, paired with auditory RI, was able to produce a greater degree of tinnitus relief compared to HD-tDCS used in isolation. Each participant underwent four HD-tDCS stimulations across two sessions (2 per session, minimum of 1 week ‘washout’ between sessions). Each stimulation was paired with one of four different RI ‘trigger’ sounds, presented in a counterbalanced order; a high-pulsed (tetanic) tonal stimulus, a low-pulsed tonal stimulus, a uniform white noise, and a ‘sham’ sound. The outcome variables were tinnitus loudness, and tinnitus associated annoyance; assessed throughout experimental proceedings using rating scales. Results: HD-tDCS resulted in a significant reduction in tinnitus loudness for 38% of participants, and in tinnitus associated annoyance for 30% of participants; however, this failed to reach statistical significance overall. The combination of HD-tDCS with high-pulsed tetanic sound resulted in a significant modulation of loudness and annoyance compared to ‘sham’ sound, although this effect was transient and had dissipated within 10 minutes. Implications: This study demonstrated that a high-pulsed tetanic auditory stimulus was able to significantly reduce tinnitus loudness and its associated annoyance, when paired with HD-tDCS of the LTA. Such a finding has not previously been reported in the literature. Future work in this area is therefore warranted, and recommendations for this are discussed.