Clinical NeuroscienceMethodology for studying the transient effects of transcranial direct current stimulation combined with auditory residual inhibition on tinnitus

- TDCS may have additive effects to auditory RI.
- MML monitoring and loudness rating changes were inconsistent with each other.
- More participants rated tinnitus loudness lower than demonstrated MML reductions.
- Monitoring of MML may cause a higher adaptation level disguising changes in tinnitus.

BackgroundTranscranial direct current stimulation (tDCS) and auditory residual inhibition (RI) are short lasting non-invasive techniques that can suppress tinnitus in some people. A pilot study was undertaken to evaluate methods to measure a potential additive effect of these treatments.New methodTen participants (mean age 59 years) with chronic tinnitus underwent four sessions of anodal tDCS of the left temporoparietal area(LTA) combined with 1 min of auditory stimulation (broadband noise presented at tinnitus minimum masking level [MML] + 10 dB), either before, during or immediately after tDCS and in a sham controlled session. Participants continuously monitored and recorded their MML on custom software prior to stimulation, during 20 min of tDCS, and 30 min following tDCS. A tinnitus loudness rating was undertaken before and after completion of testing.ResultsNine out of ten participants completed the study, one participant dropped out after two sessions due to self-reported worsening in tinnitus symptoms. Seven out of nine participants reported transient tinnitus suppression when sound stimulation was presented along with tDCS. No reliable change in MML was observed.Comparison with existing methodThis is the first study to examine the feasibility of continuously measuring MML while providing a combination of auditory and noninvasive brain stimulation simultaneously.ConclusionsWe propose modifications in the protocol used, to find ways to increase the duration of auditory RI and investigate its underlying mechanisms. Recommendations for further research are discussed.