Different resting state brain activity and functional connectivity in patients who respond and not respond to bifrontal tDCS for tinnitus suppression

Tinnitus is an ongoing phantom percept. It has been demonstrated that bifrontal transcranial direct current stimulation (tDCS) can reduce tinnitus. In this study, one group of patients reported a substantial improvement in their tinnitus perception, whereas another group described minor or no beneficial effect at all. The objective was to verify whether the activity and connectivity of the resting brain is different for people who will respond to bifrontal tDCS for tinnitus in comparison with non-responders. Higher gamma band activity was demonstrated in right primary and secondary auditory cortex and right parahippocampus for responders. It has been shown that gamma band activity in the auditory cortex is correlated with tinnitus loudness and that the anterior cingulate is involved in tinnitus distress. People who were going to respond to bifrontal tDCS also demonstrated an increased functional connectivity in the gamma band between the right dorsolateral prefrontal cortex (DLPFC) and the right parahippocampus as well as the right DLPFC and subgenual anterior cingulate cortex (sgACC). An analysis revealed that responders to bifrontal tDCS also experienced a larger suppression effect on TMS placed over the right temporal cortex (i.e. auditory cortex) than non-responders. Responders to bifrontal tDCS seem to differ in resting brain activity compared to non-responders in the right auditory cortex and parahippocampal area. They also have a different functional connectivity between DLPFC and, respectively, the sgACC and parahippocampal area. These connectivities might explain the suppression effect for both tinnitus loudness and tinnitus-related distress.