Electrical stimulation of the auditory cortex as a treatment for tinnitus

• ¹ University Hospital Antwerp, Brain Research centre Antwerp for Innovative and Interdisciplinary Neuromodulation (BRAI²N), Belgium
• ² National Center for Scientific Research (CNRS), GIPSA-lab, France

Introduction: Recently electrical stimulation delivered to the auditory cortex through extradural electrodes, has been proposed as a therapy for tinnitus. Patients who do not respond to more conventional treatments for tinnitus, and respond to placebo controlled Transcranial Magnetic Stimulation (TMS), are candidates for this treatment in our multidisciplinary neuromodulation department. Although for most patients this treatment has proven to be successful, a small number experience no relief of the tinnitus after cortical stimulation. Bearing in mind the invasiveness of the treatment, it would be interesting to develop a prognostic tool, which can discern responders from non-responders to electrical stimulation.

Materials and Methods: Ten patients (7 male, 3 female) with implanted electrodes participated in this study (five positively responding to cortical stimulation, and five not responding to the treatment). In all patients a 19 channel EEG following .the 10-20 system, recording was performed in relaxed state over 5 minutes with eyes opened in a sound proof and electromagnetically shielded room using Synamps2 equipment (Neuroscan Inc.), (sampling rate was 500Hz). The impedances were checked to remain below 5 kΩ and the reference was a scalp electrode near the vertex. At the time of the recording no stimulation was delivered, thus all patients were strongly aware of their tinnitus during the recordings. Independent sampled t-tests on the different frequency bands of LORETA source images were performed between responders and non-responders.

Results: Responders show significant increased gamma band (30-40Hz) activity in the left parahippocampal area.

Discussion: The role of the hippocampus in tinnitus is not well established but neuroimaging studies demonstrate increased metabolism in the hippocampus in tinnitus patients. Shulman developed the idea that the amygdala - hippocampal area are involved in generating a paradoxical auditory memory for tinnitus. Animal studies with noise induced tinnitus have demonstrated that hippocampal plasticity induced through the non-classical auditory pathway may be involved in tinnitus and C-fos studies confirm this. Superselective amytal injections in the anterior choroidal artery which supply the amygdalohippocampal area are capable of suppressing tinnitus transiently, supporting the idea that some form of tinnitus might be generated in the hippocampus. We therefore hypothesize that in this subgroup of patients who do not respond to auditory cortex stimulation, hippocampal gamma band activity might be involved in the tinnitus generation. Further studies will try to further analyze this hypothesis.