Eavesdropping on Tinnitus Using MEG: Lessons Learned and Future Perspectives

Abstract

Tinnitus has been widely investigated in order to draw conclusions about the underlying causes and altered neural activity in various brain regions. Existing studies have based their work on different tinnitus frameworks, ranging from a more local perspective on the auditory cortex to the inclusion of broader networks and various approaches towards tinnitus perception and distress. Magnetoencephalography (MEG) provides a powerful tool for efficiently investigating tinnitus and aberrant neural activity both spatially and temporally. However, results are inconclusive, and studies are rarely mapped to theoretical frameworks. The purpose of this review was to firstly introduce MEG to interested researchers and secondly provide a synopsis of the current state. We divided recent tinnitus research in MEG into study designs using resting state measurements and studies implementing tone stimulation paradigms. The studies were categorized based on their theoretical foundation, and we outlined shortcomings as well as inconsistencies within the different approaches. Finally, we provided future perspectives on how to benefit more efficiently from the enormous potential of MEG. We suggested novel approaches from a theoretical, conceptual, and methodological point of view to allow future research to obtain a more comprehensive understanding of tinnitus and its underlying processes.

Keywords: Magnetoencephalography; Resting state; Review; Tinnitus; Tone stimulation.

Fig. 1

Dimensions of current tinnitus theoretical concepts to approach the generation and preservation of tinnitus. (1) Integration of tinnitus concepts along the three dimensions (A) perception-distress, (B) local-network, and (C) past-future. Dimension (A) refers to the different focus of various tinnitus frameworks either on the perceptual or on the distressing aspect of tinnitus. Dimension (B) highlights the neuroanatomical focus of the frameworks that is either particularly on the auditory system or more widespread on larger brain networks. Dimension (C) comprises the time aspect within the tinnitus frameworks, meaningly the reflections of processes in the past as well as how the current state can predict future trajectories. (2) Relevant tinnitus frameworks guiding past MEG research arranged along the two dimensions (A) perception-distress and (B) local-network

Fig. 2

Magnetoencephalography (device: whole-head MEG Triux, MEGIN Oy, Finland) within the magnetically shielded room (AK3b, Vacuumschmelze, Germany). A MEG-helmet with SQUID-sensor-unit. B Dewar with liquid helium inside. C EEG-unit with inputs for electrical signals (e.g., EOG, ECG). D Microphone. E Inputs for single EEG-electrodes. F MEG seat. G MEG bed. H Screen for visual stimulation

Fig. 3

Signal recording of the MEG sensors. We aim to capture the target activation (e.g., some aberrant neural activity as the source of the tinnitus) within the brain using MEG sensors on the scalp. However, the identification of this target activation may be obscured by multiple factors. These include not only genuine external noise, such as environmental disturbances, but also internal activity from other brain regions that are not directly relevant to the investigation of tinnitus