Neurophysiological correlates of Somatosensory tinnitus modulation
Tinnitus is the subjective perception of a sound in absence of corresponding external acoustic stimuli. Research highlights the influence of the sensorimotor system on tinnitus perception. Associated neuronal processes, however, are insufficiently understood and it remains unclear how and at which hierarchical level the sensorimotor system interacts with the tinnitus-processing auditory system. We therefore asked 23 patients suffering from chronic tinnitus (11 males) to perform specific exercises, aimed at relaxing or tensing the jaw area, which temporarily modulated tinnitus perception. Associated neuronal processes were assessed using Magnetencephalography. Results show that chronic tinnitus patients experienced their tinnitus as weaker and less annoying after completion of relaxing compared to tensing exercises. Furthermore, (1) sensorimotor alpha power and alpha-band connectivity directed from the somatosensory to the auditory cortex increased, and (2) gamma power in the auditory cortex, reduced, which (3) related to reduced tinnitus annoyance perception on a trial-by-trial basis in the relaxed state. No effects were revealed for 23 control participants without tinnitus (6 males) performing the same experiment. We conclude that the increase in directed alpha-band connectivity from somatosensory to auditory cortex is most likely reflecting the transmission of inhibition from somatosensory to auditory cortex during relaxation, where concurrently tinnitus-related gamma power reduces. We suggest that revealed neuronal processes are transferable to other tinnitus modulating systems beyond the sensorimotor one that are e.g. involved in attentional or emotional tinnitus modulation and provide deeper mechanistic insights into how and through which channels phantom sound perception might be modulated on a neuronal level. Significance StatementTinnitus describes the perception of auditory phantom sounds. Research suggests that the sensorimotor system impacts on tinnitus perception, associated neuronal mechanisms, however, have remained unclear. Here, chronic tinnitus patients performed exercises with the jaw temporarily reducing (versus increasing) tinnitus perception. Tinnitus reduction was accompanied by an increase of alpha-band connectivity directed from the somatosensory to the auditory cortex and gamma power reduction in the auditory cortex. We suggest that the increase in alpha-band connectivity, when tinnitus is reduced, reflects the transmission of inhibition from somatosensory to auditory cortex, where, in parallel, probably tinnitus related, gamma power reduces. The findings have important implications both for the understanding of phantom sound perception and, more generally, of top-down modulation in healthy and impaired cognition.