Formerly known as inhibitory: effects of 1-Hz rTMS on auditory cortex are state-dependent

Abstract

The major repetitive transcranial magnetic stimulation (rTMS) paradigm applied to the treatment of tinnitus has been the 1-Hz variant due to its alleged inhibitory effects. Clinical effects have, however, been hampered by great interindividual variability as well as the fact that TMS includes no explicit mechanism to modulate excitability in circumscribed regions of tonotopically organised auditory fields. Following studies showing that the effect of TMS depends on the activational state preceding the stimulation, participants were exposed to 10 min of either notch- or bandpass-filtered noise prior to 1-Hz rTMS applied to the left auditory cortex. A control group was additionally assessed using bandpass noise - albeit with subsequent sham stimulation - to assess whether effects were due to the differential sounds alone or to a genuine interaction between sound and rTMS. Electroencephalogram was recorded from 128 electrodes before and after the experimental treatment while participants performed an auditory intensity discrimination task. While state-dependency effects from the behavioural data are not conclusive, several condition × (sound) frequency effects (some specific to the stimulated side) could be observed. Importantly, many of these could not be explained by the use of rTMS or the filtered noise alone. The resulting patterns are, however, complex and temporally variable, which currently prohibits recommendations on how to design a clinically effective approach to treat tinnitus. Nevertheless, our study gives the first evidence that state-dependency principles can induce sound frequency-specific effects in the auditory cortex, providing a crucial proof-of-principle upon which future studies can build.