Mapping interhemispheric connectivity using functional MRI after transcranial magnetic stimulation on the human auditory cortex.

Interhemispheric interactions can be important in transcranial magnetic stimulation (TMS) studies investigating motor or cognitive brain functions, but their role in predicting the outcome of TMS is not clear. Previously, we showed that individual differences in interhemispheric functional connectivity of auditory cortices influenced the behavioral effect of repetitive TMS (rTMS) applied over auditory cortex in a melody discrimination task. Here, functional magnetic resonance imaging (fMRI) scanning with the same task was carried out before and after rTMS applied over auditory cortex to determine how rTMS affects both behavior and neural function. After rTMS applied over the right auditory cortex, we found mean increases in activation in the contralateral auditory cortex. The degree and direction of modulation of the fMRI response were correlated with behavior: the higher the contralateral increase after stimulation, the faster the response times, whereas individuals with reduced contralateral activity showed no behavioral facilitation. We also found that higher interhemispheric connectivity between auditory cortices before TMS was associated with faster response times. This study shows directly the role of functional connectivity in the auditory network on TMS-induced modulation, which could explain its often variable effects on behavior. Combined TMS and fMRI is particularly useful to promote plastic reorganization in the auditory network and has implications for research on auditory-related disorders.