Asymmetric Interhemispheric Transfer in the Auditory Network: evidence from TMS, Resting-State fMRI, and Diffusion Imaging

Abstract

Hemispheric asymmetries in human auditory cortical function and structure are still highly debated. Brain stimulation approaches can complement correlational techniques by uncovering causal influences. Previous studies have shown asymmetrical effects of transcranial magnetic stimulation (TMS) on task performance, but it is unclear whether these effects are task-specific or reflect intrinsic network properties. To test how modulation of auditory cortex (AC) influences functional networks and whether this influence is asymmetrical, the present study measured resting-state fMRI connectivity networks in 17 healthy volunteers before and immediately after TMS (continuous theta burst stimulation) to the left or right AC, and the vertex as a control. We also examined the relationship between TMS-induced interhemispheric signal propagation and anatomical properties of callosal auditory fibers as measured with diffusion-weighted MRI. We found that TMS to the right AC, but not the left, resulted in widespread connectivity decreases in auditory- and motor-related networks in the resting state. Individual differences in the degree of change in functional connectivity between auditory cortices after TMS applied over the right AC were negatively related to the volume of callosal auditory fibers. The findings show that TMS-induced network modulation occurs, even in the absence of an explicit task, and that the magnitude of the effect differs across individuals as a function of callosal structure, supporting a role for the corpus callosum in mediating functional asymmetry. The findings support theoretical models emphasizing hemispheric differences in network organization and are of practical significance in showing that brain stimulation studies need to take network-level effects into account. SIGNIFICANCE STATEMENT We investigated the lateralized functional networks involved in auditory processing and how modulation of the auditory cortex (AC) influences these networks. We demonstrated that transcranial magnetic stimulation (TMS) to the AC resulted in changes in connectivity in auditory and motor-related networks in the resting state. Such changes were strongly asymmetric, with stimulation of the right AC resulting in more widespread decreases in connectivity compared with stimulation of the left AC. Finally, we showed that individual differences in the degree of modulation of functional connectivity in AC were related to structural measures of the callosal auditory pathway. The findings support theoretical models emphasizing hemispheric differences in network organization; they also show that TMS studies need to consider network-level effects of stimulation.