Determining the cortical target of transcranial magnetic stimulation

Here, a stringent hierarchical sigmoidal model fitting approach is developed that determines the cortical target(s) from TMS mapping based on electric field calculations. Monte-Carlo simulations are used to assess the significance and the goodness-of-fit of the sigmoidal fits, and to obtain confidence regions around the calculated targets. The approach was applied to mapping data on visual suppression (N = 7). In all subjects, we reliably identified two or three neighboring targets commonly contributing to the suppression effect (average distance ± SD: 7.7 ± 2.3 mm). This demonstrates that (i) the assumption of a single CoM is not generally valid and (ii) the combination of TMS mapping with the fitting approach has a cortical resolution of < 1 cm. The estimates for the field strength necessary to achieve 50% of the maximal suppression effect vary noticeably across subjects (mean ± SD: 139 ± 24 V/m), indicating inter-individual differences in the susceptibility to TMS.