A novel approach to localize cortical TMS effects Despite the widespread use of transcranial magnetic stimulation (TMS), its precise cortical location underlying the physiological and behavioral stimulation effects are still only coarsely known. So far, mapping strategies that systematically measure TMS effects rely on center of gravity approaches and therefore localize the stimulated cortical site only approximately and indirectly. Focusing on the motor cortex, we present a new method to reliably determine the stimulated cortical site at the individual subject level. The approach combines measurements of motor evoked potentials (MEPs) at different coil positions and orientations with numerical modeling of induced electric fields. We identify sharply bounded cortical areas around the gyral crowns and rims of the motor hand area as the origin of the MEPs and show that the tangential field component and the magnitude of the electric field is most relevant for the observed effect. The identification requires relatively few measurements at different coil positions and orientations. We use extensive uncertainty and sensitivity analyses to verify the robustness of the method and identify the most critical model parameters. Our approach improves the localization of the cortex area stimulated by TMS. It is generic and can potentially be applied to other modalities such as language mapping.

Effects of repetitive transcranial magnetic stimulation on restingstate connectivity: A systematic review Abstract   The brain is organized into networks that reorganize dynamically in response to cognitive demands and exogenous stimuli. In recent years, repetitive transcranial magnetic stimulation (rTMS) has gained increasing use as a noninvasive means to modulate cortical physiology, with effects both proximal to the stimulation site and in distal areas that are intrinsically connected to the proximal target. In light of these networklevel neuromodulatory effects, there has been a rapid growth in studies attempting to leverage information about network connectivity to improve neuromodulatory control and intervention outcomes. However, the mechanismsofaction of rTMS on networklevel effects remain poorly understood and is based primarily on heuristics from proximal stimulation findings. To help bridge this gap, the current paper presents a systematic review of 33 rTMS studies with baseline and postrTMS measures of fMRI restingstate functional connectivity (RSFC). Literature synthesis revealed variability across studies in stimulation parameters, studied populations, and connectivity analysis methodology. Despite this variability, it is observed that active rTMS induces significant changes on RSFC, but the prevalent lowfrequencyinhibitionhighfrequencyfacilitation heuristic endorsed for proximal rTMS effects does not fully describe distal connectivity findings. This review also points towards other important considerations, including that the majority of rTMSinduced changes were found outside the stimulated functional network, suggesting that rTMS effects tend to spread across networks. Future studies may therefore wish to adopt conventions and systematic frameworks, such as the Yeo functional connectivity parcellation atlas adopted here, to better characterize networklevel effect that contribute to the efficacy of these rapidly developing noninvasive interventions.