Multi-locus transcranial magnetic stimulation: moving beyond the current state of the art in noninvasive brain stimulation
Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation method: a magnetic field pulse from a TMS coil can excite neurons in a desired cortical location. To change the stimulated spot, the coil needs to be moved. This inherently slow physical movement limits our ability to interact with neuronal activity. To overcome this limitation, we introduce multi-locus TMS (mTMS) that allows rapid, electronic stimulation targeting. We developed an algorithm for designing optimized mTMS transducers that allow translating and rotating the induced electric field in the cortex while keeping the profile of the shifted electric field essentially unchanged; the control is achieved by adjusting the relative current strengths in a set of overlapping coils that form the transducer. To demonstrate our approach, we designed and built a two-coil mTMS transducer and a compatible mTMS device. We stimulated nearby locations along a 30-mm-long line segment of the primary motor cortex in a paired-pulse setting with interstimulus intervals between 0.5 and 10 ms and recorded the resulting MEPs. The results inform us about the mechanisms of short-interval intracortical inhibition. The instantaneous electronic control provided by mTMS will open new possibilities for neuroscience, for example, for studying the interaction between different brain areas on the neuronal timescales, or for implementing real-time feedback-guided brain-stimulation devices, with potentially powerful research and clinical applications.