Probing brain connectivity with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a noninvasive method for exciting neurons below the TMS coil. TMS is usually applied with electromyography (EMG) to study motor evoked potentials (MEPs), but it can be combined with various other technologies, e.g., electroencephalography (EEG). When combined with EMG, TMS is applied to investigate the function of muscle pathways from the brain to the muscles. Much more about the function of the brain can be learned when EEG is used to measure the brain's response to the stimulation. TMS–EEG describes causal connectivity (effective connectivity) between different brain areas. In the first study (Paper I), we studied effective connectivity with TMS–EEG on an individual level and learned that the spreading of activation patterns in the brain varies between individuals when primary and non-primary motor areas are stimulated. However, the spreading of brain activation right after the stimulation (< 50 ms) was quite similar for the subjects when the same area was stimulated. More differences in the spreading of brain activation were observed with larger latencies (> 50 ms) when the same area was stimulated and when different areas were stimulated. In the second study (Paper II), we investigated whether acute pain caused by cold water (2–4 degrees) would alter short-interval intracortical inhibition (SICI). SICI is a measure of cortical inhibition. It was concluded that SICI increased after the acute pain had ended and the effect lasted under 18 minutes. The effect was seen only in a hand muscle exposed to the experimental pain but not in a remote upper limb muscle not exposed to the experimental pain. In the third study (Paper III), we presented that signal-space projection (SSP) combined with source-informed reconstruction (SIR) was effective to suppress artifacts from the EEG data even when lateral brain areas are stimulated. However, brain signals were suppressed near the stimulation site, but the topographies of these brain signals remained even after the cleaning. In the fourth study (Paper IV), we studied how the activation patterns spread when lateral brain areas were stimulated. We observed that brain activation spreads quickly to the other hemisphere. This Thesis provides new knowledge about brain connectivity and offers a pathway between instrumentation and method development to clinical applications.