Figure 4. Probing pathways within M1and cortical‐cortical connectivity with TMS.

A, Interregional interactions from both excitatory (open circles) and inhibitory (filled circles) neuronal populations can modulate the output of M1. For instance, the use of paired‐pulse TMS over M1 can probe inhibitory processes like short‐ and long‐interval intracortical inhibition (SICI and LICI), which have been shown to modulate late I‐waves. Moreover, triple‐pulse TMS protocols can be used to see how different neural populations interact with one another (e.g. LICI vs. SICI). Using this strategy, LICI has been shown to reduce the effect of SICI (Sanger et al., 2001), a phenomenon that has been suggested to occur by presynaptic GABA_B receptor (LICI)‐mediated inhibition (Sanger et al., 2001; McDonnell et al., 2006; Muller‐Dalhaus et al., 2008). B, modulation of the MEP has also been found when a conditioning pulse is administered in a temporally specific manner to the cerebellum and parietal, premotor, or supplementary motor cortices. In the example above, one can use paired‐pulse TMS to study the connectivity between the cerebellum and M1 via the cerebellar–thalamic tracts (Spampinato & Celnik, 2018), which have been found to interact with both excitatory and inhibitory interneurons (Daskalakis et al., 2004; Fong et al., 2021).