Transcranial Magnetic Mapping of the Short-Latency Modulations of Corticospinal Activity from the Ipsilateral Hemisphere during Rest

Abstract

Skilled hand function relies heavily on the integrity of the primary motor cortex (M1) and on a web of cortico-cortical connections projecting onto it. We used a novel explorative paradigm to map the origin of cortico-M1 pathways assessed by dual transcranial magnetic stimulation (TMS) in three healthy participants. Subthreshold conditioning TMS (cTMS) was delivered over a grid of ≈100 spots. Covering the left hemisphere, and was followed by suprathreshold test (tTMS) delivered over the ipsilateral M1. Grid points were tested eight times, with inter-stimulus intervals between cTMS and tTMS of 4 and 7 ms. Participants were asked to stay relaxed with no particular task. Motor evoked potentials (MEPs) from cTMS + tTMS were normalized to MEPs from tTMS alone and were compared to the value expected from tTMS alone using t-statistics. The t-values from each grid point were then used to plot statistical maps. Several foci of significant cortico-M1 interactions were found in the dorsal–medial frontal cortex, in the ventral frontal cortex, in the superior and inferior parietal lobules and in the parietal operculum. The majority of active foci had inhibitory effects on corticospinal excitability. The spatial location of the network of different subjects overlapped but with some anatomical variation of single foci. TMS statistical mapping during the resting state revealed a complex inhibitory cortical network. The explorative approach to TMS as a brain mapping tool produced results that are self-standing in single subjects overcoming inter-individual variability of cortical active sites.

The dorsal premotor cortex exerts a powerful and specific inhibitory effect on the ipsilateral corticofacial system: a dual-coil transcranial magnetic stimulation study

“…The temporal specificity is consistent with corticocortical connections. In fact, other studies exploring corticocortical connections between areas the distances between which are similar to that between the PMCd and the mouthM1 have found interactions at ISIs around 6 ms. (Baumer et al 2009;Cattaneo and Barchiesi 2011;Davare et al 2008Davare et al , 2009Davare et al , 2010. It should be noted that the physical distance between the dorsal premotor cortex and the mouthM1 is of around 6 cm, entirely compatible with a direct corticocortical connection.…”

“…The analysis of the dual-pulse condition by normalizing with the sliding window is relatively novel (Cattaneo and Barchiesi 2011). Hence, we decided post hoc to perform, alongside to the main analysis, a conventional analysis based on averaging all MEPs within conditions and computing the ratio between the grand averages of the dualpulse trials and that of the single-pulse trials as is generally done in dual-coil TMS experiments (Davare et al 2009;Koch et al 2006;Mars et al 2009;O'Shea et al 2007b).…”

“…To do so, a procedure of normalizing single trials of dualpulse MEPs to the average of single-pulse MEPs in a sliding window that followed the dual-pulse trials was adopted (Cattaneo and Barchiesi 2011;Maule et al 2015). The two single-pulse trials immediately adjacent to the dual-pulse trial were used as the sliding window.…”

“…We provide evidence for a specific short-latency inhibitory effect of the The connectivity between the PMC and the ipsilateral M1 in humans is largely hypothesized by analogy with the non-human primates. Still, direct evidence for it has been obtained by means of the dual-coil paradigm of transcranial magnetic stimulation (TMS) (Baumer et al 2009;Cattaneo and Barchiesi 2011;Civardi et al 2001;Davare et al 2008Davare et al , 2009Davare et al , 2010Koch et al 2006;Mars et al 2009). This protocol involves the use of two magnetic stimulators connected to two distinct coils, both of which are used simultaneously on the participant's scalp.…”

Corticospinal Excitability Modulation During Action Observation

“…Little is known regarding how the inflexible tendency to match observed actions onto our motor system can be reconciled with the request to prepare nonidentical responses. In this respect, some researchers showed that the automatic effects of mirroring can be abolished following incompatible training: mirror and counter-mirror responses seem to follow the same timecourse 27,28 . Interestingly, in contrast to previous studies, MEPs induced by spTMS were recently used to assess spontaneous corticospinal activation while video-clips evoking emulative or nonidentical complementary gestures were being simply observed 29,30 .…”