MRI-based visualization of rTMS-induced cortical plasticity in the primary motor cortex

Abstract

Repetitive transcranial magnetic stimulation (rTMS) induces changes in cortical excitability for minutes to hours after the end of intervention. However, it has not been precisely determined to what extent cortical plasticity prevails spatially in the cortex. Recent studies have shown that rTMS induces changes in “interhemispheric” functional connectivity, the resting-state functional connectivity between the stimulated region and the symmetrically corresponding region in the contralateral hemisphere. In the present study, quadripulse stimulation (QPS) was applied to the index finger representation in the left primary motor cortex (M1), while the position of the stimulation coil was constantly monitored by an online navigator. After QPS application, resting-state functional magnetic resonance imaging was performed, and the interhemispheric functional connectivity was compared with that before QPS. A cluster of connectivity changes was observed in the stimulated region in the central sulcus. The cluster was spatially extended approximately 10 mm from the center [half width at half maximum (HWHM): approximately 3 mm] and was extended approximately 20 mm long in depth (HWHM: approximately 7 mm). A localizer scan of the index finger motion confirmed that the cluster of interhemispheric connectivity changes overlapped spatially with the activation related to the index finger motion. These results indicate that cortical plasticity in M1 induced by rTMS was relatively restricted in space and suggest that rTMS can reveal functional dissociation associated with adjacent small areas by inducing neural plasticity in restricted cortical regions.

Quadripulse stimulation (QPS)

“…Resting-state functional magnetic resonance imaging (fMRI) showed that QPS5 decreased interhemispheric functional connectivity between the bilateral M1, whereas QPS50 increased it (Watanabe et al 2014). The QPSinduced after-effects were restricted to the small M1 area innervating the target muscle (Tamura et al 2019). This is consistent with recent findings in monkeys (Romero et al, 2019).…”

Parallel cognitive processing streams in human prefrontal cortex: Parsing areal-level brain network for response inhibition

“…For example, as commonly experienced in TMS experiments, in stimulating the first dorsal interosseous (FDI) representation of the primary motor area (M1) with the strength near to the resting motor threshold, evoked potential is observed almost selectively in the FDI muscle even though other M1 regions are also stimulated. Another example is the changes in functional connectivity after rTMS to the M1 in our previous study (Tamura et al, 2019), in which it was demonstrated that the spatial extent of functional connectivity changes was limited to approximately 1 cm around the target. Assuming that the functional connectivity changes correlate with neuronal excitation, the spatial extent of the excitation effects of TMS can be considered very limited.…”

“…Although prominent resting-state functional connectivity was observed between the vpIFC and dpIFC, which are only 16 mm apart, no effective interaction between them during response inhibition could be detected by combining spTMS and rTMS (Figure 5). Our previous study has revealed that the spatial extent of the changes in cortical excitability after rTMS was approximately 3 mm in half width at half maximum, which is restricted to the size of one parcel (Tamura et al, 2019). Thus, the lack of an effective interaction between the vpIFC and dpIFC may be explained by the task demand for which two cortical processing streams are recruited in a parallel manner at the prefrontal stage, although the two streams may interact at downstream areas.…”

A causal role of anterior prefrontal-putamen circuit for response inhibition revealed by transcranial ultrasound stimulation in humans

“…Brain activity was measured using fMRI while the subjects performed a motor task to identify FDI representation in M1 (FDI-M1) in the left hemisphere in each subject (Figure 2A). The task was designed to activate the right FDI-M1 in the left hemisphere (Tamura et al, 2019). Activation was confined to the region anterior to the central sulcus (Figure 2B).…”