Dual-site TMS as a tool to probe effective interactions within the motor network: a review

Abstract

Dual-site transcranial magnetic stimulation (ds-TMS) is well suited to investigate the causal effect of distant brain regions on the primary motor cortex, both at rest and during motor performance and learning. However, given the broad set of stimulation parameters, clarity about which parameters are most effective for identifying particular interactions is lacking. Here, evidence describing inter- and intra-hemispheric interactions during rest and in the context of motor tasks is reviewed. Our aims are threefold: (1) provide a detailed overview of ds-TMS literature regarding inter- and intra-hemispheric connectivity; (2) describe the applicability and contributions of these interactions to motor control, and; (3) discuss the practical implications and future directions. Of the 3659 studies screened, 109 were included and discussed. Overall, there is remarkable variability in the experimental context for assessing ds-TMS interactions, as well as in the use and reporting of stimulation parameters, hindering a quantitative comparison of results across studies. Further studies examining ds-TMS interactions in a systematic manner, and in which all critical parameters are carefully reported, are needed.

Test-retest reliability of intrahemispheric dorsal premotor and primary motor cortex dual-site TMS connectivity measures

“…Variable study designs (stimulation intensities, placement of CS coil, coil focality, etc.) might explain these diverse results (Van Malderen et al, 2023). Furthermore, this study encompassed 20 datapoints in the final analysis.…”

“…Previous research has shown both facilitatory and inhibitory PMd–M1 interactions at rest. However, responses to stimulation during motor tasks seem to lean more towards facilitation, whereas inhibition seemed to be more pronounced in the context of action withholding (Van Malderen et al, 2023). Hence, it is not possible to predict whether the investigated ISI in task-based protocols would yield comparable results in terms of modulation and reliability.…”

“…One way to investigate PMd–M1 connectivity is dual-site transcranial magnetic stimulation (dsTMS), a tool that allows studying the effective or causal functional connectivity between two brain areas at rest, but also during task preparation or execution within a temporal resolution of milliseconds (Van Malderen et al, 2023). To do so, a conditioning stimulus (CS) over a brain area of interest is followed by a test stimulus (TS) over the M1 representation a few milliseconds later, and the peak-to-peak amplitude of these conditioned motor-evoked potentials (MEP) is compared to unconditioned MEPs emerging from M1 (Kujirai et al, 1993).…”

“…To do so, a conditioning stimulus (CS) over a brain area of interest is followed by a test stimulus (TS) over the M1 representation a few milliseconds later, and the peak-to-peak amplitude of these conditioned motor-evoked potentials (MEP) is compared to unconditioned MEPs emerging from M1 (Kujirai et al, 1993). A smaller or larger conditioned MEP as compared to unconditioned MEP, points towards an inhibitory or facilitatory influence of the conditioned brain area on M1, respectively (Van Hoornweder et al, 2021, Van Malderen et al, 2023). However, the nature of this effect is dependent on the stimulation protocol, depending on parameters such as the choice of the CS and TS intensity, the coil placement, and the interstimulus interval (ISI).…”

Contribution of neural circuits tested by transcranial magnetic stimulation in corticomotor control of low back muscle: a systematic review

“…Cortical (e.g., SMA and PM) or subcortical areas (cerebellum) have not been investigated in low back muscles although they are critical in postural control ( Figure 4 ) ( Massion, 1992 ; Coffman et al, 2011 ; Takakusaki, 2017 ). The involvement of SMA, PM and cerebellum have been successfully tested with dual coil protocol for hand muscles [for review see: ( Van Malderen et al, 2022 )]. Therefore, a specific involvement could also be expected in low back muscles.…”