Anatomical and physiological organization of the non-primary motor cortex
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Premotor cortex
Temporal interference (TI) could stimulate deep motor cortex and induce movement without affecting the overlying cortex in previous mouse studies. However, there is still lack of evidence on potential TI effects in human studies. To fill this gap, we collected resting-state functional magnetic resonance imaging data on 40 healthy young participants both before and during TI stimulation on the left primary motor cortex (M1). We also chose a widely used simulation approach (tDCS) as a baseline condition. In the stimulation session, participants were randomly allocated to 2 mA TI or tDCS for 20 minutes. We used a seed-based whole brain correlation analysis method to quantify the strength of functional connectivity among different brain regions. Our results showed that both TI and tDCS significantly boosted functional connection strength between M1 and secondary motor cortex (premotor cortex and supplementary motor cortex). This is the first time to demonstrate substantial stimulation effect of TI in the human brain.
Premotor cortex
Transcranial Direct Current Stimulation
Human brain
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Premotor cortex
Stimulus (psychology)
Pyramidal tracts
Evoked potential
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Premotor cortex
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Premotor cortex
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Paired transcranial magnetic stimulation (TMS) has been applied as a probe to test functional connectivity within distinct cortical areas of the motor system. Depending on the intensity of a conditioning stimulus applied to different areas of the cortical motor network both facilitation and inhibition may be detected in the primary motor cortex (M1), ipsilaterally or contralaterally to the site of conditioning stimulation. Civardi (2001) and our group (Koch; unpublished data) reported that conditioning stimuli applied to the dorsal premotor cortex (PMd) may induce distinct effects on ipsilateral M1 depending on the intensity of stimulation. Low conditioning intensities provoked inhibition with a maximum at 90% active motor threshold (AMT) which turned into facilitation when higher intensities (120% AMT and 110% RMT, respectively) were applied.
Premotor cortex
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Motor area
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The effectiveness of transcranial direct current stimulation (tDCS) placed over the motor hotspot (thought to represent the primary motor cortex (M1)) to modulate motor network excitability is highly variable. The premotor cortex-particularly the dorsal premotor cortex (PMd)-may be a promising alternative target to reliably modulate motor excitability, as it influences motor control across multiple pathways, one independent of M1 and one with direct connections to M1. This double-blind, placebo-controlled preliminary study aimed to differentially excite motor and premotor regions using high-definition tDCS (HD-tDCS) with concurrent functional magnetic resonance imaging (fMRI). HD-tDCS applied over either the motor hotspot or the premotor cortex demonstrated high inter-individual variability in changes on cortical motor excitability. However, HD-tDCS over the premotor cortex led to a higher number of responders and greater changes in local fMRI-based complexity than HD-tDCS over the motor hotspot. Furthermore, an analysis of individual motor hotspot anatomical locations revealed that, in more than half of the participants, the motor hotspot is not located over anatomical M1 boundaries, despite using a canonical definition of the motor hotspot. This heterogeneity in stimulation site may contribute to the variability of tDCS results. Altogether, these preliminary findings provide new considerations to enhance tDCS reliability.
Premotor cortex
Transcranial Direct Current Stimulation
Supplementary motor area
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