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The dynamic modulation of inter-hemispheric inhibition during bimanual grip force control

Karabanov A, Mogensen J, Lundell H and Siebner H

2019

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Transcallosal connections between motor cortices are pivotal for bimanualcoordination and previous work demonstrated that inter-hemisphericinhibition is modulated by symmetry of bimanual movements [1]. Theamount of modulation during movement preparation and the role of‘dynamic force context’ (increase vs decease) remain unknown.The ipsilateral silent period (iSP) (ISP area /pre-stimulus EMG) quantifiedinter-hemispheric inhibition during dynamic bimanual grip force control.The iSP was elicited by a TMS pulse to the left primary motor hand areawhile participants (N¼17) performed a visually cued bimanual pinch-griptask, requiring to match two changing semi-circles by increasing ordecreasing pinch force in the left and right hand. The influence of force wastested in three conditions: (1) steady symmetric pinch (baseline), (2) leftled asymmetric pinch (left hand increases force, right simultaneously releases) (3) right-led asymmetric pinch (right hand increases force, lefthand releases). Three time points during movement preparation wereassessed (100ms, 150ms, 200ms post visual-cue change).An ANOVA showed a significant main effect for Time (p¼ 0.03) and Condition (p¼ 0.04) but no Time x Condition interaction. Post-hoc testingrevealed that iSP was stronger during baseline compared to left-led (p¼0.02) and right-led (p ¼ 0.07) movement types with no significant difference between the two asymmetric conditions (p¼0.4). Generally, iSPwas strongest when assessed at 100ms post-cue change across conditions.A Pearson’s product-moment correlation indicated negative correlationbetween iSP modulation during asymmetric movements and the reactiontime of the left hand, reaching significance in the left-led condition (r¼-0.5p¼0.02).Asymmetric isometric grip force generation decreases inter-hemisphericinhibition during movement preparation. The magnitude of down-regulating transcallosal inhibition is tied to faster reaction times. For the timewindow studied, our data provide no evidence in support of the dynamicforce context on inter-hemispheric interactions as expressed by the iSP.
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