The direction of force twitches evoked by TMS in a passive limb shift according to the direction of impending contralateral muscle activation
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1
The University of Queensland, Centre for Sensorimotor Neuroscience, School of Human Movement Studies, Australia
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2
Université Bordeaux Segalen, Institut de Neurosciences Cognitives et Intégratives d'Aquitaine, France
Corticospinal excitability is modulated for muscles on both sides of the body during unilateral movement preparation. In the moving limb, there is a progressive increase in corticospinal excitability and a shift in the twitch direction evoked by transcranial magnetic stimulation (TMS) towards the upcoming movement direction. While corticospinal excitability in the passive limb is known to decrease, directional changes have not been characterised. Here, we used TMS to evoke consistent right wrist (passive limb) movements in near vertical orientations (i.e. up or down) in participants (n=12). Participants made movements with their left wrist towards one of four targets in a choice reaction time task. We assessed whether preparation of left wrist forces (active limb) to horizontal targets shifted the twitch direction in the right wrist towards the direction of action of homologous muscles, or towards the direction of opposite limb movement in extrinsic space.
The TMS-evoked twitch direction in the passive limb remained unchanged from baseline for stimuli delivered earlier than 20ms prior to EMG onset in the active limb. However, for stimuli delivered from 20ms before EMG onset to ~100ms after onset, TMS-evoked twitch direction shifted significantly from baseline towards the pulling direction of the muscles homologous to those responsible for force generation in the active limb (p<0.05).
The twitch direction in the passive limb remained unchanged for most of the contralateral movement preparation period, suggesting early unilateral motor preparation has limited directionally-specific influence on ipsilateral motor cortex. However, the twitch direction deviated sharply toward the impending contralateral movement in muscle space late in preparation, before afferent feedback could possibly be involved. Both the timing and coordinate frame of the effects suggest that the twitch biases in the passive limb are related to the release of descending commands to muscles.
Keywords:
motor learning,
TMS,
motor preparation,
ipsilateral motor cortex,
force twitches
Conference:
XII International Conference on Cognitive Neuroscience (ICON-XII), Brisbane, Queensland, Australia, 27 Jul - 31 Jul, 2014.
Presentation Type:
Poster
Topic:
Motor Behaviour
Citation:
Chye
L,
Riek
S,
De Rugy
A and
Carroll
T
(2015). The direction of force twitches evoked by TMS in a passive limb shift according to the direction of impending contralateral muscle activation.
Conference Abstract:
XII International Conference on Cognitive Neuroscience (ICON-XII).
doi: 10.3389/conf.fnhum.2015.217.00159
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Received:
19 Feb 2015;
Published Online:
24 Apr 2015.
*
Correspondence:
Prof. Tim Carroll, The University of Queensland, Centre for Sensorimotor Neuroscience, School of Human Movement Studies, Brisbane, Australia, tcarroll@hms.uq.edu.au