Unilateral movement preparation causes task-specific modulation of TMS responses in the passive, opposite limb.
J Physiol. 2018-06-19; 596(16): 3725-3738
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Chye L(1), Riek S(1), de Rugy A(1)(2), Carson RG(1)(3)(4), Carroll TJ(1).
(1)Centre for Sensorimotor Performance, School of Human Movement and Nutrition
Sciences, University of Queensland, Brisbane, Australia.
(2)Institut de Neurosciences Cognitives et Intégratives d’Aquitaine, Université
de Bordeaux, CNRS UMR 5287, Bordeaux, France.
(3)Trinity College Institute of Neuroscience and School of Psychology, Trinity
College Dublin, Dublin, Ireland.
(4)School of Psychology, Queen’s University Belfast, Belfast, UK.
KEY POINTS: Activity in the primary motor cortices of both hemispheres increases
during unilateral movement preparation, but the functional role of ipsilateral
motor cortex activity is unknown. Ipsilateral motor cortical activity could
represent subliminal ‘motor planning’ for the passive limb. Alternatively, it
could represent the state of the active limb, to support coordination between the
limbs should a bimanual movement be required. Here we assessed how preparation of
forces toward different directions, with the left wrist, alters evoked responses
to transcranial magnetic stimulation of left motor cortex. Preparation of a
unilateral movement caused excitability increases in ipsilateral motor cortex
that reflected forces produced with the active limb in an intrinsic
(body-centred), rather than an extrinsic (world-centred), coordinate system.
These results suggest that ipsilateral motor cortical activity prior to
unilateral action reflects the state of the active limb, rather than subliminal
motor planning for the passive limb.
ABSTRACT: Corticospinal excitability is modulated for muscles on both sides of
the body during unilateral movement preparation. For the effector, there is a
progressive increase in excitability, and a shift in direction of muscle twitches
evoked by transcranial magnetic stimulation (TMS) toward the impending movement.
By contrast, the directional characteristics of excitability changes in the
opposite (passive) limb have not been fully characterized. Here we assessed how
preparation of voluntary forces towards four spatially distinct visual targets
with the left wrist alters muscle twitches and motor-evoked potentials (MEPs)
elicited by TMS of left motor cortex. MEPs were facilitated significantly more in
muscles homologous to agonist rather than antagonist muscles in the active limb,
from 120 ms prior to voluntary EMG onset. Thus, unilateral motor preparation has
a directionally specific influence on pathways projecting to the opposite limb
that corresponds to the active muscles rather than the direction of movement in
space. The directions of TMS-evoked twitches also deviated toward the impending
force direction of the active limb, according to muscle-based coordinates,
following the onset of voluntary EMG. The data indicate that preparation of a
unilateral movement increases task-dependent excitability in ipsilateral motor
cortex, or its downstream projections, that reflects the forces applied by the
active limb in an intrinsic (body-centred), rather than an extrinsic
(world-centred), coordinate system. The results suggest that ipsilateral motor
cortical activity prior to unilateral action reflects the state of the active
limb, rather than subliminal motor planning for the passive limb.
© 2018 The Authors. The Journal of Physiology © 2018 The Physiological Society.
PMID: 29775218 [Indexed for MEDLINE]