Abstract
Previous studies have shown modulation of corticospinal output of the agonist muscle when a known-movement is prepared but withheld until a response signal appearance, reflecting motor preparation processes. However, modulation in the antagonist muscles has not been described, despite the fact that reaching movements require precise coordination between the activation of agonist and antagonist muscles. In this study, participants performed an instructed-delay reaction time (RT) task, with randomized elbow flexion and extension movements. The aim was to assess the time course modulation of corticospinal output in two antagonist muscles, by simultaneously quantified the amplitude of motor evoked potentials (MEPs) in biceps brachii and triceps brachii, and the amplitude and direction of elbow movements evoked by transcranial magnetic stimulation (TMS). Depending on the prepared movement direction, a specific modulation of corticospinal output was observed, MEPs and TMS-evoked movements amplitude being relatively greater for extension compared to flexion. At the end of motor preparation, a decrease in MEPs amplitude was observed for both biceps brachii and triceps brachii, regardless of the prepared movement direction. In contrast, the probability of evoking movement in the flexion direction and the amplitude of TMS-evoked movement decreased at the end of preparation for flexion, but not for extension. Together, these results confirm the existence of inhibitory processes at the end of the motor preparation, probably to avoid a premature motor response. Moreover, they provide evidence of differences in the corticospinal control of elbow flexor and extensor muscles with patterns of modulation that are not necessarily reciprocal during motor preparation.
Highlights
Each of our voluntary movements requires a preparatory phase called action preparation, during which our brain elaborates motor commands in order to produce an optimal motor behavior
Mean motor evoked potentials (MEPs) amplitudes did not significantly differed between the Baseline condition at the beginning vs. the end of the experiment (for biceps brachii (BB): Baseline 1 (0.31 ± 0.02 mV) Baseline 2 (0.38 ± 0.03 mV) [t(17) = -1.5, p = .15]; for triceps brachii (TB): Baseline 1 (0.11 ± 0.01 mV), Baseline 2 (0.12 ± 0.01 mV) [t(11) = -.47, p = .65]), indicating that corticospinal excitability in BB and TB did not change after the completion of the instructed-delay RT task
Results on MEPs for both muscles showed that corticospinal excitability is on average lower during preparation for flexion compared to extension regardless of transcranial magnetic stimulation (TMS)-Intervals, with a decrease of excitability just prior to the response signal regardless of the Direction
Summary
Each of our voluntary movements requires a preparatory phase called action preparation, during which our brain elaborates motor commands in order to produce an optimal motor behavior (for a review see [1]). An instructed-delay reaction-time (RT) task is commonly used to assess action preparation [2,3]. In this task, two signals are presented. In order to quantify corticospinal excitability, the MEPs peak-to-peak amplitude was measured during Baseline conditions and during the instructed-delay RT task for each Direction, each TMS-Interval and each muscle tested. Arm kinematics data necessary to determine the amplitude and direction of TMS-evoked movement were obtained from the KINARM motor encoders and sampled at 1000 Hz. Peak velocity of the fingertip displacement in the horizontal plane was first identified in a 150 ms window after TMS application to preclude any voluntary movement contamination. Positive values correspond to a TMS-evoked movement in the extension direction whereas negative values correspond to a TMS-evoked movement in the flexion direction
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