Timing of cortical excitability changes during the reaction time of movements superimposed on tonic motor activity
Timing of cortical excitability changes during the reaction time of movements superimposed on tonic motor activity
Seated subjects were instructed to react to an auditory cue by simultaneously contracting the tibialis anterior (TA) muscle of each ankle isometrically. Focal transcranial magnetic stimulation of the leg area of the motor cortex (MCx) was used to determine the time course of changes in motor-evoked potential amplitude (MEP) during the reaction time (RT). In one condition the voluntary contraction was superimposed on tonic EMG activity maintained at 10% of maximal voluntary contraction. In the other condition the voluntary contraction was made starting from rest. MEPs in the TA contralateral to the stimulation coil were evoked at various times during the RT in each condition. These were compared to the control MEPs evoked during tonic voluntary activity or with the subject at rest. The RT was measured trial by trial from the EMG activity of the TA ipsilateral to the magnetic stimulus, taking into account the nearly constant time difference between the two sides. The MEPs became far greater than control MEPs during the RT (mean = 332%, SD = 44 %, of control MEPs, P < 0.001) without any measurable change in the background level of EMG activity. The onset of this facilitation occurred on average 12.80 ms (SD = 7.55 ms) before the RT. There was no difference in the onset of facilitation between the two conditions. Because MEPs were facilitated without a change in the background EMG activity, it is concluded that this facilitation is specifically due to an increase of MCx excitability just before voluntary muscle activation. This conclusion is further reinforced by the observation that MEPs evoked by near-threshold anodal stimuli to the MCx were not facilitated during the RT, in contrast to those evoked by near-threshold transcranial magnetic stimulation. However, several observations in the present and previous studies indicate that MEP amplitude may be more sensitive to alpha-motoneuron activity than to motor cortical neuron activity, an idea that has important methodological implications.
magnetic stimulation, motor cortex, cortical excitability
2220-2227
Schneider, Cyril
fe903f40-11d5-454d-9d17-514bbc717864
Lavoie, Brigitte A.
8acd6a26-0768-41c6-874b-27c226764692
Barbeau, Hugues
6c8cba5c-1d3f-483f-9f3c-c071d155a58e
Capaday, Charles
0f389225-5082-4b7d-bc4d-cb35e90385a5
2004
Schneider, Cyril
fe903f40-11d5-454d-9d17-514bbc717864
Lavoie, Brigitte A.
8acd6a26-0768-41c6-874b-27c226764692
Barbeau, Hugues
6c8cba5c-1d3f-483f-9f3c-c071d155a58e
Capaday, Charles
0f389225-5082-4b7d-bc4d-cb35e90385a5
Schneider, Cyril, Lavoie, Brigitte A., Barbeau, Hugues and Capaday, Charles
(2004)
Timing of cortical excitability changes during the reaction time of movements superimposed on tonic motor activity.
European Journal of Applied Physiology and Occupational Physiology, (6), .
(doi:10.1152/japplphysiol.00542.2004).
Abstract
Seated subjects were instructed to react to an auditory cue by simultaneously contracting the tibialis anterior (TA) muscle of each ankle isometrically. Focal transcranial magnetic stimulation of the leg area of the motor cortex (MCx) was used to determine the time course of changes in motor-evoked potential amplitude (MEP) during the reaction time (RT). In one condition the voluntary contraction was superimposed on tonic EMG activity maintained at 10% of maximal voluntary contraction. In the other condition the voluntary contraction was made starting from rest. MEPs in the TA contralateral to the stimulation coil were evoked at various times during the RT in each condition. These were compared to the control MEPs evoked during tonic voluntary activity or with the subject at rest. The RT was measured trial by trial from the EMG activity of the TA ipsilateral to the magnetic stimulus, taking into account the nearly constant time difference between the two sides. The MEPs became far greater than control MEPs during the RT (mean = 332%, SD = 44 %, of control MEPs, P < 0.001) without any measurable change in the background level of EMG activity. The onset of this facilitation occurred on average 12.80 ms (SD = 7.55 ms) before the RT. There was no difference in the onset of facilitation between the two conditions. Because MEPs were facilitated without a change in the background EMG activity, it is concluded that this facilitation is specifically due to an increase of MCx excitability just before voluntary muscle activation. This conclusion is further reinforced by the observation that MEPs evoked by near-threshold anodal stimuli to the MCx were not facilitated during the RT, in contrast to those evoked by near-threshold transcranial magnetic stimulation. However, several observations in the present and previous studies indicate that MEP amplitude may be more sensitive to alpha-motoneuron activity than to motor cortical neuron activity, an idea that has important methodological implications.
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Published date: 2004
Keywords:
magnetic stimulation, motor cortex, cortical excitability
Identifiers
Local EPrints ID: 27709
URI: http://eprints.soton.ac.uk/id/eprint/27709
ISSN: 0301-5548
PURE UUID: 114a2ca9-523f-48c9-a51d-7d15d5e36919
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Date deposited: 27 Apr 2006
Last modified: 15 Mar 2024 07:20
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Contributors
Author:
Cyril Schneider
Author:
Brigitte A. Lavoie
Author:
Hugues Barbeau
Author:
Charles Capaday
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