The Journal of Neurophysiology Vol. 86 No. 3 September 2001, pp. 1195-1201
Copyright ©2001 by the American Physiological Society
Time Course of Determination of Movement Direction in the
Reaction Time Task in Humans
Martin
Sommer,1,2
Joseph
Classen,1,3
Leonardo G.
Cohen,1 and
Mark
Hallett1
1Human Motor Control Section, National
Institute of Neurological Disorders and Stroke, National Institutes of
Health, Bethesda, Maryland 20892-1428;
2Department of Clinical Neurophysiology,
University of Göttingen, 37075 Gottingen; and
3Department of Neurology, University of Rostock,
18147 Rostock, Germany
Sommer, Martin,
Joseph Classen,
Leonardo G. Cohen, and
Mark Hallett.
Time Course of Determination of Movement Direction in the
Reaction Time Task in Humans. J. Neurophysiol. 86: 1195-1201, 2001. The primary motor cortex produces motor
commands that include encoding the direction of movement. Excitability
of the motor cortex in the reaction time (RT) task can be assessed
using transcranial magnetic stimulation (TMS). To elucidate the timing
of the increase in cortical excitability and of the determination of
movement direction before movement onset, we asked six right-handed,
healthy subjects to either abduct or extend their right thumb after a go-signal indicated the appropriate direction. Between the go-signal and movement onset, single TMS pulses were delivered to the
contralateral motor cortex. We recorded the direction of the
TMS-induced thumb movement and the amplitude of motor-evoked potentials
(MEPs) from the abductor pollicis brevis and extensor pollicis brevis
muscles. Facilitation of MEPs from the prime mover, as early as 200 ms before the end of the reaction time, preceded facilitation of MEPs from
the nonprime mover, and both preceded measurable directional change.
Compared with a control condition in which no voluntary movement was
required, the direction of the TMS-induced thumb movement started to
change in the direction of the intended movement as early as 90 ms
before the end of the RT, and maximum changes were seen shortly before
the end of reaction time. Movement acceleration also increased with
maxima shortly before the end of the RT. We conclude that in concentric
movements a change of the movement direction encoded in the primary
motor cortex occurs in the 200 ms prior to movement onset, which is as
early as increased excitability itself can be detected.
