Abstract: Background: Disruption of a cortical region can paradoxically
improve behavior. After unilateral damage to the primary motor cortex (M1),
increased excitability of the unaffected M1 has been shown. The M1 plays a
critical role in motor performance and also early aspects of motor skill
learning. Repetitive transcranial magnetic stimulation (rTMS) of one motor
cortex can lead a temporary reduction in cortical excitability. We hypothesize
that unilateral suppression of one M1 by rTMS may increase excitability of the
unaffected motor cortex and thus improve motor performance and motor skill
learning with the ipsilateral hand by releasing the contralateral motor cortex
from transcallosal inhibition. Methods: Forty healthy volunteers
participated in our study; 16 for the experiment I and 24 for the experiment
II. In the experiment I, after practicing a sequential simple key-pressing task
with the index finger, their motor performance was monitored before and after
slow-frequency (1Hz) rTMS, applied on the M1 ipsilateral or contralateral to
the hand, ipsilateral premotor area or vertex (Cz). In the experiment II,
participants were randomly divided into three stimulation groups: i)
ipsilateral M1; ii) contralateral M1; and iii) Cz. rTMS was
applied before the initiation of practice and learning of a simple motor skill.
Mean execution time and error rate were recorded in 4 sessions distributed over
2 days. Results: In experiment I: rTMS of M1 shortened execution time of
the motor task with the ipsilateral hand, without affecting performance with
the contralateral hand. This effect outlasted rTMS by at least 10 min, and was
most prominent for M1 stimulation. In experiment II, disruption of M1 with rTMS
slowed down skill acquisition with the contralateral hand, but paradoxically
accelerated learning with the ipsilateral hand. This effect was evident during
the first of 2 days of practice in the group with rTMS over the ipsilateral M1
compared to the other two groups (Cz and contralateral M1). Conclusions: Our results support the notion of an interhemispheric competition, and
demonstrate the utility of rTMS to explore the functional facilitation of the
un-stimulated counterpart M1 with effects on motor execution and learning,
which may have implications for neurorehabilitation.
Keywords: Ipsilateral motor area, transcranial magnetic stimulation, motor learning, interhemispheric inhibition
