B. Brouwer et al., Asymmetry of motor cortex excitability during a simple motor task: relationships with handedness and manual performance, EXP BRAIN R, 138(4), 2001, pp. 467-476
Transcranial magnetic stimulation (TMS) was used to assess the relative con
tribution of the corticospinal (CS) pathway in activating the first dorsal
interosseous (FDI) muscle in each hand of 16 right- (RH) and 16 left-handed
(LH) subjects with varied degrees of hand preference. It was hypothesised
that asymmetry in corticospinal activation of the two hands may be related
to hand preference and interlimb differences in manual performance. Subject
s performed isometric index finger abduction at force levels of 0.5 N, 1 N
and 2 N while TMS was applied at resting threshold intensity (T), 0.9T, or
0.8T. The amount of contraction-induced facilitation of the muscle evoked p
otential (MEP) was used as an estimate of corticospinal involvement in the
task. Patterns of MEP facilitation in each hand were compared with measures
of manual performance (finger tapping speed, Purdue pegboard, maximal FDI
strength). Threshold TMS intensities for an MEP in FDI at rest were similar
in LH and RH subjects, and did not vary between hands. Facilitation of the
MEP with voluntary activation was larger overall on the left side (P <0.05
), but the asymmetry was dependent on the degree of lateralisation of hand
preference. For subjects with consistent hand preference (either LH or RH),
MEP facilitation in active FDI was larger for the left hand. For non-consi
stent RH subjects, contraction-induced MEP facilitation was larger in the r
ight FDI muscle than the left. Asymmetry of MEP facilitation was not correl
ated with differences between hands in finger tapping speed or performance
in the pegboard task, but was associated with relative differences in FDI s
trength. MEP facilitation tended to be larger in the stronger FDI muscle of
the pair. We conclude that corticospinal involvement in the command for in
dex finger abduction is generally greater when the left hand is used, altho
ugh in RH subjects the asymmetry is influenced by the degree of lateralisat
ion of hand preference. The corticospinal asymmetry is not related to speed
or dexterity of finger movements, but the association with muscle strength
suggests that it may be influenced in part by preferential use of one hand
for tasks which strengthen the FDI muscle.