Mirror movements and the role of handedness: isometric grip forces changes.

Co-activation of both hemispheres seems to play a critical role in the generation of mirror movements, involuntary muscle contractions opposite to voluntary movements of one hand. The relationship between handedness und mirror movements is unclear. We measured repetitive grip force changes performed at slow and fast frequencies with the active hand and recorded mirror activity in the opposite inactive hand in 17 healthy left-handed (LH) and 17 right-handed (RH) participants. Data suggest significant differences between RH and LH with respect to the temporo-spatial characteristics of mirror activity, but not in general mirror movement strength. Mirror activity correlation was more pronounced when the non-dominant hand was active in RH, while LH did not show such an asymmetry. We suggest that the dependence of mirror activity on the active hand in right-handers but not in left-handers reflects the more general finding of stronger manual asymmetry in brain activation as well as in functional performance in right-handers.