Muscular or directional preferences in finger movement as a function of handedness.

Right handed, left handed, and "ambiguous" male and female subjects performed circular index finger movements. Analysis of preferred direction of movements showed that strong right handers tend to move both left and right index fingers in the same direction, and familial left handers tend to move them in opposite directions. Since right handers tend to possess a strongly dominant left hemisphere, while familial left handers exhibit a high degree of cerebral lateral equipotentiality, the result indicates that interhemispheric interference in a motor skill consists of activation of inappropriate muscles of the non-preferred hand by the dominant ipsilateral hemisphere as its attempts to force that hand to conform to the direction of movement preferred by the dominant hand.