Human handedness: is there a difference in the independence of the digits on the preferred and non-preferred hands?

In non-human primates, comparative studies show that greater dexterity is associated with greater independent control over the digits. Studies of humans, however, show no difference in the degree of independent control of single digits on the more dexterous preferred hand and its less dexterous partner. We wondered whether there might be a difference in the degree of independent control on the two hands during performance of functionally relevant tasks. Many object manipulation tasks require the ability to produce and control forces with more than one finger at the same time. We hypothesized that asymmetrical independence, with greater independence on the preferred than the non-preferred hand, would be evident with a task that requires the simultaneous production of force in two digits. We examined digit individuation when subjects produced flexion forces with a single digit in isolation, and simultaneous flexion forces in all ten combinations of two digits. Consistent with previous studies, we found no difference between the individuation of the digits on the preferred and non-preferred hands during force production with single digits in isolation. Similarly, no asymmetry was present when forces were produced by two digits. However, separation of two-digit forces into thumb-to-digit opposition forces and non-opposition forces showed that although there was no difference in individuation between the two hands for non-opposition forces, the digits on the preferred hand were more independent than those on the non-preferred hand for opposition forces. We suggest that this independence asymmetry is not itself the underlying cause of the dexterity differences between the hands, but rather arises from a difference in the capacity for use-dependent reorganization of the motor cortical circuits controlling each hand, and that this difference might underlie the dexterity differences between the hands.