Handedness: coordination of within- and between-hand alternating movements

Two models of coordination of within- and between-hand alternating movements are compared. Model 1 assumes that control of bimanual movement passes back and forth between two independent systems, each of which guides one hand, and that the performance of alternating tapping movements (e.g., right index, right middle fingers) within a hand is very different from performance of alternating movements between hands (right index, left index fingers). The model suggests that right/left performance asymmetries on given tasks reflect the characteristics of the independent systems and would therefore predict that factor analysis would yield a "hand" factor. Model 2 assumes that a single system initiates and terminates movement in two subsystems, each of which guides one hand, and it suggests that the task of guiding alternating tapping movements within a hand is very similar to the task of alternating movement between hands. Model 2 would not predict an independent "hand" factor in the factor analysis of performances of tasks yielding left/right asymmetries; the asymmetries presumably arise because the single higher order system has better access to one of the subsystems than to the other. Analyses of performance of within- and between-hand alternations, single-finger tapping movements, and a pursuit rotor task in 92 subjects lend clear support to Model 2.