Generalized motor programs for rapid bimanual tasks: a two-level multiplicative-rate model.

A model is proposed for the timing of rapid bimanual movements. It combines (a) the notion of a generalized motor program (GMP) with invariant relative timing, (b) the two-level concept of timing control with a central level of control and a peripheral level where the observations are made, and (c) the hypothesis that a single GMP simultaneously controls both limbs. Our method is based on the analysis of temporal intervals measured among landmarks taken from the bimanual kinematic traces. We show that sets of tetrad ratios--each composed of two pairs of covariances among four temporal intervals in the actions--should be equal to 1.0 if the hypothesis is correct. In addition, we show that these tetrad ratios should deviate systematically from 1.0 under certain, biologically realizable violations of the model. Data from human subjects show that the results generally conform to the basic model. Simulations are used to illustrate other violations of the model and to explore characteristics of the sampling distribution of the tetrad ratios under the model.