The coordination of limb movements with different kinematic patterns

The principles underlying the coordination of limb movements with different spatiotemporal features were explored. After an initial training session in which the same unidirectional movement had to be performed with both upper limbs, subjects attempted to coordinate two different movements in a second session, i.e., the learned unidirectional movement in the left limb and a new double reversal movement in the right limb. The findings uncovered a wide variety in patterns of interlimb dependence among and within subjects, going from a high degree of dependence to relative independence. The relationship between limbs was studied by means of a detailed analysis of the displacement and acceleration patterns and the electromyographic activity of the major muscles involved. The general underlying principle that appeared to account for the diversity in movement organization was this: higher independence between limb movements is achieved when subjects initiate the movements to be coordinated successively. This asynchrony in movement onset can possibly be viewed as an attempt to safeguard against interference.     

The adjacency principle and induced movement

The apparent movement of a stationary point of light was investigated as a function of the apparent distance of the point of light with respect to the inducing frame of frames. When two frames were presented simultaneously, they were at different distances and physically moved at opposite phase in frontoparallel planes. When one frame was presented alone, it was positioned at different times at each of the two distances. The point of light was presented stereoscopically at the distance of either the near or far frame or midway in depth between these distances. With the single frame, it was found that the magnitude of the induced movement decreassed as the point of light was increasingly far in front of a frame but decreased less or remained approximately constant for distances be hind a frame. With the two frames presented simultaneously, it was found that as the depth between a particular frame and the point of light decreased, the contribution of that frame to the induced movement increased. The results illustrate the interaction of perceptions, in this case perceived depth and perceived motion, and are consistent with the adjacency principle.