Parallel direction and extent specification of planar reaching arm movements in humans

The present study was undertaken to clarify further whether the programming of direction and extent of the same motor trajectory is accomplished by parallel or serial processing channels. We reasoned that if direction and distance are independently specified, then the time course of each process should not be influenced by the need for simultaneously specifying the other parameter. If direction and distance are specified in a serial order, the need to specify two parameters rather than only one should prolong the process of response specification. For this purpose experiments were run on the same six subjects, using our time-response paradigm and the data obtained were compared. To avoid any possible effect attributable to the Hick and Hyman law, the number of targets was maintained invariant and equal to four. Our major finding is that the time course of directional specification of reaching movements is not influenced by the need for simultaneously specifying extent. This is consistent with our view of parameterization of planning and executing movements, in which the two parameters (direction, extent) can be specified in parallel.