DISCRETE AND CONTINUOUS PLANNING OF HAND MOVEMENTS AND ISOMETRIC FORCE TRAJECTORIES

We have previously demonstrated that, in preparing themselves to aim v oluntary impulses of isometric elbow force to unpredictable targets, s ubjects selected default values for amplitude and direction according the range of targets that they expected. Once a specific target appear ed, subjects specified amplitude and direction through parallel proces ses. Amplitude was specified continuously from an average or central d efault; direction was specified stochastically from one of the target directions. Using the same timed response paradigm, we now report thre e experiments to examine how the time available for processing target information influences trajectory characteristics in two-degree-of-fre edom forces and multijoint movements. We first sought to determine whe ther the specification of force direction could also take the form of a discrete stochastic process in pulses of wrist muscle force, where d irection can vary continuously. With four equiprobable targets (two fo rce amplitudes in each of two directions separated by 22 degrees or 90 degrees), amplitude was specified from a central default value for bo th narrow and wide target separations as a continuous variable. Direct ion, however, remained specified as a discrete variable for wide targe t separations. For narrow target separatons, the directional distribut ion of default responses suggested the presence of both discrete and c entral values. We next examined point-to-point movements in a multijoi nt planar hand movement task with targets at two distances and two dir ections but at five directional separations (from 30 degrees to 150 de grees separation). We found that extent was again specified continuous ly from a central default. Direction was specified discretely from alt ernative default directions when target separation was wide and contin uously from a central default when separation was narrow. The specific ation of both extent and direction evolved over a 200-ms time period b eginning about 100 ms after target presentation, As in elbow force pul ses, extent was specified progressively in both correct and wrong dire ction responses through a progressive improvement in the scaling of ac celeration and velocity peaks to the target. On the other hand, moveme nt time and hand path straightness did not change significantly in the course of specification. Thus, the specification of movement time and linearity, global features of the trajectories, are given priority ov er the specific values of extent and direction. In a third experiment, we varied the distances between unidirectional target pairs and found that movement extent is specified discretely, like direction, when th e disparity in distances is large. The implications of these findings for contextual effects on trajectory planning are discussed. The indep endence of extent and direction specification and the prior setting of response duration and straightness provide critical support for the h ypothesis that point-to-point movements are planned vectorially.