Planning and preparing expected and unexpected movements: reexamining the relationships of arm, direction, and extent of movement.

Using variations of the movement precue method, this study sought to define the operational characteristics of motoric decisions that govern the planning and preparation of arm, direction, and extent of movement. Experiment 1 examined how these parameters are programmed when the precue method does not confound motoric and nonmotoric decision processes. Experiment 2 examined how an already planned and prepared response is modified (reprogrammed) when an unexpected response must be executed in its place. The collective results of both experiments demonstrated that (a) these parameters were planned and prepared in a specific order; knowledge about direction was necessary for the programming of arm or extent; (b) arm and extent were reprogrammed independently from direction but changes in direction caused all parameters to be reprogrammed, and (c) programming and reprogramming processes operated in a parallel mode when two or more parameters were involved. The results also showed that these parameters were organized within a hierarchical structure. The present findings were discussed in relation to those reported in previous precue studies and existing models of response programming.