On the Accuracy of Movement in Space-Time

Abstract

In this commentary, we consider several of the issues addressed by Schmidt (1994) in his critique of our accompanying experimental article (K. M. New- ell, L. G. Carlton, & Kim, 1994) that provides data supporting a space-time approach to the movement speed-accuracy relation (Hancock & K. M. New- ell, 1985). Our perspective is that the impulse variability model (Schmidt, Ze- laznik, Hawkins, Frank, & Quinn, 1979) provided the essential insight into a potential link between force variability and movement outcome variability, but this model did not provide a space-time approach to movement error. In addition, the predictions of the impulse variability model regarding the form of the movement error function relations are not general. A key consideration in a space-time framework to movement accuracy is the complementarity of the frames of reference used to measure errors of time and space. In a unified space-time frame of reference, one cannot trade movement speed for accu- racy, only spatial error for temporal error.