Fitts' Theorem and Movement Time Dissociation for Amplitude and Width Manipulations: Replying to Hoffmann

ABSTRACT

The commentary by Errol Hoffmann in this issue asserts that previous work by our group provides the spurious conclusion that amplitude and width manipulations to a movement environment elicit dissociable relations between movement time (MT) and P. M. Fitts' (1954) index of difficulty (ID). Hoffmann concludes that any such dissociation is the result of actions evoked entirely as ballistic. In this reply, we demonstrate that Hoffmann's commentary is a clear misrepresentation of the study goals and conclusions stated by our group. Additionally, we provide kinematic evidence that actions involving online trajectory amendments are associated with dissociable MT–ID relations for amplitude versus width manipulations. Finally, we contend that the kinematic analyses of movement trajectories, and Hoffmann's failure to acknowledge its importance, is an important step in further understanding speed–accuracy relations in human movement.

Keywords:

• Fitts' theorem
• reaching
• saccade
• speed–accuracy