Behavioral and Brain Sciences

Open Peer Commentary
Plamondon & Alimi: Speed/accuracy trade-offs in target-directed movements

Separating A and W effects: Pointing to targets on computer displays

Christine L. MacKenzie a1 and Evan D. Graham a1
a1 School of Kinesiology, Simon Fraser University, Burnaby, B.C., Canada, V5A 1S6


We address two main issues: the distinction between time-constrained and spatially constrained tasks, and the separable A and W effects on movement time (MT) in spatially- constrained tasks. We consider MT and 3-D kinematic data from human adults pointing to targets in human-computer interaction. These are better fit by Welford's (1968) two- part model, than Fitts' (1954; Fitts & Peterson 1964) ID model. We identify theoretical and practical implications.