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 [email protected] [email protected]


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.