Human on-line response to target expansion

McGuffin and Balakrishnan (M&B) have recently reported evidence that target expansion during a reaching movement reduces pointing time even if the expansion occurs as late as in the last 10% of the distance to be covered by the cursor. While M&B massed their static and expanding targets in separate blocks of trials, thus making expansion predictable for participants, we replicated their experiment with one new condition in which the target could unpredictably expand, shrink, or stay unchanged. Our results show that target expansion occurring as late as in M&B's experiment enhances pointing performance in the absence of expectation. We discuss these findings in terms of the basic human processes that underlie target-acquisition movements, and we address the implications for user interface design by introducing a revised design for the Mac OS X Dock.