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Abstract

To hit moving targets, one not only has to arrive at the right place but also at the right time. Moving quickly decreases the spatial precision (Fitts law) but increases temporal precision (e.g. Newell et al., 1979). The latter may explain why people generally move quicker towards fast targets (speed coupling), as arriving at the right time is more important when hitting fast targets than when hitting slow ones. The timing demand depends not only on the target's speed but also on the target's elongation in the direction of motion: the timing demand decreases with increasing elongation. Here we compare manipulations of the timing demands caused by varying speed and elongation. We asked subjects to hit targets that moved from the left to the right as quickly as possible with their index finger. The targets varied in elongation in the direction of motion (influencing both spatial and temporal demands), in elongation in the orthogonal direction (influencing spatial demand) and in speed (influencing temporal demand). In a first experiment, the targets were presented in random order. Subjects hit fast targets faster than slow ones. Their movement time was influenced by the target's size (the spatial demand), but not by the direction of the elongation. In a second experiment, we presented blocks of trials with identical targets to allow subjects to optimize their strategy for each target separately. In this case the subjects did also consider the direction of the elongation. We conclude that people do not use the object's shape to estimate the timing demands of an interception task, but that they use the perceived speed and their experience on previous trials.