Speed overestimation in launching and triggering events.

In launching and triggering events [Michotte, 1946/1963 The Perception of Causality (London: Methuen)] two objects moving sequentially at speeds V1 and V2 are spontaneously described as causally related. We previously showed (Parovel and Casco Vision Research under revision) that the speed of the second moving object is largely overestimated when the delay between the two movements is short (40 ms), not when it is long (1040 ms). The ratio between the speed over- estimation of the second object (DV) and the speed of the first (V1 ) is constant in launching, and decreases as V1 increases in triggering. This dissociation indicates that overestimation follows Weber's law in launching but not in triggering. We ran two experiments in order to understand if speed overestimation is tied to causal perception, or there is overestimation in events without delay that are not causal. In experiment 1 we changed the relative directions of the trajectories, maintaining the delay at 40 ms. Subjects reported no causality and psychometric functions showed no overestimation. Experiment 2, instead, still showed overestimation when the two trajectories are in opposite directions (inverse launching). We conclude that overestimation is specific to percep- tion of causality and cannot be generalised to other consecutive speeds not involving causality.