Temporal integration of speed in perceived causality

Causal relation between two movements is evident in the launching paradigm [Michotte, 1946/1963 The Perception of Causality (London: Methuen)], in which the first moving object (S1) appears to cause the motion of the second object (S2). The causal relation requires a short delay between S1 and S2 (40 ms, launching) and fades when it is long (1040 ms, non-launching). We recently showed that in the launching condition the speed of S2 is overestimated by 34% with respect to the non-launching condition (Parovel and Casco Vision Research submitted). Here, we demon- strated the general properties of the spatio-temporal integration mechanism underlying speed overestimation in the causality phenomenon. We manipulated the trajectory-to-trajectory alignment [4 experiments], the spatio-temporal coincidence between S1 and S2 [3 experiments], the duration of the whole event [1 experiment], and the speed ratio [2 experiments]. A two-interval forced-choice task was used to measure the point of subjective equality between S2 speeds in launching versus non-launching conditions. Data support an integrative mechanism with different properties than motion averaging, motion trajectory integration, and sequential recruitment; indeed, S2 speed overestimation also occurs when S1 is slower than S2, independently from the trajectory-to- trajectory alignment or the spatial coincidence. The mechanism underlying perceived causality specifically relies on two temporal factors: it requires a short interval between S1 and S2 movements, and also it increases with their short durations. Moreover, we found that S2 speed overestimation in launching determines a displacement of psychometric functions rather than a change in slope, demonstrating a perceptual rather than a decisional effect, in agreement with Michotte's interpretation that the relationship of causality in launching events is directly perceived, without the mediation of high-level processes.