When identical visual targets move directly toward and then past one another, they appear either to stream past one another or to bounce off each other. Bertenthal et al (1993 Perception 22 193-207) accounted for the relative strengths of these two percepts by invoking a directional bias, arising from cooperative interactions within a network of motion detectors. We tested this explanation by devising conditions that would enhance or diminish the strength of such a directional bias. In separate experiments we varied (i) the presence or absence of temporal transients (pausing, disappearance, occlusion); (ii) the distances travelled by the targets; and (iii) their acceleration or deceleration before and after collision. The tendency to see the objects stream past one another was not related to the strength of an hypothesized directional bias, suggesting that the perception of this ambiguous motion display was not mediated by directional recruitment. Instead, the results suggest that perceived direction reflects the operation of neural constraints that mirror the constraints operating upon moving objects in the three-dimensional natural world.
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