Combination of image expansion and known size explains time-to-passage of biological motion

Abstract

It is important in everyday tasks to estimate the time it takes an object to reach an observer, time-to-contact (TTC), or when it will pass us, time-to-passage (TTP). However, we do not solely interact with inanimate objects but also with people. It is known that the visual system does not integrate biological motion (BM) linearly over space and time with constant efficiency (Neri Morrone and Burr, 1998 Nature 395 894-896), but little is known about judging TTC/TTP of BM. In a TTP task we presented expanding point-light walker (PLW) animated with either biological or rigid motion (RM). Subjects had to decide whether the PLW passed them before or after a reference time (1 s) signaled by a tone. Due to its higher complexity, BM conveyed a poor expansion pattern with more noise. Subjects judged that BM passed sooner than RM. Even so BM allows judging TTP well enough in so far as we did not find significant differences on precision and reaction time between BM and RM. Moreover, we show that observer knowledge of walker size combined with image expansion influences their interceptive judgments, as it happens for rigid objects [López-Moliner Field and Wann, 2007 Journal of Vision 7(13):11, 1-8].