TIME-TO-PASSAGE JUDGMENTS IN NONCONSTANT OPTICAL FLOW FIELDS

The time until an approaching object will pass an observer (time to pa ssage, or TTP) is optically specified by a global flow field even in t he absence of local expansion or size cues. Kaiser and Mowafy (1993) h ave demonstrated that observers are in fact sensitive to this global f low information. The present studies investigate two factors that are usually ignored in work related to TTP: (1) non-constant motion functi ons and (2) concomitant eye rotation. Non-constant velocities violate an assumption of some TTP derivations, and eye rotations may complicat e heading extraction. Such factors have practical significance, for ex ample, in the case of a pilot accelerating an aircraft or executing a roll. In our studies, a flow field of constant-sized stars was present ed monocularly on a large screen. TTP judgments had to be made on the basis of one target star. The flow field varied in its acceleration pa ttern and its roll component. Observers did not appear to utilize acce leration information. In particular, TTPs with decelerating motion wer e consistently underestimated. TTP judgments were fairly robust with r espect to roll, even when roll axis and track vector were decoupled. H owever, substantial decoupling between heading and track vector led to a decrement in performance, in both the presence and the absence of r oll.