Visual Judgement of Collisions and Near Collisions: The Sorry State of Tau Theory

Heiko Hecht

Massachusetts Institute of Technology,
Man-Vehicle Lab

Abstract:
I will present a critical assessment of an elegant theory and prime example of ecological optics, namely tau theory. The theory claims that time-to-contact (TTC) is optically specified by the relative change over time of local expansion, information to which observers are highly sensitive (Lee, 1976).  Likewise, time-to-passage (TTP) can be optically specified by a global flow field even in the absence of local expansion or size cues. Kaiser and Mowafy (1993) demonstrated that observers are in fact sensitive to this global flow information.  Typically, local and global variables have been studied in isolation and under a set of constancy assumptions (e.g. constant approach velocity of the object). In realistic situations, however, both local and global cues are usually present, but constancies are often violated.  We investigated performance in situations where local but not global expansion cues were disturbed, or where the constant velocity assumption was violated.  TTP/TTC judgments had to be made based on expanding or non-expanding targets.  In some cases, the targets underwent local rotation, such that global information remained intact but local information was disturbed. Observers were not able to ignore local disturbance, even when global information adequately specified TTP.  Also, observers did not appear to utilize acceleration information. Neither were they able to update TTC estimates when the stimulus was  intermittently occluded.  Different remedies for the ailing tau theory will be discussed, ranging from ad hoc patches to thorough demolition.

The lecture will take place :

in the Lecture Hall, Room 203, 44 Cummington St.
on Wednesday, February 21, 2001
at 4:00 pm