The shape of things to come: effects of object shape and rotation on the pickup of local Tau

Tresilian (1991) identified two mathematical forms of Local Tau as candidates for perceiving time to-contact information. Local Tau 1 (LT1) only requires the physical distance between two points on the approaching object to remain constant in relation to each other over time. Local Tau 2 (LT2) requires the complete outline of the object to remain constant over time. Subjects in this study were required to intercept a spherical or an oval ball (rotating or nonrotating). Movement initiation time was expected to remain constant for both types of ball if LT1 solely provided time to contact. If LT2 was being used, an oval ball rotating around its short axis should perturb the dilation of the image and result in a later movement initiation time. Results showed that movement initiation time was significantly later when the ball rotated. This finding questions the use of LT1 as the sole source of time-to-contact information and provides some support from a real world task that LT2 may be used in interceptive timing.