The influence of optic expansion rates when judging the relative time to contact of familiar objects.

Previous research has shown that size-dependent errors in time-to-contact (TTC) judgments can be attenuated when approaching objects are familiar to the observer and have a known size. We describe two experiments that show that the effect of size on relative TTC judgments can be modeled on observers' reliance on the instantaneous optic expansion rates of the approaching objects. This reliance on optic expansion rates occurred independently of object familiarity and when the actual TTC of the approaching objects was relatively brief or relatively long. However, observers' sensitivity to differences in TTC was improved for familiar objects when TTC was large. These results are consistent with other research showing that optic expansion rate is a critical variable for judging TTC.