Perceiving the Severity of Contacts Between Two Objects

The tau-dot hypothesis states that when tau-dot greater than or equal to -0.5, impact will be soft; when tau-dot < -0.5, impact will be "hard." Four experiments tested the usefulness of tau-dot for observers of collisions between 2 objects (rather than collisions between the observer and an object). Observers watched collisions depicted on a computer as 1 object approaching another. Results conformed to the tau-dot hypothesis and were consistent with previous research wherein observers watched collisions as participants. Results suggest that the information specifying collision severity is the rate of the relative rate of optical variation, a quantity that remains invariant over differing collision path trajectories and over differing perceivers (i.e., spectators or participants, viewing from different vantage points, moving or stationary).