Visual control of braking: a test of the tau hypothesis.

Deceleration during braking could be controlled by (a) using the time derivative of the relative rate of optical expansion, relative to a -0.5 margin value of tau-dot (D.N. Lee, 1976) or (b) computing the required deceleration from spatial variables (i.e., perceived distance, velocity, or object size). Participants viewed closed-loop displays of approach to an object and regulated their deceleration with a brake. The object appeared on a checkerboard ground surface (providing velocity, distance, and size information) or with no background (providing only optical expansion). Mean tau-dot during braking was -0.51, and estimates of the critical value of tau-dot based on brake adjustments were -0.44 and -0.52, close to the expected value. There were no overall effects of the ground surface or object size. The results are consistent with a tau-dot strategy, where the direction and magnitude of brake adjustments are regulated using tau-dot.