Optical information about the severity of upcoming contacts.

The time derivative (tau) of the inverse of the relative rate of optical expansion (tau) may have critical values with potential implications for controlling activity. The present research addresses the particular hypothesis that tau < -0.5 specifies "unsafe" collision courses and tau > or = -0.5 specifies "safe" collision courses. Optical expansion patterns were simulated on a computer with -1.0 < or = tau < 0 and judged as suggesting a "hard" or "soft" collision. tau < -0.5 led to significantly different decisions from tau > or = -0.5, but the critical value of -0.5 was not perceived reliably as soft, a deviation possibly due to discretely approximating continuous functions. Additional experiments evaluated terminal rates of change and display duration and examined the effects of biasing the presented displays toward the soft or the hard end of the tau continuum. The results were consistent with the tau hypothesis.