Sensitivity to the acceleration of looming stimuli.

The aim of this study was to determine if observers could discriminate between looming stimuli simulating targets approaching the observers at either constant or non-constant speeds. Discrimination between accelerating and constant-speed approaches improved after 70-90 trials for accelerations >2 m/s2. For lower accelerations the ability to discriminate was poor regardless of the trial number. Following the learning phase, observers were able to identify accelerating targets from constant-speed approaches fairly consistently at performance levels of 70-75% for accelerations as low as 4 m/s2 and at 80-96% for accelerations of 6-14 m/s2. Observers' accuracy in identifying decelerating from constant-speed targets did not increase as a function of increasing deceleration. In fact, observers had a slight bias to select the constant-speed stimulus as being the decelerating stimulus. In summary, the sensitivity to acceleration for simulated motion in depth is poor, but increases as acceleration increases and sensitivity to acceleration is far greater than for deceleration.