Fig. 1. Illustration of the generation and properties of the visual looming stimulus. (A) An LCD projector projects a black disk that grows in size with time on to a translucent white screen. This produces a growing image on the retina, and its size can be quantified as the view angle =2xtan^-1 (d/2xs), where d is the diameter of the projected disk on the screen and s is the distance from the screen to the eye. (B) Two looming stimuli 1 and 2, plotted as vs time. Both have the same final view angle, but stimulus 2 grows more slowly than 1.