Abstract
The perceptibility of relative motion in depth (sagittal motion) of a stimulus light is studied as a function of the angular subtense of the light at onset, the exposure duration and the direction of movement (direct approach or recession). The relevant perceptual cue is a change in either the apparent size or brightness of the light, accompanying a change in viewing distance. Velocity thresholds are determined by means of a signal detection method. It is concluded that size-brightness cues are relatively ineffective in the detection of relative motion.