Motion perception and spatial orientation.

Abstract

Traditionally, visually perceived motion is attributed to three cues, acting singly or in combination. (See Goldstein, 1996, chap. 7, for a good review of traditional motion perception theory and research.) First, the visual system monitors the positional changes of a moving image across a stationary retina and attributes this systematic displacement to movement of the stimulus. Second, pursuit eye movements, and in particular the command to move the eyes, provide a cue for perceived motion. Thus, a bird in flight that is fixated by the observer will appear to move because the brain monitors the command to move the eyes.Retinal displacement and pursuit eye movement cues are usually considered together. The basic idea is that the brain interprets retinal displacement cues differently depending on whether or not the eyes have been directly commanded to move. Thus, the bird in flight described previously, which is relatively stationary on the observer's retina, is nevertheless seen to move because the image's stability on the retina is attributed to tracking eye movements. Conversely, the truly stationary environment does not seem to move when we move our eyes because the brain attributes the displacement of the environment across the retina to the eye movements.Third, another cue for motion perception involves not the observer, as do the first two cues, but rather the stimulus array. In particular, the dynamic stimulus relationship cue accounts for motion perception in terms of the changing spatial relationships among stimuli. A stationary observer with stationary eyes sees a running dog as moving, for example, because the dog changes its relationship systematically with respect to other stimuli in the field of view. The importance of the dynamic stimulus relationship cue for motion perception is well established and indeed some (Gibson and other ecological psychologists; see Blake, 1994) argue that it is the primary cue.Many phenomena of motion perception can be explained in terms of these three cues. Space limitations permit only a few examples. (a) As would be expected, thresholds for perceiving motion are lower when all 46