Relative Motion and the Adjacency Principle

The perception of motion of physically moving points of light was investigated in terms of the distinction between absolute and relative motion cues and the change in the effectiveness of the latter as a function of the frontoparallel separation between the points. In situations in which two competing relative motion cues were available to determine the perceived path of motion of a point of light, it was found that the relative motion cue between more adjacent points was more effective than the relative motion cue between more separated points. In situations in which only one relative motion cue was available to determine the perceived motion of a point it was found that the effectiveness of this cue as compared with the absolute motion cue decreased with increased separation. These results are predictable from the adjacency principle which states that the effectiveness of cues between objects is an inverse function of object separation. Some consequences of the study for the theory of motion perception are discussed.