Having walked around it, the formless invariants that underlie the changing perspectives can be picked up. The Extending of Perception by Locomotion . Head-turning allows the individual to perceive the environment behind his head. Head-displacements or short locomotions allow the individual to perceive the solidity and superposition of objects and the surfaces behind the occluding edges. Locomotion extends perception.
Long locomotions extend it farther out than short locomotions.
Note that during locomotion, perception is both prospective and retrospective . The observer has foresight of what is to come and hindsight of what is past. He perceives over time. Note that the perception of what is ahead and of what is behind are continuously connected with the perception of what is here during locomotion. The perceiving of events in the world, i.e., the motions of objects, is extended in time, of course, whether or not the observer himself moves.