p296fig08.07 When two flashes turn on and off out of phase with the correct range of interstimulus intervals, and not too far from one another, then either beta motion of phi motion are perceived.<BR>||  Beta and Phi motion percepts.  Beta motion: percepts of continuous motion of a well-defined object across empty intervening space.  Phi motion: sense of "pure" motion without a concurrent percept of moving object. (Exner 1875) http://www.yorku.ca/eye/balls.htm