Ambiguous apparent motion in exchanging disks

Abstract

A large and a small disk with radii R and r, side by side, exchange positions repetitively at 1.33 Hz. The motion is ambiguous. If r/ R < 0.8 then observers perceive a single large disk jumping back and forth across a small static disk. But if r/ R > 0.8, observers perceive two static disks that expand and contract in counter phase, with no motion across the gap between the disks. Observers respond to the movement of the centroid, which is greatest when r/ R < 0.8.