Ambiguous apparent motion in exchanging disks

TL;DR

This study explores how people perceive the motion of two disks exchanging positions, showing that perception changes based on the size ratio of the disks.

Contribution

The paper reveals a new perceptual phenomenon where disk size ratio alters perceived motion from jumping to expanding/contracting.

Findings

When the small disk is less than 80% the size of the large one, observers see a single disk jumping.

When the small disk is over 80% the size, observers see two disks expanding and contracting.

Perceived centroid movement is strongest when the small disk is less than 80% the size of the large one.

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.