The 'shrinking circle illusion' is a shrinking ellipse.

Area of Science:

• Visual perception
• Human psychophysics
• Motion perception

Background:

• The 'shrinking circle illusion' describes perceptual distortions of circular paths.
• Previous studies often overlooked the vertical component of perceived motion paths.

Purpose of the Study:

• To investigate the perceived size of horizontal and vertical extents of a rotating target's path.
• To compare perceptual distortions during smooth pursuit eye movements versus fixation.

Main Methods:

• Twenty-four subjects observed a circularly rotating target at varying velocities (0.18–1.13 cycles s-1).
• Judgments of path extents were recorded during smooth pursuit eye movements and central fixation.

Main Results:

• Underestimation of the target path increased with velocity.
• Perceptual underestimation was greater for the horizontal than the vertical component.
• These velocity-dependent distortions were more pronounced during smooth pursuit than fixation.

Conclusions:

• The perceived path of a rotating target is elliptical, not circular.
• The eccentricity of this perceived ellipse increases with target velocity, particularly during smooth pursuit.