[Müller-Lyer illusion and colour contrast]

Area of Science:

• Visual Perception
• Psychophysics
• Color Vision

Background:

• The Müller-Lyer illusion is a well-known optical illusion where two lines of equal length appear different due to the direction of arrow-like fins.
• Previous research has explored various factors influencing the strength of this illusion, including line length, angle, and luminance contrast.

Purpose of the Study:

• To investigate the effect of luminance contrast on the strength of the Müller-Lyer illusion.
• To determine if the illusion's magnitude changes at isoluminance, a state where perceived brightness is equal across different colors.

Main Methods:

• Psychophysical experiments were conducted where participants adjusted a test line segment to match a reference line segment within the Müller-Lyer figure.
• The illusion was measured across a range of wing lengths (7-35 min arc) and internal angles (40-180 degrees).
• Stimuli were generated using a VSG2/3 system on a gamma-corrected monitor, with chromatic contrast provided by CRT primary colors. Isoluminance was determined using the two-color gratings method.

Main Results:

• The strength of the Müller-Lyer illusion significantly increased, by 1.5 to 2 times, at isoluminance.
• This enhancement of the illusion at isoluminance was observed irrespective of the specific color pairs used.
• The magnitude of the illusion varied systematically with the length and internal angle of the figure's wings.

Conclusions:

• Isoluminance conditions significantly amplify the Müller-Lyer illusion, indicating a strong influence of chromatic processing on this visual phenomenon.
• The findings support interpretations based on spatial-frequency filtering, suggesting the involvement of low-pass chromatic filters in visual perception.
• Chromatic contrast plays a critical role in modulating the perceived length of lines in the Müller-Lyer figure.