The Bourdon illusion occurs with straight-, right-angle-, and parallel-edge figures.

Area of Science:

• Visual perception
• Geometric illusions
• Psychophysics

Background:

• The Bourdon illusion involves the apparent bending of a straight line within a figure composed of two apex-to-apex elongated triangles.
• Previous research primarily focused on the standard apex-to-apex configuration.

Purpose of the Study:

• To investigate the Bourdon illusion's behavior with different component arrangements (base-to-base, right angles, parallel).
• To determine how variations in component configuration influence the direction and presence of the illusion.

Main Methods:

• Three experiments were conducted using modified geometric figures to elicit the Bourdon illusion.
• Component arrangements were systematically varied (apex-to-apex, base-to-base, right angles, parallel).
• Supplementary observations assessed the impact of component size, separation, and edge curvature.

Main Results:

• The Bourdon illusion occurred in the opposite direction when components were arranged base-to-base instead of apex-to-apex.
• The illusion was observed with components at right angles and parallel arrangements, with directional changes based on configuration.
• Illusion strength was influenced by component size and separation; curved edges eliminated the illusion.

Conclusions:

• The direction and occurrence of the Bourdon illusion are highly sensitive to the geometric arrangement of its constituent components.
• Variations in component configuration offer new ways to study the mechanisms underlying this visual illusion.
• Understanding these geometric influences provides insights into the principles of visual processing and spatial perception.