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Related Experiment Video

Updated: Jun 22, 2026

Generating Strictly Controlled Stimuli for Figure Recognition Experiments

Generating Strictly Controlled Stimuli for Figure Recognition Experiments

Published on: March 18, 2019

The triangle-bisection illusion.

Stuart Anstis¹, Richard Gregory, Priscilla Heard

¹Department of Psychology, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0109, USA. sanstis@ucsd.edu

|June 3, 2009

Summary

This summary is machine-generated.

The triangle-bisection illusion makes a dot appear higher than its actual midpoint. This visual perception may be due to processing the triangle's center of area, not its height.

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Area of Science:

Visual perception
Geometric illusions
Psychophysics

Background:

The triangle-bisection illusion demonstrates a discrepancy between a dot's actual position and its perceived location within a triangle.
Previous research has explored geometric illusions and their impact on visual perception.

Purpose of the Study:

To investigate the triangle-bisection illusion and its underlying perceptual mechanisms.
To determine if observers respond to the triangle's geometric center (center of area/gravity) or its height.

Main Methods:

Presenting participants with equilateral and concave triangles with a dot positioned at the half-height.
Testing the illusion with different triangle types, including those defined by stereo depth and equiluminous texture.
Analyzing observer responses to gauge the perceived location of the dot relative to the triangle's height.

Main Results:

The illusion, where the dot appears significantly higher than halfway, was consistently observed.
The illusion was strongest for equilateral triangles and even more pronounced for concave triangles.
The effect was present for triangles defined by stereo depth and equiluminous texture.

Conclusions:

The findings suggest that visual perception of the dot's position is influenced by the triangle's overall shape and area.
Observers likely rely on the triangle's center of area or center of gravity rather than its linear half-height for spatial judgment.
This research contributes to understanding how the brain processes geometric information and spatial relationships in visual stimuli.