The Bicycle Illusion: A new look at acuity, form, and motion interactions in conscious experience

Abstract

A bicycle rolling on a level surface alongside a fence of horizontally sagging chains appears to move up and down with the chains at certain viewing distances. In this bicycle illusion the relative vertical position of the bike follows the path of the chains. The illusion is different from motion capture because here a static form (fence) influences the perception of a moving form (bicycle), rather than the other way around. It is also distinguished from motion contrast because the moving bike follows the inducing chains rather than repelling from them. We took this sidewalk visual science into the lab by moving a disc horizontally on the screen and on top of two sinusoidal wavy lines. Our discoveries include: 1. The direction of the illusion is acuity dependent: at far viewing distances the disc appears to wiggle vertically in phase with the wavy lines (assimilation illusion) whereas at nearer viewing distances it appears to wiggle in counterphase to the lines (contrast illusion). 2. The assimilation illusion depends on the relative acuity of the static lines and the moving disc: the illusion is strengthened when the luminance contrast of the disc is reduced relative to the luminance contrast of the lines. 3. The contrast illusion is less dependent on the relative contrast of the disc and the lines. The bicycle illusion is therefore a novel preparation for studying the interactions between acuity, form and motion in conscious visual perception.