A thickness illusion: Horizontal is perceived as thicker than vertical

Jasper M. de Waard*, Erik Van der Burg, Christian N.L. Olivers

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review


We report two psychophysical experiments that investigate a visual illusion that is considered common knowledge among type designers, but has never been studied scientifically. Specifically, the thickness of a horizontal line is overestimated in relation to that of a vertical line. Experiment 1 confirmed the existence of the illusion. In Experiment 2, we replicated the effect and showed that the illusion is closely related to the vertical-horizontal illusion, in which the length of a vertical line is overestimated in comparison to a horizontal one. Both the overestimation of thickness and length is larger when the stimulus is surrounded by a horizontally elongated frame, as opposed to a vertically elongated frame. We discuss potential explanations for the thickness illusion and its relation to the vertical-horizontal illusion.

Original languageEnglish
Article number1
Pages (from-to)1-11
Number of pages11
JournalVision (Switzerland)
Issue number1
Early online date4 Jan 2019
Publication statusPublished - Mar 2019


  • Illusion
  • Orientation
  • Perception
  • Vision


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