from Michael’s Visual Phenomena & Optical Illusions


What to do

The vertical part of the inverted ‘T’ (right) changes its length. Click on it, the movement will stop, and you can now drag the size until you are satisfied that the vertical and horizontal parts are identical in length. Then pressing the “Show result” button, the length in percent is displayed and the veridical length is overlaid in green.


Most explanations don’t convince me particularly (but see the new reference below {M&T 2013} for good background information in addition to nice experiments).

I would group this into “size constancy”. Size constancy is not well developed for vertical distances. An example: When you’re on a tower and look down, people & things look tiny, like toyland (in other words, their distance is underestimated). But the same distance horizontally is well compensated for by size constancy. Now apply these thoughts to the vertical & horizontal parts in the T-illusion.

“Top hat illusion” (Wundt-Fick variant)

Also known as the “Oppel-Kundt illusion”. A nice variant on the bottom right: the thin dashed lines, indicating height & width of the top hat, indeed span identical distance.


Oppel JJ (1855) Über geometrisch-optische Täuschungen. Jahresbericht des Physikalischen Vereins zu Frankfurt 37–47

Mikellidou K, Thompson P (2013) The vertical-horizontal illusion: Assessing the contributions of anisotropy, abutting, and crossing to the misperception of simple line stimuli. JOV 13:8,7

The zero-gravity parabolic flight will test the T-illusion (more details).

Landwehr K (2022) The Prospects of Utilizing Geometrical Visual Illusions as Tools for Neuroscience. Symmetry 14:1687