Delboeuf Illusion
What to see
On the above there are two blue disks, and two black circles. One circle is a little
larger than the blue disks, the other much larger. The black circles exchange place
every few seconds, and you may notice that the blue disks seem to change in size.
This effect is moderately strong but fully illusory.
What to do
With the slider on the very right you can adjust the relative sizes of the blue
circles. As soon as you move the slider, the automatic circle interchange stops
and you can adjust at your leisure. If they look sufficiently equal, tap “ex”
(for exchange) and the circles will switch places. In all likelihood, your careful
adjustment will now look totally out of place. The number to the right of the “ex” button gives (in percent) the current size difference
of the blue disks.
Comments
As Coren et al. (1976) put it “(This) is some sort of cognitive contrast effect,
where in general the apparent difference in the size of the central test element
and adjacent or surrounding elements is accentuated.” Helmholtz referred to it as
“size contrast.” These illusory effects depend upon cognitive information processing
strategies. Into the same class belongs the Ebbinghaus-Titchener illusion (as Coren
et al. show with a factor analytical approach), but not size illusions which relate
to depth perception / perspective. For me this illusion is rather fleeting: Its
strength varies, sometimes it does not seem to be present at all. As such these
cognitive size illusions are less “cognitively impenetrable” than the perspective-related
size illusions.
If I find this illusion rather “mild” and open to cognitive modulation (and did
not even read the original French publication), why did I go to the effort to include
it in my collection? Because there is this titillating paper by Ittersum & Wansing
(2012), which to me rather convincingly demonstrates that plate size influences
the amount we eat – even nutritional experts fall for it. Plate size has increased
over the last 100 years by ≈15%, and this may in part explain why most of us (well,
me at least) eat too much.
So, in a cafeteria, choose a smaller plate!
I composed the neighbouring food example with exactly identical burgers on both
plates – I can't believe it myself. Why is this stronger than the animation above?
Sources
Delboeuf, Franz Joseph (1865) Note sur certaines illusions d’optique: Essai d’une
théorie psychophysique de la manière dont l’oeil apprécie les distances et les angles
[Note on certain optical illusions: Essay on a psychophysical theory con cerning
the way in which the eye evaluates distances and angles]. Bulletins de l’Académie
Royale des Sciences, Lettres et Beaux-arts de Belgique, 19, 2nd ser. 195–216
Coren S, Girgus JS, Ehrlichman H, Hakstian AR (1976). An empirical taxonomy of visual
illusions. Perception and Psychophysics, 20, 129-137 [PDF]
van Itersum
K, Wansing B (2012) Plate Size and Color Suggestibility: The Delboeuf Illusion’s
bias on serving and eating behavior. J Consumer Res 39:(print version appears in
August 2012) [PDF]
Created: 2012-05-01
Last update: 2013-10-04