Hermann Grid
The widely known Hermann-grid illusion (Hermann 1870).
Dark patches appear in the street crossings, except the ones which you are directly
looking at.
Rather weak, but in every textbook…
See below for the classical explanation. HOWEVER: see the
next page for a convincing rejection of this explanation.
[This page is also available in Czech language.]
The classical explanation
1. Why do we see the dark patches?
Look at the left part of the left diagram and assume an on-center retinal ganglion
cell. Its receptive field is indicated by the reddish disk. When the ganglion cell
is, by chance, looking at the grating so that its centre ('+') is positioned at
a crossing (left-top), there are 4 bright patches in the inhibitory surround. A
ganglion cell looking at a street (left-bottom) however only gets 2 inhibitory patches,
so it will have a higher spike rate then the one at the crossings. This was measured
by Baumgartner (1960) in Freiburg, see picture on the right.
2. Why don't we see the patches when we look right at them?
Because then we direct the fovea at the crossings, and in the fovea the receptive
fields are much smaller (see the small reddish disks on the right of the left figure).
With such small receptive fields it obviously does not matter whether they are at
the crossings or not.
3. Why is this explanation, so plausible it sounds –and it is in every textbook–
not the full story?
See the next page for a simple refutation.
Sources
Hermann L (1870) Eine Erscheinung simultanen Contrastes. Pflügers Archiv für die
gesamte Physiologie 3:13–15
Hartline HK, Wagner HG, Ratliff F (1956) Inhibition in the Eye of Limulus. J Gen
Physiol 39:651–673
Baumgartner G (1960) Indirekte Größenbestimmung der rezeptiven Felder
der Retina beim Menschen mittels der Hermannschen Gittertäuschung. Pflügers
Arch ges Physiol 272:21–22
Spillmann L (1994) The Hermann Grid Illusion: a Tool for Studying Human Perceptive
Field Organization. Perception 23:691–708
Lingelbach, B & Ehrenstein, WH Jr (2002) Das Hermann-Gitter und die Folgen.
DOZ 5:13–20,
www.leinroden.de/304herfold.htm
Corney D, Lotto
RB (2007) What Are Lightness Illusions and Why Do We See Them? PLoS Comput
Biol 3:e180
This interesting paper shows that the Herman grid illusion “automatically” occurs
as a by-product when an artificial neural network is trained for brightness constancy.
Bach M (2008)
Die Hermann-Gitter-Täuschung: Lehrbucherklärung widerlegt. Der Ophthalmologe
Created: 1999-07-18
Last update: 2013-10-04