Geier J, Bernáth L, Hudák M, Séra L, 2008, "Straightness as the main factor of the Hermann grid illusion" Perception 37(5) 651 – 665
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Straightness as the main factor of the Hermann grid illusion
János Geier, Lászlo Bernáth, Mariann Hudák, Lászlo Séra
Received 5 April 2006, in revised form 4 July 2007; published online 1 May 2008
Abstract. The generally accepted explanation of the Hermann grid illusion is Baumgartner’s hypothesis that the illusory effect is generated by the response of retinal ganglion cells with concentric ON – OFF or OFF – ON receptive fields. To challenge this explanation, we have introduced some simple distortions to the grid lines which make the illusion disappear totally, while all preconditions of Baumgartner’s hypothesis remain unchanged. To analyse the behaviour of the new versions of the grid, we carried out psychophysical experiments, in which we measured the distortion tolerance: the level of distortion at which the illusion disappears at a given type of distortion for a given subject. Statistical analysis has shown that the distortion tolerance is independent of grid-line width within a wide range, and of the type of distortion, except when one side of each line remains straight. We conclude that the main cause of the Hermann grid illusion is the straightness of the edges of the grid lines, and we propose a theory which explains why the illusory spots occur in the original Hermann grid and why they disappear in curved grids.
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