Cite as:
Geier J, Séra L, Bernáth L, 2005, "Distortion tolerance of the Hermann grid" Perception 34 ECVP Abstract Supplement

Distortion tolerance of the Hermann grid

J Geier, L Séra, L Bernáth

Earlier we showed (Geier et al, 2004 Perception 33 Supplement, 53) that the Hermann grid illusion disappears by applying certain distortions to the lines. In the absence of an explanation for this new phenomenon, we used psychophysical measurements. The dependent variable was the 'distortion tolerance': at a given distortion type, starting from straight lines and increasing the distortion step by step, we wished to establish a measure when the illusion disappeared for a given subject. The independent variables were the distortion type and the line width. The applied distortion types were sinusoid lines (sinus curves while the intersections rotated, but remaining right angled), wavy lines (sine curves with one period in the middle), humped lines ('humps' in the middle), knotted lines (thicker middles of the lines), and one-sided knotted lines (thicker middles on one side only). Stimuli were shown in random order on a computer monitor: d = 15 inches , 1024 × 768 pixels, viewing distance 60 cm, gazing with free eye movements. Distorted Hermann grids included 7 × 5 white lines on a black background, with 3 different line widths (11, 17, and 23 pixels), with constant line spacing (102 pixels). There were twenty-two subjects. A two-way ANOVA showed that the main effect of distortion type is highly significant ( p < 0.01); the main effect of line width is not significant; and interaction is not significant. Pairwise comparison showed that differences between the one-sided knotted line and every other line type were significant; differences between all other pairs were not significant. Our conclusion is that the main cause of the Hermann grid illusion is the straightness of the black - white edges; the line width plays no significant role. Documentation in support of these results can be found at http://www.geier.hu/ECVP2005.

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