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D. Bodkin, J.P. Comerford, F. Thorn; The Effects of Tilt on the Chromatic Hermann Grid Illusion . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4573.
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Purpose: With a Hermann Grid, illusory dots are perceived at the intersection of horizontal and vertical bars. Previous work has shown there to be a diminished illusion for achromatic grids when they are presented at oblique angles. The purpose of this study is to investigate this phenomenon in the Chromatic Hermann Grid Illusion. We believe there will be a weakening of the illusion the more obliquely it is presented based on previous work that has shown similar visual processing for the achromatic and chromatic versions of the illusion. Methods: A circular Hermann Grid consisting of 21 intersections and a 50% contrast level was presented to subjects on a computer monitor. 3 sets of 10 grids were shown to 5 subjects. The 3 sets were defined by grey grids on a grey background, grey on red, and grey on blue. Each set was presented at 0°, ±5°, ±10°, ±15°, ±30°, and 45°. The subjects used a magnitude estimation technique, based on a 98% contrast standard grid, to rate the illusion seen. To control for the oblique effect, 2 additional achromatic sets were tested. These 2 sets consisted of 11 different contrast levels (5 positive, 5 negative, and isoluminant). They were presented at 0° and 45°. In this control test, the subjects reported whether or not they were able to observe a grid (not the illusion) at each contrast level. The experiment was run at 2 separate times for each subject and grids were presented in a random order within each set. Results: All 5 subjects demonstrated the same contrast threshold for 0° and 45°. All subjects perceived the Hermann Grid Illusion for all grids presented. The illusion diminished significantly in magnitude the more obliquely it was presented for all (100%) subjects. Conclusions: There is a tilt effect observed for the perception of the Chromatic Hermann Grid Illusion. This effect is similar to that for the achromatic version of the illusion. Since chromatic visual processing has been found by others to have little if any orientation selectivity we believe that this tilt effect is caused primarily by the achromatic cortical processing involved in the perception of the illusion.
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