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Todd Macuda, Kevin Johnston, Brian Timney; A direct estimate of the size of the illusory spots in the Hermann Grid Illusion. Journal of Vision 2003;3(9):754. doi: 10.1167/3.9.754.
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© ARVO (1962-2015); The Authors (2016-present)
The Hermann Grid Illusion has been used to investigate the properties of human perceptive fields, the psychophysical analogue of physiological receptive fields. Spillman (1994) suggested that the bar width at which the illusion is strongest corresponds to the perceptive field centre size at a given retinal location. Alternatively, Troscianko (1982) argued that this underestimated centre size. He proposed that a closer approximation could be achieved by multiplying the bar width by the square root of two. We investigated these hypotheses using a psychophysical matching procedure in which observers estimated the size of the illusory spots at the intersections of a Hermann Grid. Observers viewed a 6x6 grid in the centre of which was presented a circular field set at the same luminance as the grid bars. Centred within this field was a circular comparison patch with a gaussian luminance profile. We first obtained estimates of the perceived magnitude of the illusion at a range of bar widths. Observers were instructed to match the contrast of the comparison patch with that of the illusory spots. We then obtained estimates of the size of the illusory spots by asking observers to match the size of the comparison patch with that of the spots present at the grid intersections. For this condition, the contrast of the comparison patch was set equal to the observers' initial contrast estimates. Size estimates were obtained for black on white and white on black grids, at bar widths ranging from 6 to 60 arcmin-1. Estimates for both grids at all bar widths closely matched the actual width of the grid bars. These data suggest that the bar width at which the strongest illusion occurs represents an accurate measure of perceptive field centre size.
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