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J.B. Ares, M.J. Cox, I.E. Pacey, J.M. Gilchrist, G.T. Mahalingam; Asymmetry as an Investigative Tool for the Origin of the Hermann Grid Illusion . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5354.
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Neuroanatomical studies have reported that midget and parasol retinal ganglion cells lying in the nasal retinal quadrant have smaller dendritic field diameter than those in the temporal, upper, and lower quadrants in human (Dacey & Petersen, 1992) and macaque (Watanabe & Rodieck, 1989). The purpose of this study is to investigate spatial tuning asymmetry of perceptive fields by means of the Hermann grid illusion (HGI). The HGI was used to estimate the perceptive field centre size (PFCS) in the inferior–nasal and inferior–temporal retinal quadrants of the human eye with a newly developed methodology based on a nulling technique. Confirmation of an asymmetry would indicate a retinal origin for the HGI with limited cortical modification.
A computer–monitor based psychophysical test using randomly interleaved staircases measured the minimum intensity of a patch of light required to null the HGI at different bar widths (range 3.6 arc min to 60 arc min) for three different retinal eccentricities (1.5, 3, and 6 deg) in three normal subjects in the inferior–nasal and inferior–temporal retinal quadrants. The stimulus consisted of an isolated intersection of a Hermann grid with a vertical/horizontal bar contrast chosen to maximise illusion strength.
The results indicate that PFCS for an optimum HGI strength increases with increasing retinal eccentricity in the inferior–nasal (approximately 17 arc min, 18 arc min and 30 arc min at 1.5, 3 and 6 deg eccentricity, respectively) and the inferior–temporal (approximately 16 arc min, 21 arc min and 35 arc min at 1.5, 3 and 6 deg eccentricity, respectively) retinal quadrants. This compares with neuroanatomical data where midget and parasol retinal ganglion cells lying in the nasal retinal quadrant were approximately a 30% smaller than those in the temporal, upper, and lower quadrants at 6 deg eccentricity in macaque (Watanabe & Rodieck, 1989) and human (Dacey & Petersen, 1992).
Psychophysical PFCS estimations qualitatively, but not quantitatively, match the predictions from neuroanatomical studies at the retina. This result may be interpreted as an indication of limited participation of cortical mechanisms in this contrast illusion.
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