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J.M. Ash, J.P. Comerford, F. Thorn; The Effect of Head Tilt on Orientation Tuning of the Hermann Grid Illusion . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4090.
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Purpose: We have found that patients with Parkinsonism or diabetes are impaired in seeing the Hermann Grid illusion and we have assumed that this result was due to a local effect of these diseases on retinal neurons. On the other hand, we have found that the Hermann Grid illusion may be modified by non-retinal factors, such as stimulus tilt. In this research we wish to measure an orientation tuning function for the illusion and demonstrate that orientation tuning may be modified by a non-retinal factor, head tilt. Methods: Six observers where presented with a circular Hermann Grid viewed either with the observer's head in a normal orientation or tilted 45°. In order to produce head tilt, the observer's whole body was tilted using an ophthalmic chair. Under either condition, observers were presented with 14 conditions of orientation of a Hermann Grid. The illusion was measured by canceling it out with smudges of opposite contrast to the illusory smudges of the Hermann Grid illusion. To control for the effects of ocular counter-rotation with head tilt, subjects were tested monocularly, and an afterimage technique was used to ensure that the eyes were tilted as intended. As a further control, the observers' eyes were photographed in both conditions of head tilt. Results: With the head held straight or tilted, the Hermann Grid illusion was dependent on grid orientation (P < 0.0001) with significant inter-subject differences for the strength (P < 0.0001) and orientation tuning (P < 0.0001) of the effect. The illusion was strongest when the grid was vertical or near vertical on the retina. However, orientation tuning was much greater when the head was held vertical than when it was tilted 45°. Conclusions: Our results suggest that the Hermann Grid illusion shows an orientation tuning function. Our results also suggest that the vestibular input may modify the orientation selectivity of this illusion. The possible influence of ocular counter-rotation cannot explain these results. These results are inconsistent with the generally accepted theory that the illusion depends on center-surround interaction within concentric receptive fields, and support the theory that this illusion is based, in part, on interactions beyond the retina. Supported by the New England College of Optometry Research Fund.
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