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U. Wolfe, J.A. Comee, E.M. Arvidson, B. Sherman; A Visually Induced Somatosensory Illusion . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4832.
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Purpose: We report a somatosensory illusion induced by asymmetric dark-adaptation of the two eyes. When only one eye is dark-adapted (DA) and both eyes are exposed to a dark visual environment, a strong illusory feeling of "sagging" in the eyelid of the non-dark-adapted (NDA) eye is induced. We tested the hypothesis that the illusion arises as a result of the incongruency between the senses of vision and somatosensation: while vision indicates that the two eyes are exposed to different lighting conditions, somatosensation signals no information that could explain the difference between the two eyes. Specifically, we predicted that the illusion would weaken or disappear when somatosensory information is introduced that eliminates this incongruency. Methods: Subjects (n=37) were dark-adapted for 30 minutes. They then wore goggles that blocked light from one eye while the other eye was exposed to light for 1 minute. Both eyes were then exposed to a dark environment and subjects removed the goggles. Subjects were asked to report any unusual feelings in either of the eyelids. They were next asked to report any change in the sensation as they a) closed each eye individually, b) covered each eye individually with their hand without closing it and c) put on goggles that were either normal or blocked light from one of the eyes. Results: 30 out of 37 subjects (81 percent) reported the illusory feeling of sagging or heaviness in the lid of the NDA eye. The feeling persisted for 2.5-4.5 minutes. Out of the 30 subjects reporting the illusion, 23 (77%) reported that the illusion was eliminated or reduced when they closed one eye. 27 subjects (90%) reported that the illusion was eliminated or reduced when they covered one eye with their hand. 29 subjects (93%) reported a reduction or disappearance when wearing one of the sets of goggles. Conclusions: Our results are consistent with the hypothesis that the illusory somatosensation arises as a result of the incongruency between vision and somatosensation. We show that the illusion is eliminated or weakened in the presence of somatosensory information that reduces this incongruency. Our findings indicate that when faced with cross-modal incongruency between vision and somatosensation, the perceptual system attempts to create a unified perceptual interpretation that is consistent across the two senses.
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