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18 March 2005 Visual illusions: pointing the finger at the Judd illusion
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Proceedings Volume 5666, Human Vision and Electronic Imaging X; (2005)
Event: Electronic Imaging 2005, 2005, San Jose, California, United States
Attempts to demonstrate dual route processing of vision for action and vision for perception, have yielded mixed results. Early work suggested that motor actions were, unlike perceptual responses, unaffected by visual illusions. However, it has been argued that these experiments were methodologically flawed and that the evidence actually supports a unitary representation account. We have examined perception of and pointing at the Judd illusion. In Experiment 1 we compared immediate object orientated pointing with perceptual line-matching at the ends and unmarked midpoints of the left and right facing Judd illusion. In Experiment 2 we compared immediate and delayed (4s) pointing and line-matching performance using the right facing Judd. We found that although pointing and matching were affected by the illusion, the pattern and magnitude of the errors were different across modality. Immediate pointing performance was generally less accurate (bigger errors) than line-matching performance; delayed pointing accuracy improved (errors were reduced) while line-matching accuracy remained unchanged. We argue that these data do not fit either the unitary or the standard dual route account, and are best understood in the context of a two-stage dual route model. We suggest that looking for differences in the pattern of results might serve a more useful approach than focussing on null effects in the motor task.
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Andrew Dunn and Peter Thompson "Visual illusions: pointing the finger at the Judd illusion", Proc. SPIE 5666, Human Vision and Electronic Imaging X, (18 March 2005);

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