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Irene Pepperberg, Jennifer Vicinay, Patrick Cavanagh; Processing of the Mïller-Lyer illusion by a Grey Parrot (Psittacus erithacus). Journal of Vision 2007;7(9):601. doi: 10.1167/7.9.601.
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Alex, a Grey parrot (Psittacus erithacus) who identifies the bigger or smaller of two objects by reporting its color or matter using a vocal English label and who states “none” if they do not differ in size, was presented with two-dimensional Mïller-Lyer figures (Brentano form) in which the central lines were of contrasting colors. His responses to “What color bigger/smaller?” demonstrated that he saw the standard length illusion in the Mïller-Lyer figures in 32 of 50 tests where human observers would also see the illusion and reported the reverse direction only twice. He did not report the illusion when (a) arrows on the shafts were perpendicular to the shafts or closely approached perpendicularity, (b) shafts were 6 times thicker than the arrows, or (c) after being tested with multiple exposures, conditions that also lessen or eliminate the illusion for human observers. These data suggest that parrot and human visual systems process the Mïller-Lyer figure in analogous ways despite a 175 fold difference in the respective sizes of their brain volumes, and that parrots are viable candidates for further testing on visual illusions.
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