In this paper, I report my efforts to investigate what animals perceive, with a focus on one of my comparative cognitive studies in which my colleagues and I examined whether pigeons would perceive Ebbinghaus–Titchener illusion. The results of the first two experiments were consistent with the previous studies, supporting the hypothesis that pigeons, known for their local precedence of visual information, might perceive a reversed version of Ebbinghaus-Titchener illusion (i.e., the size of the circle surrounded by larger circles is perceived larger than surrounded by smaller circles). However, further investigation in the two subsequent experiments provided a more plausible, non-illusory account for both previous and present results. Hasty acceptances of evidence that appears to align with the hypothesis or prediction can be a “pitfall”, whereas careful investigation with a healthy dose of skepticism to avoid such pitfalls can contribute to studies involving human participants, for whom verbal instructions and introspection are believed to work well, as well studies involving nonhuman animal subjects.
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