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Everything is subjective under water surface, too: visual illusions in fish


The study of visual illusions has captured the attention of comparative psychologists since the last century, given the unquestionable advantage of investigating complex perceptual mechanisms with relatively simple visual patterns. To date, the observation of animal behavior in the presence of visual illusions has been largely confined to mammal and bird studies. Recently, there has been increasing interest in investigating fish, too. The attention has been particularly focused on guppies, redtail splitfin and bamboo sharks. Overall, the tested species were shown to experience a human-like perception of different illusory phenomena involving size, number, motion, brightness estimation and illusory contours. However, in some cases, no illusory effects, or evidence for a reverse illusion, were also reported. Here, we review the current state of the art in this field. We conclude that a wider investigation of visual illusions in fish is fundamental to form a broader comprehension of perceptual systems of vertebrates. Furthermore, we believe that this type of investigation could help us to address general important issues in perceptual studies, such as the role of ecology in shaping perceptual systems, the existence of interindividual variability in the visual perception of nonhuman species and the role of cortical activity in the emergence of visual illusions.

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This work was carried out within the scope of the project "use-inspired basic research", for which the Department of General Psychology of the University of Padova has been recognized as "Dipartimento di eccellenza" by the Ministry of University and Research.


This study was funded by STARS 2017 grant (acronym: ANIM_ILLUS) from the University of Padova and by PRIN grant no. 2015′ (prot.: 2015FFATB7) from ‘Ministero dell’Istruzione, Universita` e Ricerca’ (MIUR, Italy) to C. Agrillo.

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Agrillo, C., Santacà, M., Pecunioso, A. et al. Everything is subjective under water surface, too: visual illusions in fish. Anim Cogn 23, 251–264 (2020).

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  • Comparative perception
  • Optical illusions
  • Motion illusions
  • Subjective contours
  • Fish