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Brief Report

Susceptibility to Size Visual Illusions in a Non-Primate Mammal (Equus caballus)

Department of General Psychology, University of Padova, 35131 Padova, Italy
Department of Biomedical Sciences, University of Padova, Via Bassi 58, 35131 Padova, Italy
Padua Neuroscience Center, University of Padova, Via Orus 2, 35131 Padova, Italy
Author to whom correspondence should be addressed.
Animals 2020, 10(9), 1673;
Received: 30 July 2020 / Revised: 10 September 2020 / Accepted: 13 September 2020 / Published: 17 September 2020
(This article belongs to the Special Issue Companion Animal Cognition, Communication, and Behavior)
Visual illusions are commonly used by researchers as non-invasive tools to investigate the perceptual mechanisms underlying vision among animals. The assumption is that, if a species perceives the illusion like humans do, they probably share the same perceptual mechanisms. Here, we investigated whether horses are susceptible to the Muller-Lyer illusion, a size illusion in which two same-sized lines appear to be different in length because of the spatial arrangements of arrowheads presented at the two ends of the lines. Horses showed a human-like perception of this illusion, meaning that they may display similar perceptual mechanisms underlying the size estimation of objects.
The perception of different size illusions is believed to be determined by size-scaling mechanisms that lead individuals to extrapolate inappropriate 3D information from 2D stimuli. The Muller-Lyer illusion represents one of the most investigated size illusions. Studies on non-human primates showed a human-like perception of this illusory pattern. To date, it is not clear whether non-primate mammals experience a similar illusory effect. Here, we investigated whether horses perceive the Muller-Lyer illusion by using their spontaneous preference for the larger portion of carrot. In control trials, we presented horses with two carrot sticks of different sizes, and in test trials, carrot sticks of identical size were shown to the subjects together with arrowheads made of plastic material and arranged in a way meant to elicit the Müller-Lyer illusion in human observers. In control trials, horses significantly discriminated between the smaller and larger carrot stick. When presented with the illusion, they showed a significant preference for the carrot that humans perceive as longer. Further control trials excluded the possibility that their choices were based on the total size of the carrot stick and the arrowheads together. The susceptibility of horses to this illusion indicates that the perceptual mechanisms underlying size estimation in perissodactyla might be similar to those of primates, notwithstanding the considerable evolutionary divergence in the visual systems of these two mammalian groups. View Full-Text
Keywords: visual illusions; cognitive ethology; comparative perception visual illusions; cognitive ethology; comparative perception
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MDPI and ACS Style

Cappellato, A.; Miletto Petrazzini, M.E.; Bisazza, A.; Dadda, M.; Agrillo, C. Susceptibility to Size Visual Illusions in a Non-Primate Mammal (Equus caballus). Animals 2020, 10, 1673.

AMA Style

Cappellato A, Miletto Petrazzini ME, Bisazza A, Dadda M, Agrillo C. Susceptibility to Size Visual Illusions in a Non-Primate Mammal (Equus caballus). Animals. 2020; 10(9):1673.

Chicago/Turabian Style

Cappellato, Anansi, Maria E. Miletto Petrazzini, Angelo Bisazza, Marco Dadda, and Christian Agrillo. 2020. "Susceptibility to Size Visual Illusions in a Non-Primate Mammal (Equus caballus)" Animals 10, no. 9: 1673.

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