Abstract
Simple SummaryVisual 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.
Highlights
Understanding how animals see the world is one of the main issues of cognitive ethology.To achieve this goal, the neurobiological investigation of photoreceptors and the study of neural circuits of vision are fundamental to delineate which information is captured by the retina and how Animals 2020, 10, 1673; doi:10.3390/ani10091673 www.mdpi.com/journal/animalsAnimals 2020, 10, 1673 this information is processed in the brain
We conducted a linear mixed model (LMM) with condition (Control A/B/C/D/Illusory trials/Control for overall size) as within-subjects factors on the proportion of choice for the physically or subjectively larger carrot stick that revealed a significant effect of the Condition (F(5, 54) = 3.367, p = 0.010) on the horses’ performance
Horses selected the food portion presented with inward-pointing arrows more frequently than they might by chance, t(9) = 6.626, p < 0.001, d = 2.095 (Figure 2)
Summary
Understanding how animals see the world is one of the main issues of cognitive ethology.To achieve this goal, the neurobiological investigation of photoreceptors and the study of neural circuits of vision are fundamental to delineate which information is captured by the retina and how Animals 2020, 10, 1673; doi:10.3390/ani10091673 www.mdpi.com/journal/animalsAnimals 2020, 10, 1673 this information is processed in the brain. Understanding how animals see the world is one of the main issues of cognitive ethology. To achieve this goal, the neurobiological investigation of photoreceptors and the study of neural circuits of vision are fundamental to delineate which information is captured by the retina and how Animals 2020, 10, 1673; doi:10.3390/ani10091673 www.mdpi.com/journal/animals. Animals 2020, 10, 1673 this information is processed in the brain This type of investigation gives limited insight into how each animal sees the surrounding world. With respect to this issue, visual illusions represent a unique opportunity for understanding the perceptual laws that underlie vision in humans and in other species. They occur when individuals misperceive the size of an object as a function of the context (e.g., the surrounding background)
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