Susceptibility to geometrical visual illusions has been tested in a number of non-human animal species, providing important information about how these species perceive their environment. Considering their active role in human lives, visual illusion susceptibility was tested in domestic dogs (Canis familiaris). Using a two-choice simultaneous discrimination paradigm, eight dogs were trained to indicate which of two presented circles appeared largest. These circles were then embedded in three different illusory displays; a classical display of the Ebbinghaus-Titchener illusion; an illusory contour version of the Ebbinghaus-Titchener illusion; and the classical display of the Delboeuf illusion. Significant results were observed in both the classical and illusory contour versions of the Ebbinghaus-Titchener illusion, but not the Delboeuf illusion. However, this susceptibility was reversed from what is typically seen in humans and most mammals. Dogs consistently indicated that the target circle typically appearing larger in humans appeared smaller to them, and that the target circle typically appearing smaller in humans, appeared larger to them. We speculate that these results are best explained by assimilation theory rather than other visual cognitive theories explaining susceptibility to this illusion in humans. In this context, we argue that our findings appear to reflect higher-order conceptual processing in dogs that cannot be explained by accounts restricted to low-level mechanisms of early visual processing.

In humans, geometrical illusions are thought to reflect mechanisms that are usually helpful for seeing the world in a predictable manner. These mechanisms deceive us given the right set of circumstances, correcting visual input where a correction is not necessary. Investigations of non-human animals' susceptibility to geometrical illusions have yielded contradictory results, suggesting that the underlying mechanisms with which animals see the world may differ across species. In this review, we first collate studies showing that different species are susceptible to specific illusions in the same or reverse direction as humans. Based on a careful assessment of these findings, we then propose several ecological and anatomical factors that may affect how a species perceives illusory stimuli. We also consider the usefulness of this information for determining whether sight in different species might be more similar to human sight, being influenced by contextual information, or to how machines process and transmit information as programmed. Future testing in animals could provide new theoretical insights by focusing on establishing dissociations between stimuli that may or may not alter perception in a particular species. This information could improve our understanding of the mechanisms behind illusions, but also provide insight into how sight is subjectively experienced by different animals, and the degree to which vision is innate versus acquired, which is difficult to examine in humans.

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-One way to uncover visual capabilities in animals is to assess perception of geometric illusions. Recently, we found that dogs did not demonstrate susceptibility to the Ponzo illusion when it was presented in a variety of contexts, a unique result as all other published reports of nonhuman animal species tested on the illusion have demonstrated human-like susceptibility. Two important variables were not considered in our previous study. First, the stimuli were presented horizontally, whereas the more traditional presentation is vertical. Second, it is not known whether dogs can differentiate physical size differences small enough to facilitate perception of the Ponzo illusion. To investigate these issues, we tested the same dogs from our previous study on a vertical version of the Ponzo illusion and on a size discrimination task. Dogs did not demonstrate illusion susceptibility at the group level, although one dog was susceptible. In general, they were better able to detect size differences when the absolute size of the stimuli was large. Nonetheless, with stimuli approximately the same size as those used to test susceptibility to the Ponzo illusion, all eight dogs were able to discriminate between circles that differed in length by 20%, with four discriminating 10% size differences and none able to discriminate 5% differences. These findings suggest that at least some dogs are capable of perceiving the average size difference that humans perceive when observing the Ponzo illusion, but that susceptibility to this illusion is variable and weak, regardless of whether it is presented in either a vertical or horizontal format.

-While domestic dogs (Canis familiaris) play a large role in human daily lives, little is known about how they perceive the visual world. Recent research suggests that dogs may perceive certain visual illusions differently than humans. To further evaluate geometric illusion susceptibility, eight dogs were assessed on their susceptibility to the Ponzo illusion. Four experiments were conducted: 1) a presentation of the Ponzo illusion with target circles in a 'grid inducer context', 2) a re-test of Experiment 1 after additional training, 3) a presentation of the Ponzo illusion with target rectangles in a 'grid inducer context' and 4) a presentation of the Ponzo illusion with target circles in a 'converging lines context.' A one-sample t-test of the dogs' responses to the Ponzo stimuli in Experiment 1 demonstrated illusion susceptibility at the group level; however, no individual dog performed significantly above chance in binomial tests. In Experiments 2, 3, and 4, one-sample t-tests found no significant results at the group level, although one or more dogs did demonstrate a small but significant effect. Taken together, then, there was limited evidence for dogs' susceptibility to the Ponzo illusion in a two-choice discrimination paradigm. As most animals tested previously have demonstrated human-like susceptibility to the Ponzo illusion, these findings have implications for theoretical explanations. The divergence of results between dogs and humans/other animals suggest that mechanisms underlying perception of the Ponzo illusion may differ across species and that care should be taken when using visual paradigms to test dogs' cognitive skills.