Preliminary study to investigate the Delboeuf illusion in ring-tailed lemurs (Lemur catta): Methodological challenges

Visual illusions are commonly used in animal cognition studies to compare visual perception among vertebrates. To date, researchers have focused their attention mainly on birds and mammals, especially apes and monkeys, but no study has investigated sensitivity to visual illusions in prosimians. Here we investigated whether lemurs (Lemur catta) perceive the Delboeuf illusion, a well-known illusion that occurs when subjects misperceive the relative size of an item because of its surrounding context. In particular, we adopted the spontaneous preference paradigm used in chimpanzees and observed lemurs’ ability to select the larger amount of food. In control trials, we presented two different amounts of food on two identical plates. In test trials, we presented equal food portion sizes on two plates differing in size: If lemurs were sensitive to the illusion, they were expected to select the food portion presented on the smaller plate. In control trials, they exhibited poor performance compared to other mammals previously observed, being able to discriminate between the two quantities only in the presence of a 0.47 ratio. This result prevented us from drawing any conclusion regarding the subjects’ susceptibility to the Delboeuf illusion. In test trials with the illusory pattern, however, the subjects’ choices did not differ from chance. Our data suggest that the present paradigm is not optimal for testing the perception of the Delboeuf illusion in lemurs and highlight the importance of using different methodological approaches to assess the perceptual mechanisms underlying size discrimination among vertebrates

The influence of visual illusion perception on numerosity estimation could be evolutionarily conserved: exploring the numerical Delboeuf illusion in humans (Homo sapiens) and fish (Poecilia reticulata)

Discriminating between different quantities is an essential ability in daily life that has been demonstrated in a variety of non-human vertebrates. Nonetheless, what drives the estimation of numerosity is not fully understood, as numerosity intrinsically covaries with several other physical characteristics. There is wide debate as to whether the numerical and spatial abilities of vertebrates are processed by a single magnitude system or two different cognitive systems. Adopting a novel approach, we aimed to investigate this issue by assessing the interaction between area size and numerosity, which has never been conceptualized with consideration for subjective experience in non-human animals. We examined whether the same perceptual biases underlying one of the best-known size illusions, the Delboeuf illusion, can be also identified in numerical estimation tasks. We instructed or trained human participants and guppies, small teleost fish, to select a target numerosity (larger or smaller) of squares between two sets that actually differed in their numerosity. Subjects were also presented with illusory trials in which the same numerosity was presented in two different contexts, against a large and a small background, resembling the Delboeuf illusion. In these trials, both humans and fish demonstrated numerical biases in agreement with the perception of the classical version of the Delboeuf illusion, with the array perceived as larger appearing more numerous. Thus, our results support the hypothesis of a single magnitude system, as perceptual biases that influence spatial decisions seem to affect numerosity judgements in the same way

Red-footed tortoises (Chelonoidis carbonaria) do not perceive the Delboeuf illusion

Visual illusions have been widely used as a tool to study animal visual perception. In many cases, identical experimental procedures were adopted to make highly controlled interspecific comparisons. However, reducing methodological variability may prevent reliable comparisons because a certain methodology could be more suitable for some species than others. This study sought to build on previous work that investigated the perception of the Delboeuf illusion in reptiles. Reptiles were presented with trials composed of 2 different-sized food portions on 2 identical plates in which they were expected to maximize their food intake. In contrast to the bearded dragons (Pogona vitticeps), tortoises (Chelonoidis carbonaria) performed poorly in all conditions and therefore no firm conclusion regarding their perception of the illusion could be made. Such results could be due to cognitive challenges or due to the experimental setup, because descending a ramp is demanding for the tortoises. In this study, we adopted the same experimental paradigm but in a flat apparatus. Tortoises significantly discriminated the larger food portions in baseline trials, however, their performance did not differ from chance in illusory trials revealing that, under these conditions, they are not sensitive to the Delboeuf illusion. This nonperception could be ascribed to different factors, such as poorer discrimination ability or a low sensitivity to contrast and assimilation phenomena. Our study highlights the importance of additional investigation to better understand the nature of null results, taking in consideration the ecological needs of the species before drawing any conclusions about its abilities. (PsycInfo Database Record (c) 2020 APA, all rights reserved)