Can reptiles perceive visual illusions? Delboeuf illusion in red-footed tortoise (Chelonoidis carbonaria) and bearded dragon (Pogona vitticeps).

Abstract

Optical illusions have been widely used to compare visual perception among vertebrates because they can reveal how the system is able to adapt to visual input. Sensitivity to visual illusions has never been studied in reptiles. Here, we investigated whether red-footed tortoises, Chelonoidis carbonaria, and bearded dragons, Pogona vitticeps, perceive the Delboeuf illusion. This illusion involves the misperception of the size of a target circle depending upon the context in which it is presented. We adopted the same size discrimination for both species to compare their performance. Animals were presented with two different types of trials. In control trials, they received two different-sized food portions on two plates of the same size. In test trials, they received two same-sized food portions but presented on two different-sized plates. If they perceived the illusion in the same way as humans, we expected them to select the food portion presented on the smaller plate. The tortoises exhibited poor performance in the control trials, which prevented us from drawing any conclusions about their perception of the Delboeuf illusion. In contrast, the bearded dragons selected the larger amount of food in control trials. In test trials, they selected the portion presented on the smaller plate significantly more often than chance, suggesting a human-like sensitivity to the Delboeuf illusion. Our study provides the first evidence of the perception of a visual illusion in a reptile species, suggesting that rather than simply detecting visual input, they interpret sensory information captured by photoreceptors.

Delboeuf Illusion

“…It is important to note that the lemurs did not reliably discriminate food portions during control trials, suggesting that they may have failed to discriminate quantities at a given ratio necessary to perceive the Delboeuf illusion (Santacà et al 2017). A similar finding was reported for red-footed tortoises (Chelonoidis carbonaria) who also failed to maximize in control trials, limiting the interpretation of illusory performance (Santacà et al 2019).…”

“…Several other animal species have been presented with the Delboeuf illusion using the spontaneous choice paradigm. Positive evidence for the illusion has been documented in a variety of taxa, including domestic cats (Felis silvestris catus: Szenczi et al 2019), bearded dragons (Pogona vitticeps: Santacà et al 2019), and damselfish (Chromis chromis: Fuss and Schluessel 2017), which also fall prey to the highly similar Ebbinghaus-Titchener illusion (Fuss and Schluessel 2017). Alternatively, lemurs (Lemur catta) were not sensitive to the Delboeuf illusion using the spontaneous choice task, showing indifference in illusion trials with equal likelihood to select food presented on a large or small plate.…”

Methodological Challenges in the Assessment of Dogs' (Canis lupus familiaris) Susceptibility of the Ebbinghaus-Titchener Illusion Using the Spontaneous Choice Task

“…To date, there is no such criteria utilized within the spontaneous choice task; however, the current methodology may represent a major constraint for replication and generalizability. We leave this question open to further debate, exploration, and investigation, but highlight that at least two other studies evaluating illusion susceptibility via a spontaneous choice task have questioned the adequacy of this methodology (Santacà et al, 2017(Santacà et al, , 2019. In the interim, a possible solution to mediate the issue regarding control trial performance, is to set criteria that subjects must meet before moving on to illusion trials or until study exclusion criteria is met.…”

Size Illusion

“…A number of species presented with the Ebbinghaus and Delboeuf illusions respond in a humanlike direction, perceiving stimuli surrounded by large inducers as being smaller and vice versa, including redtail splitfin (Sovrano et al 2015), damselfish (Fuss and Schluessel 2017), bearded dragons (Santacà et al 2019), dolphins (Murayama et al 2012), domestic cats (Szenczi et al 2019), capuchin monkeys and rhesus macaques (Parrish et al 2015), and chimpanzees (Parrish and Beran 2014). A number of these species also have been assessed for their perceptual processing mode and reveal a global precedence, including redtail splitfin, dolphins, and chimpanzees.…”