Pigeons perceive the Ebbinghaus-Titchener circles as an assimilation illusion.

Abstract

A target circle surrounded by larger "inducer" circles looks smaller, and one surrounded by smaller circles looks larger than they really are. This is the Ebbinghaus-Titchener illusion, which remains one of the strongest and most robust of contrast illusions. Although there have been many studies on this illusion in humans, virtually none have addressed how nonhuman animals perceive the same figures. Here the authors show that the Ebbinghaus-Titchener figures also induce a strong illusion in pigeons but, surprisingly, in the other direction; that is, all five successfully trained pigeons judged the target circle surrounded by larger circles to be larger than it really is and vice versa. Further analyses proved that neither the gaps between target and inducer circles nor the cumulative weighted surface of these figural elements could account for the birds' responses. Pigeons are known to show similarities to humans on various cognitive and perceptual tasks including concept formation, short-term memory, and some visual illusions. Our results, taken together with pigeons' previously demonstrated failure at visual completion, provide strong evidence that pigeons may actually experience a visual world too different for us to imagine.

Why do animals differ in their susceptibility to geometrical illusions?

“…In support of this explanation, Rosa Salva et al (2013) implemented a more naturalistic training and testing protocol and found that 4-day-old chicks were susceptible to the Ebbinghaus illusion in the same direction as humans, contradictory to findings in six-month-old chickens by Nakamura et al (2008Nakamura et al ( , 2014. Further investigation into potential factors responsible for the different findings is required.…”

“…Moreover, Nakamura, Watanabe, and Fujita (2008) found that pigeons appeared to experience an opposite illusion, perceiving the circle surrounded by large inducer circles to be larger than the circle surrounded by small inducer circles. In a size classification task, these pigeons judged a target circle surrounded by large inducers to be larger than its true size and a target circle surrounded by small inducers to be smaller than its true size.…”

“…In a size classification task, these pigeons judged a target circle surrounded by large inducers to be larger than its true size and a target circle surrounded by small inducers to be smaller than its true size. Subsequent control tests were conducted to rule out the possibility that the pigeons were making choices based on overall size or the size of the gap between the target circle and inducers (Nakamura et al, 2008).…”

To Err Is (Not Only) Human: Fallibility as a Window Into Primate Cognition

“…Baboons do not experience the illusion (Parron & Fagot, 2007) despite showing other visual illusions (e.g., Barbet & Fagot, 2002;Benhar & Samuel, 1982). This might suggest human-uniqueness in seeing this illusion, as would the reported reversed Ebbinghaus-Titchener illusions in pigeons (Nakamura, Watanabe, & Fujita, 2008) and bantam chickens (Nakamura, Watanabe, & Fujita, 2014). However, the story again is not that simple, as other nonprimate species have shown evidence of perceiving this illusion in the direction shown by humans (dolphin: Murayama, Usui, Takeda, Kato, & Maejima, 2012;domestic chicks: Rosa Salva, Rugani, Cavazzana, Regolin, & Vallortigara, 2013; redtail splitfin fish: Sovrano, Albertazzi, & Rosa Salva, 2015).…”

Do Dogs Demonstrate Susceptibility to a Vertically Presented Ponzo Illusion?

“…Some species demonstrate human-like susceptibility, some demonstrate no susceptibility, and some demonstrate reversed susceptibility (e.g., Agrillo, Parrish, & Beran, 2014;Fujita, 1996Fujita, , 1997Murayama, Usui, Takeda, Kato, & Maejima, 2012;Nakamura, Watanabe, & Fujita, 2008;Sovrano, Albertazzi, & Salva, 2014;Watanabe, Nakamura, & Fujita, 2011. Moreover, even within a species, mixed findings have been observed in susceptibility to the same illusion (e.g., Nakamura et al, 2008Nakamura et al, , 2014Salva, Rugani, Cavazzana, Regolin, & Vallortigara, 2013). One species in which such findings have recently been observed is the domestic dog.…”