Is the susceptibility to visual illusions related to the relative brain size? Insights from small-brained species

The study of visual illusions represents a useful tool in different research fields. Visual illusions are commonly used to establish the perceptual mechanisms underlying our perception of static and dynamic events (for a review, see Eagleman, 2001; Gregory, 1997). Visual illusions have become a valid tool in clinical psychology as a noninvasive screening for detecting schizophrenic and other psychopathological traits (e.g., Gori, Molteni, & Facoetti, 2016; Notredame, Pins, Deneve, & Jardri, 2014; Pessoa, Monge-Fuentes, Simon, Suganuma, & Tavares, 2008). Last, they represent a powerful set of stimuli to compare the vision of human and non human animals. With respect to this issue, the past two decades have been characterized by a growing number of studies finding that apes, old-world monkeys, and new-world monkeys are susceptible to many visual illusions. Nonhuman primates perceive size illusions (e.g., chimpanzees, Pan troglodytes: Parrish & Beran, 2014; capuchin monkeys, Sapajus apella: Parrish, Brosnan, & Beran, 2015; Suganuma, Pessoa, Monge-Fuentes, Castro, & Tavares, 2007), depth illusions (baboons, Papio papio: Barbet & Fagot, 2002), orientation illusions (rhesus monkeys, Macaca mulatta: Agrillo, Parrish, & Beran, 2014b), numerosity illusions (rhesus monkeys: Beran, 2006), and motion illusions (rhesus monkeys: Agrillo, Gori, & Beran, 2015).