Consistently Inconsistent Perceptual Illusions in Nonhuman Primates: The Importance of Individual Differences

SIMPLE SUMMARY: Visual illusions fascinate humans, in large part because we realize how such experiences disconnect how we perceive the world from reality. The discovery that other animals also experience some of these illusions has provided a compelling comparative story about the role that perception plays in sometimes misrepresenting the nature of the real world. What has also become apparent from comparative studies is that not all animals experience illusions the same way, and sometimes the same individual may not experience some illusions while experiencing others. A survey of the results of 14 experiments from 10 different studies with two monkey species that performed numerous illusion tasks reinforces the idea that individual differences are a rich source of information for better understanding any universal principles of visual perception. Future research should focus more effort towards understanding the causes and effects (on other behaviors) of experiencing (or not experiencing) these illusions. ABSTRACT: Perceptual illusions, and especially visual illusions, are of great interest not only to scientists, but to all people who experience them. From a scientific perspective, illusory visual experiences are informative about the nature of visual processes and the translation of sensory experiences to perceptual information that can then be used to guide behavior. It has been widely reported that some nonhuman species share these illusory experiences with humans. However, it is consistently the case that not all members of a species experience illusions in the same way. In fact, individual differences in susceptibility may be more typical than universal experiences of any given illusion. Focusing on research with the same nonhuman primates who were given a variety of perceptual illusion tasks, this “consistent inconsistency” is clearly evident. Additionally, this can even be true in assessments of human illusory experiences. Individual differences in susceptibility offer an important avenue for better understanding idiosyncratic aspects of visual perception, and the goal of isolating any possible universal principles of visual perception (in primates and beyond) should address these individual differences.