Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Michael J Beran

Second Advisor

Sarah F Brosnan


The perceptual system operates ideally to reveal rapidly processed, accurate and functional information to an organism. However, illusory phenomena emerge when there is discontinuity between sensory input and perception on the basis of misleading contexts. Because illusions emerge as a byproduct of an otherwise functional and efficient perceptual system, they provide a means to understand better mechanisms of perception within and across species. Beyond anatomical and functional similarities in the visual system across primates, nonhuman primate species reveal intriguing similarities in the perception of visual illusions with one another and humans.

This dissertation explored visual illusions across the Order Primates, including human adults and children, chimpanzees, rhesus monkeys, and capuchin monkeys, with a focus on when, why and for whom geometric illusions emerge. Geometric illusions occur when a target’s perceived properties (e.g., size, shape, color) are impacted by an illusory context. Specifically, I focused on similarities and differences in perceptual mechanisms across primate species and on the role of attention in illusion emergence. I also assessed the translational impact of visual illusions on everyday decision-making (i.e., food-choice behavior).

Overall, this research demonstrated that illusions are complex, and they emerged differently across species as a function of processing mode (i.e., global versus local processing) and attentional control. Further, not all illusions were perceived equally. External factors including illusory array design (i.e., the relationship between target and inducer stimuli) and testing paradigm directly impacted illusion perception. Response competition emerged if the illusory array was too heavily weighted towards inducing stimuli, such that the inducers were the more salient element within the array relative to the target stimulus. These methodological challenges proved to be especially true for local processors (e.g., monkeys) that first perceived the individual elements within an array prior to perceiving the global figure. The manner in which illusions are presented to pre- and non-verbal species can constrain or perhaps create a scaffold for illusory perception. Comparative research as in this dissertation provides a deeper understanding of how context influences perception and choice and will shed light on how we see and subsequently interact with our world.