Visual Illusions in the Human and in the Animal Minds

Visual Illusions in the Human and in the Animal Minds

Oxána Bánszegi y Peter Szenczi

Vision, one of the most important senses in many vertebrates, serves two main purposes: it allows assessing qualities of objects from a distance and provides information about their location to help navigate the world. The eyes of vertebrates are the result of millions of years of evolution and are often considered among the most sophisticated organs nature has produced. Nevertheless, the image formed on the retina cannot fully determine reality. Light bouncing off many different objects—positioned at various angles and distances—can produce the same pattern of light in our eyes. Size and distance are also interconnected: an image on the retina could result from either a small object nearby or a large one farther away. 

Despite these ambiguities, visual processing must be both fast and generally accurate. To achieve this, the brain relies on shortcuts and assumptions—such as strategies that speed up perception and help resolve uncertainty. Sometimes, senses rely also on external cues. For example, determining an object’s color involves assumptions about ambient lighting, while distance ambiguities are often resolved using cues from other objects of known size in the visual field. Other times, the corrections come from internal information such as on previous experiences about the world. Even with these, the brain’s visual interpretation can lead to errors. 

How general are these assumptions, used to form precise mental representations of the environment? Are they unique to humans, while animals perceive the world in entirely different ways? How can we find out? Since the time of the ancient Greeks, we’ve known that humans are susceptible to visual illusions. In fact, 
humans have even invented optical tricks that exploit these perceptual mechanisms to fool the visual system. These illusions can serve as tools to prove how visual perception works, because the errors they produce offer clues about the underlying processes of normal perception. If animals were to also be susceptible to the same illusions as humans, it would mean that their visual systems may rely on similar mechanisms. This, in turn, implies that these processes were shaped by natural selection and have evolved to solve common perceptual challenges across species. 

While the discovery and description of visual illusions flourished during the 19th and 20th centuries, animals were long viewed by scientists as simple machines, and their perception was largely ignored. With few exceptions, this perspective persisted until the 21st century, when researchers began to recognize that animal cognition might be more like that of humans than previously thought. The first documented study of visual illusions in animals dates back to the 1920s and involved a chicken. The bird was trained to choose the smaller of two objects. Once it reliably performed this task, two identical objects were presented embedded in a visual illusion. The chicken consistently chose the one that appeared smaller, suggesting that it was susceptible to the illusion in the same way humans are. 

In the last two decades, comparative studies of illusion perception in humans and non-human animals have flourished. These studies are increasingly recognized as powerful, non-invasive tools for exploring the mechanisms and evolution of visual perception. From this body of research, we now know that most animals are generally susceptible to geometric illusions—though there are notable exceptions. A 2024 meta-analysis conducted by our group concluded that illusion perception is a widespread phenomenon among non-human animals, including some invertebrates. However, the degree of susceptibility appears to vary across animal taxonomy and groups. For example, birds tend to show stronger responses to visual illusions than other animal groups, and species with laterally positioned eyes appear more prone to certain illusions than those with forward-facing eyes, such as primates and humans. 

Some researchers even suggest that certain species actively manipulate their environment to create visual illusions. In many animals, mate choice involves comparing locally available individuals based on features that signal quality—such as size or coloration. As a result, individuals may attempt to increase their perceived attractiveness by displaying in contexts that make them appear more impressive relative to nearby rivals. 

This phenomenon has been observed in peacock spiders during courtship. Males raise their abdominal flaps and their third pair of legs in a posture that makes them appear taller and wider—an effect reminiscent of the Delboeuf illusion, in which size perception is altered by surrounding shapes. These findings suggest that some animals not only perceive illusions but may also use them to manipulate others in social interactions. A similar observation has been made on guppies. In this species, females prefer to mate with colorful males that have large color patches. Researchers found that male guppies preferentially positioned themselves next to males with smaller color patches—lower-quality competitors—thereby enhancing their own apparent attractiveness. This behavior appears to exploit a context-dependent size comparison similar to the Ebbinghaus illusion, where an object appears larger or smaller depending on the size of surrounding objects. 

The Developmental Psychobiology Lab at the Institute of Biomedical Research with cooperation of the National Institute of Psychiatry Ramón de la Fuente Muñiz is currently investigating the development of the perception of visual illusions in a number of animal models, and possible implementation of these methods to better understand certain neurodevelopmental disorders in humans.