Visual Illusions. What Can We Learn from Them?

For us, seeing is simple and effortless—we do it all the time when we are awake. Without a doubt, vision is one of our most important senses. However, no two people see the same environment in exactly the same way, because we often perceive not objective reality, but the parts our brains deem important. Seeing feels so natural that we rarely think about the complex and still poorly understood mechanisms behind it. Our eyes work like cameras: when light enters the eye, it activates the light-sensitive photoreceptors in the retina. This light energy is then converted into electrical signals that travel to the brain. In the visual cortex, a representation of what we see emerges. But now we know that it’s not just the visual cortex involved—at least 30 different areas of the primate brain have been identified as participating in visual processing.

So, if seeing is so easy, why does it require so much brainpower? Because every second we are awake, our brain must build a representation of the three-dimensional world based on information that is two-dimensional, incomplete, and constantly changing. In addition, this information must be compared in real time with our past experiences and memories. In other words, our visual system interprets the world around us. But this also means we don’t see the world as it truly is—we see it as it is useful for us to see. However, like every complex system, vision is far from perfect. How many times have you heard the expression “Are my eyes deceiving me?” Or thought to yourself “Is it an illusion, or is it real?” Sometimes, a discrepancy arises between our visual perception and reality. In these moments, we misperceive the true physical characteristics of an object. This is what we call a visual illusion. 

NO TWO PEOPLE SEE THE SAME ENVIRONMENT IN EXACTLY THE SAME WAY

Visual illusions are thought to result from the long-term adaptation of our visual system to the environment. Illusory perception often feels just as real as actual perception and is usually indistinguishable from reality. Even when we know that what we’re seeing isn’t real, we can’t simply “turn off” the illusion. Our brains persist in interpreting the visual input in the way they’ve been trained to over time. 

One such illusion that has been observed for thousands of years is the “moon illusion”. The moon appears larger when it is near the horizon than when it is high in the sky. However, this apparent size difference is purely an illusion—it happens only in our minds. But why do we see it this way? In truth, nobody fully knows. One possible explanation is that our brain interprets the moon’s size relative to other objects in the landscape, such as trees or buildings, whose sizes we are familiar with. When the moon is near these objects, our brain may assume it is closer and thus scales its size up. When the moon is high in the sky with no nearby reference points, this scaling becomes difficult, and the moon appears smaller. 

The history of visual illusions stretches back to ancient times. In the 5th century BCE, Greek philosophers began to explore this phenomenon. Epicharmus was one of the first to suggest that while the mind may understand things clearly, the senses can deceive us, resulting in illusions. In contrast, Protagoras believed that the environment itself is what fools us, not our senses. Later, Aristotle contributed a more nuanced view, agreeing with Protagoras that we can generally trust our senses to provide a reliable picture of reality, but also acknowledging that the senses can be easily tricked. The 19^th century marked a golden age for the scientific study of visual illusions. During this time, researchers focused heavily on geometric optical illusions—visual phenomena in which the perception of an object’s size, length, angle, color, or tilt is distorted by the presence of other visual elements. Many of these illusions were named after the researchers who described them, such as Ebbinhaus or Titchener (1901), Delboeuf (1865), Mach (1865), Müller-Lyer (1889), Poggendorff (1860), and Zöllner (1860).

VISUAL ILLUSIONS ARE THOUGHT TO RESULT FROM THE LONG-TERM ADAPTATION OF OUR VISUAL SYSTEM TO THE ENVIRONMENT

Why is it important for researchers, scientists, psychologists, or doctors to study visual illusions? Because individuals vary in how they process sensory information. People with neurodevelopmental conditions such as autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), or schizophrenia often experience the world differently, in part due to sensory processing differences. These differences may reflect imbalances in how information is integrated and interpreted by the brain, leading to unique perceptual experiences compared to those of neurotypical individuals. In our research group, we routinely use visual illusions to study the development and neural basis of perception in both humans and non-human animals. Through a collaborative effort between UNAM’s Institute of Biomedical Research, the National Institute of Psychiatry Ramón de la Fuente Muñiz, and the Children’s Psychiatric Hospital Doctor Juan N. Navarro, we are currently investigating how sex (referring to biological differences) and gender (referring to socially constructed roles and identities), age, screen time, and lifestyle factors affect susceptibility to specific visual illusions.

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