Why do our brains fall for optical illusions?

A: Our brains fall for optical illusions because of the ways in which they perceive the world, including using contextual information, shortcuts, and predictions. Among other patterns, the brain assumes light comes from above, fills missing edges, and exaggerates contrasts.

While these tricks help us navigate the world, sometimes they also produce rational mistakes, where the brain interprets ambiguous information in ways that depart from physical reality. For example, identical colours may look different against different backgrounds and lines of equal length may appear unequal when framed differently.

A new study in Nature Neuroscience has deepened this picture. Researchers studied how the brain handles illusory contours, including shapes such as the Kanizsa triangle, where we see edges that are not there. Using advanced imaging and optogenetics in mice, the researchers found that special neurons called IC-encoders in the primary visual cortex respond to these illusory shapes as if they were real edges. The IC-encoders do this by integrating predictions from higher brain areas and broadcasting them in a process where the brain fills in missing parts to create a coherent whole.

When scientists stimulated these neurons, the brain produced the illusion even without a visual stimulus, showing that illusions simply ‘hack’ how perception normally works: by combining partial evidence with prior expectations to infer the most likely picture.

“Sensory systems are constantly faced with incomplete or ambiguous sensory information,” the researchers wrote in their paper. “In these situations, successful perception depends on sensory inference.”