Kanizsa’s Triangle is a well-known optical illusion that was first described by the Italian psychologist Gaetano Kanizsa in 1955. The illusion falls into a category known as illusory contours or subjective contours, where contours are perceived without a physical gradient or edge present in the image.

In Kanizsa’s Triangle, you’re presented with an image that appears to contain a bright white triangle overlaid on top of three black circles and an outlined triangle. The overlaid white triangle’s corners are marked by three cut-off or “pac-man” style discs, and its sides are suggested by the outlines of another triangle behind it.

The fascinating part is that there is no explicit white triangle drawn in the image. The edges and the interior of the white triangle don’t exist in the actual image and are instead inferred by your brain due to the arrangement of the surrounding elements. The perceived brightness of the illusory white triangle is usually more pronounced (it appears brighter) than the surrounding area, even though they are of the same brightness.

This illusion demonstrates the principle of closure in Gestalt psychology, where our brains tend to see complete figures even when part of the information is missing. In this case, our brains use the available cues to complete the shape of the triangle and even perceive contours and areas that aren’t physically present. It’s a remarkable testament to how our visual system fills in gaps based on the information it’s given, showing that perception involves more than simply processing direct visual input.

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