The Ebbinghaus illusion (sometimes called the "Titchener illusion") is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size are placed near to each other and one is surrounded by large circles while the other is surrounded by small circles; the first central circle then appears smaller than the second central circle.
The Ebbinghaus illusion, as well as numerous other visual and perceptual illusions, provide a valuable way to investigate how the eye and brain process visual information. Equally, they are used by artists for visual effect, entertaining and satisfying the endless fascination human beings have with novelty and creativity.
The Ebbinghaus illusion is named for its discoverer, German psychologist Hermann Ebbinghaus (1850-1909). Ebbinghaus is perhaps most well known for his work in the field of memory, but he also made contributions to the area of visual perception. Ebbinghaus likely introduced this illusion in the 1890s, though he did not publish it in any specific publication.
After 1957, the illusion began to be attributed to another scientist, Edward Titchener, who had never expressed any authorship whatsoever. Because of this, the illusion is sometimes called the "Titchener illusion".
The classic Ebbinghaus illusion consists of a circle surrounded in one image by smaller circles, and in another by larger circles. The viewer tends to perceive the circle surrounded by smaller circles as being larger than the circle in the other image, even though both are exactly the same size.
The Ebbinghaus illusion has played a crucial role in the debate over the existence of separate pathways in the brain for perception and action. Experiments have shown that, while adult subjects perceive the center circles as differing in size, they reach out to grasp the circle accurately. In theory, this is due to the process of perception using a different visual pathway than the process of action. While adults rarely misjudge the size of the center disk while reaching for it, experiments have found that young children do, in fact, misjudge size both perceptually and through action. Researchers have proposed that this is because young children rely on both pathways to process tasks, instead of the separate pathways that adults use.
The Ebbinghaus illusion is a useful means of studying the various effects of perception on the brain. By studying the differences in reaction to the illusion between children and adults, conclusions have been drawn about brain development and function.
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