Optical illusions are distortions in our visual system characterized by a perception that appears to differ from reality. There are three major types of optical illusions: physical, physiological, and cognitive. Physical illusions are caused by the physical environment, such as refraction making a spoon immersed in water appear to bend. Physiological illusions arise in the eye or the visual pathway and can be caused by excessive stimulation of receptors (e.g. afterimage after seeing a camera flash). Cognitive illusions , such as the one below, are the result of unconscious inferences based on assumptions about how the world should look and are the most widely known type of optical illusion.
The popular Müller-Lyer Illusion consists of equal length line segments with variable fin orientations, which causes the brain to misinterpret the actual length of the line segment. A complex example of this optical illusion is shown in the animation below. Despite the blue and black line segments remaining static, the context provided by the fin positions tricks our brain into interpreting it as changes in line segment length.
There are two hypotheses for how exactly this cognitive optical illusion manifests. The first relies upon our ability to judge depth based on perspective. The fins are misinterpreted by the brain as depth cues, which results in an erroneous assumption of a 3D scene. The second relies upon how the brain computes perceived location using the centroid of an object’s luminance. The orientation of the fins causes a shift in the centroid and results in a misjudgement of distance.
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