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Size Constancy



Have you ever seen the illusions in pictures 1 and 2🤔? 

Picture 1

Picture 2

Picture 3

Both lines A and B (picture 3) in both illusions have the same length, but we perceive them as one longer than the other. Why is that so🧐? It is due to the failure of size constancy.

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What is size constancy?

Size constancy is the tendency of our visual system to perceive the object as the same size regardless of the change of the perceived distance and size on the retina.

Size constancy failure explanation

Ponzo Illusion

In the 🔹Ponzo illusion🔹, the diagonal lines would be seen as ascending to the horizon, hence line A would be perceived as being further away from line B. If both line A and line B are the same sizes, then line A should be perceived as being shorter. However, as the length of line A exceeds the given range of the two diagonal lines, it would then be perceived as being longer compared to line B. As a result, given these two factors, line A would be perceived to be longer than line B despite them having the same length.

Müller-Lyer Illusion

For the 🔹Müller-Lyer Illusion🔹, the arrowheads in both lines (pointing outwards and inwards) would be used as an indicator for the perception of the length by our visual system to interpret depth cues. For line A, the outward-pointing arrowhead presents the actual length of line A. Whereas for line B, the inward-pointing arrowhead (line B) indicates that the line is further away. As objects that are seen as further away usually would be smaller or shorter, hence the actual object would be bigger or longer. Thus, line B would be perceived to be longer than line A.
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