Under certain circumstances, perception distorts sensation to satisfy the mind’s own inner logic. Illusions illustrate how perception is a process of fabricating mental representations which are invariably expressionistic.
The Moon Illusion
Regardless of its elevation in the sky, the Moon’s angular size at the eye remains the same. Yet the horizon Moon may appear to be nearly twice the diameter of the elevated Moon. ~ American psychologists Lloyd Kaufman & James Kaufman
The Moon appears much larger as it rises over the horizon than when it is directly overhead. Many people think this is because the Moon is closer to the Earth when rising – not so.
The eye sees the Moon at the same size regardless of azimuth. But when the Moon is on the horizon, the mind figures it is farther away. This is because interposition cues, such as trees and buildings, indicate distance. Overhead, there are no such distance cues. Consequently, though the sensation is of a selfsame size, the perception is that the Moon is more distant when on the horizon. To compensate for this incongruity, the mind inflates the size of the Moon on the horizon.
(Another Moon illusion involves pareidolia: promiscuously perceiving patterns where none exist. Whereas westerners see the dark splotches on the Moon’s surface as forming a face (the man in the Moon), easterners see a rabbit pounding mochi (for rice cakes) (in Korea and Japan), or making the elixir of life (in China).)
The horizon Moon is perceived as larger because the perceptual system treats it as though it is much farther away. ~ Lloyd Kaufman & James Kaufman
Italian psychologist Mario Ponzo posited the Ponzo illusion in 1911, based upon his insight that the mind judges an object’s size in a spatial context. His illustration was of 2 parallel horizontal lines of identical length, juxtaposed with converging lines which suggest linear perspective. The eye perceives the 2 lines as the same length, but the mind lengthens the top line, even though it is not consciously perceived as farther away.
Along the same lines, German sociologist Franz Carl Müller-Lyer devised the Müller-Lyer illusion in 1889. The original illusion was to ask viewers to place a mark at the midpoint of the arrow shown. Invariably, the mark is placed toward the tail.
The variation more commonly seen nowadays is of 2 line segments: one with fins inwards, the other with tails out. Both shafts are of equal length, but the one with 2 tails is always perceived as longer.
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A key goal of vision is to detect objects using any information that happens to be available. ~ Vilayanur Ramachan-dran & American cognitive scientist Diane Rogers-Ramachandran
To make sense of the world, the mind regularly infers. Visually, the mind completes figures from partial sources.
The mind’s bias is to see the image above as rings around a cylinder. But, with mental effort, a different take is possible: C-shaped metal arches, with sharp ends facing forward.
The mind’s completion algorithm is so strong that it may override our knowledge of the natural world. An impossibly elongated cat is seen in the figure at left.
Illusions of the Blind
Some sightless people employ echolocation to identify objects. They too are subject to optical illusions. In one experiment participants were asked to pick up 3 boxes by a string. The boxes all weighed the same but were of different sizes. Sighted individuals invariably succumbed to size-weight illusion: the bigger boxes felt lighter. Blind study subjects did not experience the illusion. But the ones who relied upon echolocation to first assess the boxes did.
Perception sets expectations and vice versa.