table of contents

a look at how humans think and see

Visual Illusions: Introduction
by David Rudd Cycleback

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'Water in the road' mirage

Visual illusions have tricked and fascinated humans for thousands of years. They have influenced history, religion and society, been studied by scientists and philosophers, used by athletes, architects, medical doctors, engineers and artists, and have amused kids and mystery lovers of all ages. Visual illusions include rare spectacular atmospheric events and mundane everyday moments.

Learning about visual illusions and how they work show us that reality and human perception of reality are different things.

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What are Visual Illusions?

A visual illusion is when the viewer misperceives what she is looking at. What she thinks she sees is different than what she is looking at.

Visual illusions include misidentifying objects, perceiving things that don't exist or not perceiving things that do exist. It also includes significantly misjudging qualities, such as color, angle, amount, shape, weight, size and distance. Visual illusions happen to birds, fish, flies, dogs and other animals.

The above are perfect circles

Humans have natural and learned ways of perceiving. These methods are good, serving our practical day-to-day needs, but are far from perfect. Mistakes, often minor, are made daily by all humans.

Visual illusions are caused by a wide variety of factors. The factors differ from illusion to illusion, and there are multiple causes working together for each illusion. General factors include:

Physiology: As with all animals with eyes, humans have strengths, weaknesses and limitations in how they detect and translate light. Humans see better during day than at night, see a limited range of light and their eyes/mind do not translate light in an entirely efficient and accurate way. This all effects our visual perception.

Physical environment: This includes the brightness and angle of light, along with atmospheric conditions like fog, smog and air temperature. Many mirages are caused by unusual atmospheric conditions that distort light. In daily events the difference between light and dark, clear and cloudy can be the difference between identification and misidentification.

Biases: Humans have conscious and subconscious, innate and learned biases that effect how they perceive. These biases are used to categorize, prioritize, label, translate and judge information. Biases often cause the viewer to perceive patterns, shapes, colors and identities that do not objectively exist. Biases cause us to place undue emphasis on trivial information, while ignoring what may be important. The proverbial missing the forest for the trees.

Personal knowledge and background: How a human perceives something is greatly affected by his knowledge, what he has been taught and past experiences. You identify a dog by having already seen animals and learned they are called dogs. Without that experience, you would be mystified by that strange creature sniffing around your neighbor's hedges. People new to geography often fall for visual traps the natives do not. In the crystal clear air of mountains, long distance objects typically appear much closer than reality. Newbies to high altitude are accustomed to seeing through the hazier air of low altitude, with distance of a far away object being judged in part by its relative haziness-the further away a building or cliff, the hazier. In an environment with clearer air, low altitude rules can deceive. In highest altitudes a mountain can both be far away and crystal clear, appearing closer than it is to the new climber. As one might imagine, misjudging distance in the mountains and cliffs can be dangerous.

Humans shape their perceptions and overcome many of their visual illusions with experience. After walking into a sliding glass door for the second time, you likely have learned not to do it again. Humans aren't omniscient and learn by trial and error, gaining knowledge as they go. In many cases visual illusions are a natural part of the learning process-- error in judgment (visual illusion), followed by realizing the error, followed by having better knowledge.



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