Are you seeing what is not?

MC Escher "Humanity

MC Escher “Humanity”

Graphics are very interesting. They can become even more interesting if we incorporate a type of optical illusion into them. Optical illusions are characterized by visually perceived images that are different from objective reality.
They can be divided into three main types:
1. Literal optical illusions create images that are different from the objects that make them.
2. Physiological illusions are effects on the eye & brain of excessive stimulation of a specific type(brightness, tilt, color, movement).
3. In cognitive illusions the eye and brain make unconscious inferences. This one is a bit more interesting for us.

The theory behind physiological illusions is that stimuli have individual dedicated neural paths in the early stages of visual processing. Repetitive stimulation of only a few channels causes a physiological imbalance that in turn alters perception. The Hermann grid illusion and Mach bands show this phenomenon.
In the Mach bands illusion we see bands of increased brightness at the edge of a color because of something called
Lateral inhibition, where in the receptive field of the retina, light & dark receptors compete with one another to become active. Once a receptor is active it inhibits neighboring receptors. This inhibition creates contrast, highlighting edges. In the grid illusion this response makes gray spots appear at the intersections because of the increased dark surround.
Cognitive illusions arise by what we assume is true about the world. This leads to an “unconscious inference”, an idea first suggested by
Hermann Helmholtz in the 19th century.
Cognitive illusions can be divided into four types:
1. Ambiguous illusions are pictures or objects that elicit a perceptual ‘switch’ between the alternative interpretation. Examples: Necker Cube; Rubin Vase
2. Distorting illusions are characterized by distortions of size, length, or curvature. Examples: Cafe wall illusion; Muler-Lyer illusion
3. Paradox illusions are generated by objects that are paradoxical or impossible. Examples: Penrose Triangle; Ascending Descending; Waterfalls
4. Fictional illusions are defined as the perception of objects that are not there to all but a single observer, such as those induced by schizophrenia or a hallucinogen. These are commonly known as hallucinations.
Perceptual organization is the term used to describe the brain’s organization of incoming sensations into information which is meaningful. This happens when it perceives sensory stimuli as a meaningful whole. Our brain has a need to see familiar simple objects and has a tendency to create a “whole” image from individual elements. It formulates what isn’t there with what is believable. This can also be explained using Gestalt Theory also known as
Gestalt Psychology.
Depth perception is the ability of the brain to see in 3 dimensions when the image hitting the retina is 2 dimensional. A clear example of this can be seen in the
Ponzo Illusion. Here converging parallel lines tell the brain that the line higher in the visual field is farther away perceiving it to be much larger when in reality they are the same length.
Motion perception can also create illusions. Animations are based on the illusion that the brain sees a series of slightly varied images produced rapidly one after the other as a moving picture. When we are moving fast, like in a car, surrounding objects may appear to also move. We also perceive big objects to be moving slower than smaller objects. A good example of how the brain perceives motion can be seen in the
Phi phenomenon.
It is thought that optical illusions are due to a neural lag that most humans experience while awake. When light hits the retina, about one-tenth of a second goes by before the brain translates the signal into a visual perception of the world.

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