How Do Visual Illusions Work?

The illusion is a very interesting concept. Some illusion amazes us to the level to dig into the understanding of perception. Let’s try to define it.

As per its definition, “An illusion is a distortion of the senses, which can reveal how the human brain normally organizes and interprets sensory stimulation.” Though these illusions distort our perception of reality, they are shared by many people. These illusions are based on some assumptions of the brain which it makes during perception. Perceptual illusion, in specific, is “the perception of something objectively existing in such a way as to cause misinterpretation of its actual nature.” The illusion occurs when the brain processes the visual data you transmit to your brain. Let’s look into some famous illusions.

     Great Optical Illusions

1. Muller Lyer Illusion:

Question- Which of the two lines is bigger, upper one or lower one?

It is a classic Muller-Lyer illusion. Perhaps, this type is the most famous of all. In this, there are two stylized arrows, one of which ends in inward-pointing arrows, the other which ends with outward-pointing arrows, like in the picture. The angle and direction of the arrowheads affect the ability to accurately perceive the length of the lines. While observing the two lines, the lower line appears to be significantly shorter than the other.

Reason – The first explanation is the size constancy explanation, which is the reason for the object to be stable as per size. This results in a better perception of the 3D world but creates errors in the 2D world. Another explanation is of depth cues, our ability to judge distance.

2. Ponzo Illusion:

Question- Which of the two lines is bigger, upper one or lower one?

In the Ponzo illusion, a pair of converging lines, like rail lines, distorts the perception of two identically sized lines. In the above figure, the two converging lines mimic railroad tracks disappearing into the distance. Two horizontal lines or bars are placed across these tracks, one above the other. When looking at the image, people perceive the upper line as larger than the lower one.

Reason – The Ponzo Illusion is explained through perspective hypothesis, where the converging lines, which are associated with distance, is the perspective feature in the figure. The framing effects hypothesis contributes to the magnitude of the distortion.

3. Hering Illusion:

Question- Are two red lines parallel?

In the Hering illusion, the two vertical lines are both straight, but they look like they were bowed outwards when presented in front of a radial background. The viewer appears to see a pair of bowed or curved lines placed in these radial lines.

Reason – It is due to the overestimation of the angle made at the points of intersection.

4. Poggendorff Illusion:

Question- Which of the following lines is the continuation of the upper line?

In the Poggendorff illusion, the perception of the interaction between diagonal lines and horizontal and vertical edges is visualized. In the above figure, the thin diagonal lines are positioned at an angle behind wider stripes. When observing these thin lines, they appear to be misaligned; the black lines appear to be a continuation of the blue line, rather it is of the red line.

Reason – It is a geometrical illusion, where the opaque block causes the acute angles between the lines and block to expand. The visual system enhances the orientation contrast at the point of intersection.

5. Zollner Illusion:

Question- Are the following lines parallel?

The Zöllner illusion is a classic optical illusion. In this, a pattern surrounding parallel lines creates the illusion that they are not parallel. The above figure is composed of a series of parallel, diagonal lines intersected by a number of short horizontal and vertical bars. The parallel lines instead of parallel appear to converge and diverge from each other.

Reason – Here, the acute angle formed by the horizontal lines and the short lines appear to expand, resulting in tilting appearances of the diagonal lines.

6. Moon Illusion:

Question- Does the moon sometimes appear bigger than other times?

The Moon illusion is a classic illusion of everyday life, where the moon in the sky appears larger near the horizon than it does while up high in the sky. On looking at the moon while it is high overhead, it appears to be smaller than when it is close to the horizon. In some instances, people fail to realize that the moon has remained the same size.

Reason – The perceived size of the moon is based on the actual size and angular size, both. As angular size varies from time to time, the perceived size also changes.

7. Ebbinghaus Illusion:

Question- Are two orange circles of the same size?

The Ebbinghaus illusion is the one of relative size perception, where two circles of identical size are placed near to each other and one is surrounded by large circles while the other is surrounded by small circles. Due to the construction, the first central circle then appears smaller than the second central circle.

Reason – It can be explained by the typical tendency of context by the Gestalt law, rather it is the proof of it.

8. Grid Illusion:

Question- Do you see the dots between the lines?

A grid illusion is any grid that deceives a person’s vision. Though there are many, there are two most common types of grid illusions- Hermann grid illusions and Scintillating grid illusions. The Hermann grid illusion is the above figure, created with a grid of black squares upon a white background. Some grey figures are perceived at the intersections of these white lines, which disappear when one looks directly at an intersection. The Scintillating grid illusion is similar than the previous, but instead of white bars, there have grey bars with white discs inserted at each intersection between them. When viewers move their eyes around the image, black dots seem to appear and disappear.

Reason – The explanation for this type of illusion is the effect of the inhibitory process in the retinal ganglion cells, the neurons that transmit signals from the eye to the brain.

References:

1. https://www.ucg.org/illusion
2. https://quizlet.com/376248409/psych-101-test-2-flash-cards/
3. https://www.newworldencyclopedia.org/entry/Ponzo_illusion
4. https://en.m.wikipedia.org/wiki/Illusion
5. https://www.newworldencyclopedia.org/entry/Ebbinghaus_ill