Critical Thinking and Learning Site

As our Eyes and Senses Deceive us, so does our Mind


Another part of the website is the execution of experiments, with emphasis on social experiments.

Why am I doing this? I though that on one hand this is an active learning process; thinking, designing, conducting the experiment and then analyzing the results would help me remember a particular part of knowledge. On the other hand, in combination with the Open Minds (serious Think Tank based in Greece), it would be a nice opportunity to conduct a research and help other people in the process of critical thinking. So a two-birds-with-one-stone thing.

The previous week, we conducted a small experiment in the form of a two-question quiz. Before I explain why we did that, and the purpose of the quiz, let’s see what those questions were.

The first question was followed by the following image, with the question “Which square is darker A or B?”

The second question  consisted of another image with the question, “which of the two lines is longer?”


The brief solution and the reason I chose those two questions.

In the first one, with the squares, A has the exact same color with B. You can check that by either testing the picture in photoshop (or another photo editing program) to see the colors’ hex, or you can simply take a piece of paper, make two holes on it, so that you can cover the entire picture and leave only the two squares uncovered.

In the second one, the two lines have the same length. You can simply take a ruler and measure them.

Another optical illusion? That is not the point.

The most amazing thing is that, even though you can check and confirm (with the methods I mentioned before) the previous statements , and even though later you will receive the scientific explanation, you keep seeing square A as darker than square B, and the first line as longer than the second one. You know it, you test it, but still your eyes deceive you.

The point that I want to illustrate and the most important part is that if that happens with our eyesight, our hearing and our senses, what makes you believe the same thing isn’t happening with our mind?

That means that our mind tricks us. It tricks us in the way we think, in the way we decide and act. And if an optical illusion causes no harm, our biased mind can affect us heavily in our lives.

The results of the first question

As you can see, in the first question the vast majority (80%) answered that square A is darker than square B, with a 13% answering they are the same and a 6% answering B is darker.

Scientific and simplified Explanation

The scientific explanation of the first one, according to Professor Adelson, the creator of  the illusion, is the following:

 “The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view. “


Long story short, the two major factors that trick our eyes are:

a)The  4 darker squares surrounding square B makes us perceive square B as lighter when compared to the other squares.

b)The second is that our visual systems try to determine (imagine) the colour of the squares without  the shadow from the cylinder. Thus,  we perceive square as brighter than it actually is.

The results of the second question

As far as this question is concerned, the things were almost 50/50. With 50% asnwerging that  the first line as longer, while the other 49% answering they are the same.

The simple explanation is that, in a two dimension world, (like the lines  in the question or cartoons) our brain makes assumptions about the depth and space of objects. So, our eyes say that the two lines have the same length but our brain says that this is a depth issue, so they argue. In this kind of battles , brain usually wins.

If you want an in depth scientific explanation, I quote part of an article from Rochester Institue of Technology (click it to see the whole exlanation)

“In the three-dimensional world, depth perception concerns judging distance. The closer an object is to the retina, the larger it is on the retina. However, in the two-dimensional world of the Muller-Lyer illusion, our brain makes assumptions about the relative depths of the two shafts based on monocular (pictorial) cues. We are used to seeing outside corners of buildings as near to us with the top and bottom of the corner sloping out and away (like the outward slanting fins of the Muller-Lyer illusion). We are used to seeing inside corners of buildings as farther from us with the top and bottom of the corner sloping in somewhat towards us (like the inward slanting fins of the Muller-Lyer illusion).

The retina is saying that the two shafts are the same length but the brain is interpreting the Muller-Lyer as a depth issue, with the shaft that looks like an outside corner being closer and the shaft that looks like an inside corner being farther away. In other words, the retina is saying “two shafts equal” and the brain is saying “outside shaft shorter than inside shaft”. The brain usually wins differences like this. Thus, the brain sees as longer than.

(Source: )

All in all, I have to repeat myself because this is a major point, and it will be the basis on the following articles to come.

If our eyesight, our hearing, and our senses trick us, what makes you believe the same things isn’t happening with our mind.




About the author

By Plato
Critical Thinking and Learning Site

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