Optical illusions: what they are and why they confuse our brain

How many times have you wondered about what you see? 😵 ‍ 💫

Optical illusions are images that challenge our visual perception and they reveal a lot about how the brain processes reality.

In this article, we explore some of our favorite visual illusions — like the famous white/gold or blue/black dress — and explain what's behind these visual confounds, based on evidence from neuroscience.

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What is an optical illusion?

An illusion of optics (or illusion of optics) consists of images that are picked up by the eyes in one way, but interpreted differently by the brain.

In other words, we see something that does not exactly correspond to what is in front of us.

The brain interprets visual stimuli based on previous experiences, context and expectation and sometimes makes mistakes.

According to an article published in Nature Reviews Neuroscience(2015), these illusions are natural manifestations of “cognitive heuristics”: shortcuts that the brain uses to quickly decipher the environment.

Although these shortcuts work in most situations, they can cause perceptual distortions.

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Our 5 favorite optical illusions

Let's get to know five fascinating visual illusions that show how the brain can be easily deceived:

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1. Pareidolia Optics Illusion: faces on the tree

This image shows a seemingly ordinary tree, but with attention it is possible to identify several faces between the roosts and the trunk.

This type of optical illusion is an example of pareidolia: a psychological phenomenon in which the human brain interprets ambiguous visual stimuli as being familiar forms, especially faces.

For example, have you ever looked at the clouds and identified an animal? Or did you find yourself noticing a wall outlet that seemed to be smiling? 😅 That's pareidolia!

The human brain has an incredible ability (and a certain tendency) to look for patterns in everything it sees.

When faced with ambiguous or indefinite stimuli, he tries to “fill in the blanks” based on his previous experiences. Sometimes he gets it right... other times, he just makes it up.

👉 But let's go to what interests you, the answer:Although some people report seeing a larger or smaller number, the most consensual interpretation indicates that it is possible to identify 10 facesdifferent in the image.

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2. Rubin Glass Optical Illusion

Created by Edgar Rubin in 1915, this illusion demonstrates how the brain perceives visual boundaries, alternating between seeing a glass in the foreground or two faces as the background.

Simply put, the brain constantly needs to decide which part of an image represents the main object and which corresponds only to the background.

Researchers at the Salk Institute have discovered that neurons in the deepest layers of the visual cortex are the first to attribute this border property. They function as “central decision-makers”, processing visual information and defining what is figure and what is background.

This ability to quickly distinguish objects from the background is essential to our perception and survival. It is what allows us, for example, to differentiate a stone from a simple shadow when driving on a road - something vital to safely interact with the environment around us. ⚠️

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3. Illusion of Dress Optics: blue and black or white and gold?

In February 2015, this simple photograph rocked the internet. For some, it was white and gold. For others, blue and black.

What seemed to be just another viral image turned into a real case study in the field of vision science.

👉 But then, after all, what is the actual color of the dress? Blue and black.

The author of the picture herself confirmed this information by showing the dress physically. And, for the most skeptical, it was enough to use design tools to confirm the bluish and brown/black tones of the original image:

The explanation?Our brains are adept at interpreting the world based on assumptions. When an image has ambiguous lighting (and this was captured with a mobile phone, in uncertain light conditions), the brain has to decide: “is it in the shade or in direct light?”

Depending on the answer, it compensates the colors differently:

• If you think that the dress is in the natural shade, the brain removes the bluish tones, and sees white and gold. ⬜🟨
• Assuming that it is artificially lit, it keeps the dark and bluish tones, and sees blue and black. 🟦⬛

A European study published in the journal Vision Research demonstrated that this variation in color perception is related to individual differences in how we interpret surfaces and lighting. In other words, each brain solves the “puzzle” of light in its own way, based on previous visual experiences.

Interestingly, other researchers suggest that their biological rhythm can also influence perception: people who wake up early (the so-called “cotovias”) tend to see the dress as white and gold, while the “corujas” (more active at night) tend to see blue and black.

Interesting, isn't it?! 🙂

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4. Illusion of Ebbinghaus Optics (or Titchener's Circles)

This illusion was first described by the German psychologist Hermann Ebbinghaus, at the end of the 19th century, and popularized by Edward Titchener (which is why it is also known as the Titchener circles illusion).

At first glance, it looks like a simple observation exercise: two central circles surrounded by other circles of different sizes.

👉 Your brain probably identified the lower black circle as being larger than the upper one. However, we guarantee that both are exactly the same size:

🧠 What happens in the brain? The illusion occurs because our brain interprets size based on comparisons. When a central circle is surrounded by large circles, it appears smaller; when it is surrounded by small circles, it appears larger.

This phenomenon can be explained by multiple simultaneous mechanisms and, in fact, scientists have not yet come to a definitive consensus:

• Some theories suggest that the brain performs a size contrast: the larger the surrounding context, the smaller the central object will appear.
• Other researchers focus on “lower” mechanisms, at the level of the visual cortex, where neurons that process close contours interact with each other, influencing the overall perception of shape.
• There are also hypotheses linked to the constancy of size: the central circle may appear more distant or closer depending on its context, which changes its perception of scale.
• More recently, researchers have pointed out that the total spatial extent of the set and the relative size of the surrounding circles are two key factors for the degree of illusion.

This is a classic example of how our brain does not evaluate objects absolutely, but according to context!

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5. Illusion of Boring Optics (or Young and Old)

This classic illusion, originally published in 1915 by cartoonist W.E. Hill, became known as Boring's Illusion after being analyzed by psychologist Edwin Boring in the 1930s.

Some see an elegant young woman in profile, looking at the background of the picture; others see an elderly lady turned to the left.

👉 Both interpretations are correct:

What each person sees in the first place depends on their individual perception, and the brain can switch between both versions as it looks at the figure, depending on the focus of attention.

This is because it is aambiguous stimulus, which can be interpreted in different ways without the physical stimulus being altered. 🙃

Although it is still a process without great concrete answers, there are 2 main theories that seek to explain this phenomenon:

• Bottom-up (sensory) theory: He argues that perception changes automatically due to the adaptation of neurons in the initial visual areas. These “get tired” of one interpretation, allowing another to emerge.
• Top-down (cognitive) theory:It suggests that change is influenced by attention, expectation, and prior knowledge. In other words, the brain actively chooses what to interpret, based on higher-order mental processes.

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Final Considerations

Optical illusions show that the reality we perceive is a mental construction, based on expectations and interpretations.

These images not only surprise but also reveal the limits and sophistication of the human brain!

At NeuroImprove Clinic, we use advances in neuroscience to promote brain and psychological health.

If you liked this article, share it with your friends — challenge them to see the world with different eyes! 🧠✨

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References

Liu, J., Li, J., Feng, L., Li, L., Tian, J., & Lee, K. (2014). Seeing Jesus in toast: Neural and behavioral correlates of face pareidolia. Cortex, 53, 60—77. https://doi.org/10.1016/j.cortex.2014.01.013

Franken, T. P., & Reynolds, J. H. (2021). Columnar processing of border ownership in primate visual cortex. eLife Sciences, 10, e72573.https://doi.org/10.7554/eLife.72573

Witzel, C., O'Regan, J. K., & Hansmann-Roth, S. (2017). A dress and individual differences in the perception of surface properties. Vision research, 141, 76—94. https://doi.org/10.1016/j.visres.2017.07.015

Wallisch, P. (2023). Here's why people saw “The Dress” differently. Slate. https://slate.com/technology/2017/04/heres-why-people-saw-the-dress-differently.html 

Kirsch, W., & Kunde, W. (2021). On the origin of the Ebbinghaus illusion: The role of figural extent and spatial frequency of stimuli. Vision Research, 189, 1—10.https://doi.org/10.1016/j.visres.2021.07.016

Kornmeier, J., & Bach, M. (2012). Ambiguous figures - what happens in the brain when perception changes but not the stimulus.Frontiers in human neuroscience, 6, 51. https://doi.org/10.3389/fnhum.2012.00051 

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