Did you know that over 50% of the cortex, the large outer layer of the brain, is devoted to vision? Optical illusions illustrate that the mind makes assumptions about the world. This gifographic covers optical illusions from primitive forms starting with the Ancient Greeks in 350BC, to 21st-century illusions in the field of theoretical neurobiology.
Visual, or optical, illusions show us that our minds tend to make assumptions about the world and what you think you see is often not the truth.
Throughout history, curious minds have questioned why our eyes are so easily fooled by these simple drawings. Illusions, we have found, can reveal everything from how we process time and space to our experience of consciousness.
Illusions have a long history, going as far back as the ancient Greeks.
In 350BC, Aristotle noted that “our senses can be trusted but they can be easily fooled”.
He noticed that if you watch a waterfall and shift your gaze to static rocks, the rocks appear to move in the opposite direction of the flow of water, an effect we now call “motion aftereffect” or the waterfall illusion.
Tracking the flow of the water seems to “wear out” certain neurons in the brain as they adapt to the motion. When you then shift your gaze to the rocks, other competing neurons over-compensate, causing the illusion of movement in the other direction.
In the 1960-70s illusions inspired a style called optical art, or “Op-Art”. Victor Vasarely is widely regarded as the father of this movement, and some of his work is studied by scientists today. For example, research using his “nested squares illusion”, similar to the image below, suggests that the brain identifies shapes using corners rather than lines.
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Today, Optical Illusion research is booming once more. Technology advances now allow scientists to peer inside our brains as we look at illusions, and to begin to understand the underlying mechanisms going on inside our head.
Functional magnetic resonance imaging (fMRI) allows researchers to analyze how the neurons in our brain respond to individual illusions.
Martinez-Conde is now building on the work of some of the 19th Century researchers. It was Helmholtz, for example, who first realized that our eyes make rapid movements called saccades.
To experience them, gently put a finger on your eyelid and move your eye. You will see that the world will start to appear jittery, like a series of snapshots. We don’t notice our eye darting about like this because our brain smoothes things out when constructing what we see.
Martinez-Conde realized that these saccades might help to explain why we see movement in this image, the snake illusion.
All of this research points to one thing: our visual system remains too limited to tackle all of the information our eyes take in. “For that our brain would need to be bigger than a building, and still then it wouldn’t be enough,” says Martinez-Conde.
And so our minds take shortcuts. Like betting on the best horse in a race, our brain constantly chooses the most likely interpretation of what we see.
Seeing, then, is certainly not always believing.
To see more of Optical Illusion topic BBC Team’s Article.