5 MOST AMAZING OPTICAL ILLUSIONS (AND HOW THEY WORK)

Optical illusions freak our mind, but in my case they do play a role in many ways, notably in training sessions. When trainees get bored or sleepy (normally after lunch break), optical illusion exercises are often used to refresh their minds or as a wake-up call. The top image is an example. In most cases, or at least initially, the illusions fool their brain and they end up more confused than before. In all cases, such exercises are meant not only to re-energize their brains but also as lessons in managing their work (e.g. perception vs reality, facts vs assumptions, etc.)

Some optical illusions have ready-made answers, while others still elude explanation. The following article presents those that are explainable. Without the explanations and without having seen them before, could you have correctly given the answers?

The Most Amazing Optical Illusions (and How They Work)
Natalie Wolchover, Life's Little Mysteries.

Introduction

Optical illusions harness the shift between what your eyes see and your brain perceives. They reveal the way your visual system edits images before you're even made aware of them - like a personal assistant, deciding what is and isn't worthy of your attention.

People were creating optical illusions long before we knew what made them work. Today, advances in neuroscience have pinpointed the visual processes that fool your brain into falling for many of them. Others still elude explanation.

Here, a selection of eye- and brain-boggling illusions, and explanations of how they work.

5. Checkered Shadow

On the checkerboard below, tile A looks much darker than tile B. Remarkably, as seen in the revised image below, A and B are actually exactly the same colour. In an image editing program, they'll both register an RGB value of 120-120-120.

Edward Adelson, a professor of vision science at MIT, created this so-called "checker shadow illusion" in 1995 to demonstrate how the human visual system deals with shadows. When attempting to determine the colour of a surface, our brains know that shadows are misleading - that they make surfaces look darker than they normally are. We compensate by interpreting shadowy surfaces as being lighter than they technically appear to the eye. [Why Do We See In 3-D?]

Thus, we interpret square B, a light checkerboard tile that is cast in shadow, as being lighter than square A, a dark checkerboard tile. In reality, the shadow has rendered B just as dark as A.

4. Disappearing Light

After staring at the blinking light in the centre of the above video for about 10 seconds, the yellow dots spaced evenly around it start to disappear. One might vanish, then reappear only to have another go away. Two or three of the dots may fade and re-emerge together. These disappearances and reappearances continue at random for as long as you stay focused on the blinking light - it's downright impossible to train your brain to keep them all in the picture.

This mind trick, called motion induced blindness, has no universally accepted explanation, but research suggests that effect arises in the primary visual cortex, the part of the brain that processes information about static and moving objects.

3. Lilac Chaser

Fixate on the crosshairs. After 20 seconds or so, the fuzzy lilac dots fade to grey. The absence of a dot, which hops around the chain, becomes a rotating dot of green.

This visual trickery is called Troxler's fading, or Troxler's effect, and was discovered by Swiss polymath Ignaz Paul Vital Troxler in 1804. The effect results from the ability of our visual neurons to switch off their awareness of things that aren't changing, and heighten their perception of things that are. In the above image, the lilac dots stay still while the absence of the dots moves. Thus, after a brief figuring-out period, the visual system transitions to focusing only the moving blank dots - which it turns green because of a second illusion at play here - and lets the immobile lilac dots fade. [Why Do We See In Colour?]

Other human sensory systems behave similarly. If a bug lands on your arm, for example, you can feel it at first. But if it stands still for a few seconds, you lose the physical sensation of its presence. Only when it keeps walking, giving varying stimulation to your tactile neurons, do you keep feeling it.

As for the other optical illusion, the blank dot turns minty green because your retina has been oversaturated with the lilac coloured dots. When the lilac is removed from the spots, you see its complementary colour (minty green) instead, which is composed of white light minus the lilac.

2. Hering Illusion

Credit: Fibonacci | Creative Commons

In this geometrical-optical illusion, discovered by the German physiologist Ewald Hering in 1861, two straight and parallel lines look as if they bow outwards. Hering ascribed the effect to our brains overestimating the angle made at the points of intersection between the radiating lines and the red ones. But why do we miscalculate? [How Do Calculators Calculate?]

Researcher Mark Changizi of Rensselaer Polytechnic Institute in New York believes it has to do with the human tendency to visually predict the near future. Because there's a lag between the time that light hits the retina and the time when the brain perceives that light, Changizi thinks the human visual system has evolved to compensate for the neural delay by generating images of what will occur one-tenth of a second into the future. He explained the Hering illusion in a 2008 article on LiveScience, a sister site to Life's Little Mysteries:

"Evolution has seen to it that geometric drawings like this elicit in us premonitions of the near future. The converging lines toward a vanishing point (the spokes) are cues that trick our brains into thinking we are moving forward - as we would in the real world, where the door frame (a pair of vertical lines) seems to bow out as we move through it - and we try to perceive what that world will look like in the next instant."

1. Gradient Illusion

Credit: Dodek | Creative Commons

The horizontal bar in the above image looks gradated, moving from light to dark grey in the opposite direction as the background. You may have already guessed it: This is just a trick of the mind. If you cover everything but the bar itself, you'll see that it's actually monochrome.

The so-called "simultaneous contrast illusion" is similar to the checker shadow illusion shown in the first slide. The brain interprets the two ends of the bar as being under different illuminations, and deduces what it thinks the bar's true shading would be (if it were lit evenly along its length). It deduces that the left end of the bar is a light grey object in dim lighting. The right end looks like a darker object that is well-lit.

[Source: Life's Little Mysteries. Edited. Top image added.]