Visual illu­sions are fun and inter­est­ing: They can tell us a lot about our brain.

The brain has two hemi­spheres, each divided into four lobes. Each lobe is respon­si­ble for dif­fer­ent func­tions. For instance the frontal cor­tex (in blue here) is respon­si­ble for deci­sion mak­ing and plan­ning; the tem­po­ral lobe (in green) for lan­guage and mem­ory; and the pari­etal lobe (in yel­low) for atten­tion and spa­tial skills. The occip­i­tal lobe (in red) is entirely devoted to vision: It is thus the place where visual illu­sions happen.

The frontal lobe rep­re­sents 41% of total cere­bral cor­tex vol­ume; the tem­po­ral lobe 22%; the pari­etal lobe 19%; and the occip­i­tal lobe 18%.

There is thus a huge part of our brain devoted to pro­cess­ing visual infor­ma­tion. How the visual sys­tem processes shapes, col­ors, sizes, etc. has been researched for decades. One way to under­stand more about this sys­tem is to look at how we can trick it, that is to look at how the brain reacts to visual illusions.

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Here are 10 visual illu­sions to com­bine fun and learn­ing about the visual system.

We know you know there is a trick since these are illu­sions… but don’t try to be smarter than your brain: Just enjoyed being tricked!

To go beyond the illu­sions, read about what hap­pens in your brain while you expe­ri­ence them.

1. Can you put the fish in the fishbowl?

Stare at the yel­low stripe in the mid­dle of the fish in the pic­ture below for about 10–20 sec. Then move your gaze to the fish bowl.

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2. Are the squares inside the blue and yel­low squares all the same color?

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3. Are the hor­i­zon­tal lines straight or crooked?

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4. Are the cir­cles sta­tic or moving?

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5. How many legs does this ele­phant have?

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6. Are the two hor­i­zon­tal lines of the same length?

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7. Do you see gray dots at the inter­sec­tions of the white lines?

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8. Are the two orange cir­cles of the same size?

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9. Does Lincoln’s face look normal?

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10. Can you see a baby?

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Check your answers and learn about what was going on in your brain while you expe­ri­enced each of these illusions:

1. Can you put the fish in the fishbowl?

Did you see a fish of a dif­fer­ent color in the bowl? You have just expe­ri­enced an after­im­age.
In the retina of your eyes, there are three types of color recep­tors (cones) that are most sen­si­tive to either red, blue or green. When you stare at a par­tic­u­lar color for too long, these recep­tors get “fatigued.” When you then look at a dif­fer­ent back­ground, the recep­tors that are tired do not work as well. There­fore, the infor­ma­tion from all of the dif­fer­ent color recep­tors is not in bal­ance. This will cre­ate the color “afterimages.”

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2. Bezold effect

The smaller squares inside the blue and yel­low squares are all the same color. They seem dif­fer­ent (magenta and orange) because a color is per­ceived dif­fer­ently depend­ing on its rela­tion to adja­cent col­ors (here blue or yel­low depend­ing on the outer square).

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3. Café Wall Illusion

The hor­i­zon­tal lines are straight, even though they do not seem straight.  In this illu­sion, the ver­ti­cal zigzag pat­terns dis­rupt our hor­i­zon­tal perception.

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4. Illu­sory Motion

The cir­cles do appear to be mov­ing even though they are sta­tic. This is due to the cog­ni­tive effects of inter­act­ing color con­trasts and shape position.

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5. How many legs does this ele­phant have?

Tricky, isn’t it?! This pic­ture is an impos­si­ble pic­ture that also con­tains some sub­jec­tive con­tours, such as the Kanizsa Tri­an­gle below: A white tri­an­gle (point­ing down) can be seen in this fig­ure even though no tri­an­gle is actu­ally drawn. This effect is known as a sub­jec­tive or illu­sory con­tour. The con­tour of the tri­an­gle is cre­ated by the shapes around it.

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6. The Mueller-Lyer Illusion

The two hor­i­zon­tal lines are of the same length, even though the one at the bot­tom seems longer.
As you know, the visual angle gets smaller with dis­tance, so the brain auto­mat­i­cally per­ceives objects at far­ther dis­tances to be big­ger.
In gen­eral, lines that have inward flaps, such as cor­ner of a build­ing, are rel­a­tively the near­est points of the over­all object. Sim­i­larly, lines with out­ward flaps are found at the longer dis­tance, as the far­thest cor­ner of a room.
So in the Mueller-Lyer illu­sion, the brain per­ceives the line with out­ward flaps to be at a far­ther point as com­pared to the line with inward flaps. Con­se­quently, the brain per­ceives the line with out­ward flaps to be longer.

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7. Her­mann grid illusion

There are not gray dots in this grid. How­ever “ghost­like” gray blobs are per­ceived at the inter­sec­tions of the white lines. The gray dots dis­ap­pear when look­ing directly at an inter­sec­tion.
This illu­sion can be explained by a neural process hap­pen­ing in the visual sys­tem called lat­eral inhi­bi­tion (the capac­ity of an active neu­ron to reduce the activ­ity of its neighbors).

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8. The Ebbing­haus Illusion

The two orange cir­cles are exactly the same size, even though the one on the left seems smaller.
This size dis­tor­tion may be caused by the size of the sur­round­ing cir­cles or by their dis­tance to the cen­ter circle.

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9. Does Lincoln’s face look nor­mal?

It seems nor­mal but now, look at it upright: Lincoln’s eyes do not look quite right!

Some neu­rons in the brain seem spe­cial­ized in pro­cess­ing faces. Faces are usu­ally seen upright. When pre­sented upside down, the brain no longer rec­og­nizes a pic­ture of a face as a face but rather as an object. Neu­rons pro­cess­ing objects are dif­fer­ent from those pro­cess­ing faces and not as spe­cial­ized. As a con­se­quence these neu­rons do not respond to face dis­tor­tions as well. This explains why we miss the weird eyes when the face is inverted.

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10. Can you see a baby?

Another great exam­ple of an illu­sory con­tour! The baby’s head is on the left, the baby’s feet are against the trunk of the tree on the right.

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I hope you had fun and learned inter­est­ing facts about your brain!

Let’s end by enjoy­ing a pic­ture cre­ated by the mas­ter of illu­sion: M. C. Escher

Be Socia­ble, Share!

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