#8. Test your Brain with these 10 Optical Illusions


Con­trast your answers with the ones below, 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.” 

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)..

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.

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.

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.

empty triangle illusion test.


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..

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)..

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..

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..

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..

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


Next in Sharp­Brains’ top 25 brain teasers series:

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  1. Steven on October 30, 2010 at 1:57

    Most of these are old, but some I haven’t seen before. I absolute­ly love opti­cal illusions.

  2. T.L. on November 2, 2010 at 5:27

    Awe­some! I like being tricked even when I know its hap­pen­ing. Too cool.

  3. Geroboam on November 2, 2010 at 6:49

    Old sal­sa.

  4. Lenny on November 8, 2010 at 7:04

    These are pret­ty neat!!! I am always so fas­ci­nat­ed by opti­cal illusions!!

  5. T. H. on November 13, 2010 at 5:27


  6. Pascale on November 15, 2010 at 8:10

    Hi T.H., There is noth­ing spe­cial to be seen in Escher’s draw­ing. It is a great exam­ple of an impos­si­ble object, just for your eyes’ pleasure!

  7. zach on November 22, 2010 at 6:26


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SHARPBRAINS is an independent think-tank and consulting firm providing services at the frontier of applied neuroscience, health, leadership and innovation.
SHARPBRAINS es un think-tank y consultoría independiente proporcionando servicios para la neurociencia aplicada, salud, liderazgo e innovación.

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