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Tease your brain with the Hermann Grid illusion

Here’s a beau­ti­ful exam­ple of the Her­mann Grid illu­sion in the real-world: You will notice faint white cir­cles appear where the hor­i­zon­tal and ver­ti­cal slats inter­sect in the win­dow frame (even if no white cir­cles are there at all!)

The Hermann Grid illusion

In this image, do you see some­thing oth­er than black and white? 

Hermann Grid

While there are only black blocks and white spaces in the grid, the high con­trast black and white areas fool the eyes into per­ceiv­ing a gray cir­cle at each inter­sec­tion. The illu­sion results from reti­nal cells adjust­ing the bright­ness of an image by adjust­ing the inten­si­ty of the light sig­nal in many small sec­tions.  The size of these small sec­tions is deter­mined by the size of neur­al recep­tive fields in the reti­na.

The fovea, an area near the cen­ter of your reti­na with the high­est res­o­lu­tion vision, has the small­est recep­tive fields and the high­est num­ber of pho­tore­cep­tor cells. Areas of your reti­na ded­i­cat­ed to periph­er­al vision have larg­er recep­tive fields, and there­fore, low­er res­o­lu­tion view­ing.

In the Her­mann Grid, the inter­sec­tion that you are look­ing direct­ly at falls onto the fovea, which has very lit­tle lat­er­al inhi­bi­tion due to the small recep­tive field and high con­cen­traHermann Grid Explanationtion of pho­tore­cep­tors. Our per­cep­tion with­in this area is fair­ly accu­rate, and we see the area as either white or light gray. In the area with­in the green cir­cle, the dark and light areas are bal­anced, so no gain adjust­ment is need­ed, and we see this area as white. At the periph­er­al inter­sec­tions (pur­ple cir­cle), most of the recep­tive field is flood­ed with white light, caus­ing strong lat­er­al inhi­bi­tion which results in reduced gain and an area that appears gray.


For more visual illusions and brain teasers:

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As seen in The New York Times, The Wall Street Journal, BBC News, CNN, Reuters,  SharpBrains is an independent market research firm tracking how brain science can improve our health and our lives.

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