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The Hermann grid optical illusion, explained

In this image, do you see some­thing oth­er than black and white? Does it mat­ter if you look direct­ly at one of the black blocks, vs. at one of the inter­sec­tions?

Hermann Grid

While there are real­ly 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, which allows you to see a wide range of both bright and dark details in the same image–unlike a com­put­er mon­i­tor or TV screen that has one bright­ness set­ting for the entire image.

Retina Anatomy 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.

 

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