How many col­ors do you see in this image?

While there are really only two col­ors 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.

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. Inhibitory lat­eral con­nec­tions between cells in the periph­eral retina help deter­mine the edges of the rough shapes per­ceived with the low res­o­lu­tion periph­eral sight. These lat­eral con­nec­tions turn down brightness.

In the Her­mann Grid, the inter­sec­tion that you are look­ing directly at (orange cir­cle) falls onto the fovea, which has very lit­tle lat­eral inhi­bi­tion due to the small recep­tive field and high con­cen­tra­tion 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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