The Hermann Grid Visual Illusion

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How many col­ors do you see in this image?

Hermann Grid


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.

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. 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.

Hermann Grid Explanation

 

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Comments

  2. Caroline says

    Ben —

    Good ques­tion. The answer is “depends”.

    Almost all vis­i­ble col­ors are cre­at­ed using one of two sys­tems of pri­ma­ry colors. 

    Trans­mit­ted col­ors (RGB) use red, green and blue. Col­or dis­plays use red, green, and blue pix­els for the dis­play screen. In this sys­tem, col­ors are added to cre­ate white and sub­tract­ed to cre­ate black.

    Reflect­ed col­ors (CMY) use cyan (light blue), magen­ta (pur­plish-red), and yel­low. Col­or print­ing uses CMY (col­ors are sub­tract­ed to cre­ate white but added to make black). In the­o­ry, equal parts of cyan, magen­ta and yel­low ink make black, but the blacks tend to be mud­dy. Thus, a pure black fourth ink is always used in the four col­or CMYK process (K for blacK).

    So, black can be all col­ors or the lack of col­or! It depends if you are reflect­ing or transmitting.

  7. Theofanis says

    two colours, the grey is not on the paper, your mind cre­ates by the com­bi­na­tion of black and white

  8. Katie says

    in all tech­ni­cal­li­ty, there are no “col­ors” in the field. There are 3 shades, black, gray and white. I did­n’t see the gray until I read on about it. It was there but I just did­n’t real­ize it was a part on the question.

  9. catiana271 says

    Looks like the total absence of col­or and all the col­ors to me
    (black and white)
    I don’t see the gray