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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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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