Stare at the black dot in the centre of this image. The longer you stare, the more the grey haze surrounding the dot will disappear.
Our focal point is the dot itself; it is sharply defined against the grey and so we are naturally drawn to it. The grey shades, by contrast, appear to be out of focus, allowing them to recede the longer we stare. Notice that the shades of grey grow darker towards the centre. Our brains focus on this area, allowing the outer circle to practically vanish. Our eyes detect light, but our brains interpret it differently.
Keep staring at the black dot...
After a while the gray haze around it will appear to shrink.
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