Question

Why the hermann grid illusion produces illusory spots in the periphery but not in the fovea?

Answer 1

Consider two regions of your retina. One region views an intersection of a white horizontal and vertical band and the other region views a white band between two intersections (the region going away from the intersection). Although, the two regions themselves receive the same amount of light, the situation in their neighboring regions is different. At the intersection, there is light coming in from all four sides, but in the white band that lies between the two intersections it is surrounded by two dark sides.

    This leads to a physiological mechanism called lateral inhibition, which has the effect of causing a bright surround to an area appear darker and, conversely, a dark surround will make an area appear lighter.