You will notice that where the white lines intersect, there is black on four sides, whereas the lines themselves are surrounded by black on only two sides. When you look at the intersections, the cells in the retina are surrounded on four sides by other cells that are also receiving light. They are therefore more inhibited than the cells focused on the lines. It is their inhibition that causes the dark spots to appear.
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Medical Student Who Is Striving Hardly for M.D. [May Allah SWT bless]