Look at the cross below. Count the dark dots you see at intersections.
Now look directly at one intersection -- the dot vanishes!

Hermann Grid

Gray ghost dots appear at intersections of the white lines -- but vanish when you look directly at them!

Lateral inhibition in retinal ganglion cells creates this effect. Each cell's receptive field is inhibited more at intersections (4 bright lines meeting) than along edges (2 bright lines), creating a darker signal at intersections in your periphery.

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