scintillating grid illusion

Harmann grid illusion

Do you see the gray spots at the intersections? If you stare directly at any one of them, it will disappear. Why? "scintillating grid illusion" and "Harmann grid illusion" are the optical illusion which are called "grid illusion" as the generic name and have the almost same effect and cause. At the intersections, there is light coming in from all four sides, but in the white band, 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.

「シンティレイト（閃光）錯視」、あるいは「ハーマン・グリッド錯視」はほぼ同じ効果と理由によって生じる錯視。格子の交差部に暗い点があらわれるのは「側枝抑制」という知覚機能が働くため。交差部では四方からの白線のため、すなわち回りが明るいため交差部自体が暗く見える。逆に黒い四角形の間の白線は両側が暗いため、明るく見える。lateral inhibition(側枝抑制）については後で説明するが、人間の視覚はこのように絶対的なトーンや色、または形を捉えているのでなく、対象を関連として捉えているのだ。