Horizontal means sideways. These eye cells stretch sideways to connect lots of light-catching cells, and they help edges look crisp.
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Horizontal cells live in the retina, the thin layer at the back of your eye.
They sit just above the rods and cones, the light catchers. Each horizontal cell spreads its branches out sideways and touches many light catchers at once.
Scroll up to the 3D model and press Share sideways to see the message spread across.
Horizontal cells compare a spot with the spots around it. If one spot is bright and its neighbors are dark, they make that difference stand out.
This helps you see where one thing ends and another begins, like the edge of a letter on a page or a ball against the sky.
Your eyes are always comparing neighbors. That is one reason a gray square can look lighter on a dark background than on a white one!