Wundt Lines

What is happening

First take in the two thick lines as a whole, comparing the gap at the center against the gap at top and bottom. This figure places fan-shaped lines extending from left and right around a pair of straight lines. Nothing about it is actually animated — the impression changes purely from a static arrangement.

Why it looks that way

When the angle at which surrounding lines cross changes from place to place, it also affects the judgment of a long line’s direction. The directional context created by thin lines can make the middle of a thick line look pulled inward. It’s a figure that contrasts a geometric straight line with a perceived shape.

How to check

Removing the context hides only the fan-shaped background, leaving the two lines in the same place. Bringing the display back doesn’t move the endpoints. Since the Hering illusion uses a background radiating outward from the center, you can go back and forth between the two figures to compare which direction the curve appears in.

Try a related illusion

For reference: explanation and research. The figure here is an original implementation.

First described

After Wilhelm Wundt

Described in the late 19th century. It pairs with the Hering illusion, with the parallel lines appearing to curve in the opposite direction. The exact year of first description is not settled.

See it in the timeline