Straight, bent or tilted?
Tilt illusion
Are the stripes in the middle patches straight up and down?
Yes, though they look tilted: away from the stripes around them on the left, toward them on the right.
Why? The nerve cells that sense the direction of lines nudge each other. The surrounding stripes change which direction the middle ones seem to point.
Picture: Drawn by Science Playground, after Qcheng · Wikipedia(opens in a new tab)