The posted image didn't do it for me, but found a link to another example in the comments: https://upload.wikimedia.org/wikipedia/commons/1/1b/Troxler-Effekt.j

The posted image didn't do it for me, but found a link to another example in the comments: https://upload.wikimedia.org/wikipedia/commons/1/1b/Troxler-Effekt.jpg https://upload.wikimedia.org/wikipedia/commons/1/1b/Troxler-... Having an X to focus on may have helped induce it - a surprising visual effect.