It's been called the expectation effect. Is it a B or a 13? It appears that how we see depends on the context and what we expect to see.

This neat example is from a 1955 experiment by Jerome Bruner and A. Leigh Minturn. One of the findings, other than people seamlessly identifying a B or 13 when surrounded appropriately by letters or numbers, was that when asked to draw what they saw "as is" participants were more inclined to draw a closed or partially closed B when it was surrounded by letters. What participants expected to see appeared to affect what they did see.

Context is king

The reason, generally, is simply that all those times previously we weren’t tuned in to notice what’s top of mind now. The frequency likely hasn’t changed a bit just our noticing of it has.

Also known as the Baader-Meinhof Phenomenon.

The frequency illusion: we notice what’s top of mind

The moon seems smaller when up in the sky and bigger when on the horizon. I was surprised to learn that the real cause is still disputed.

The moon illusion

The Hermann Grid, the Necker cube, and the Blivet

Sketchplanations tagged with illusion