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.
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