[fanfan]

I suspect one can't see something if they don't expect it to be there.

It's like love.

I can't prove it exists, but I know it does.

[BenLurkin]

I'll stipulate that when faced with an ambiguous stimulus, the human mind often fills in the unknown with what it expects (or hopes) to see.

Consequently, person A, expecting to see a flying saucer, is more likely to see a flying saucer when faced with an unknown or ambiguous visual stimulus.

Person B, not expecting a flying saucer, is much less likely to see a saucer when faced with an unknown or ambiguous visual stimulus.