Have you ever experienced to taste something invisible without actually tasting it? Well, I have with something sour and acidy, like lemons, grapefruits, oranges... In most cases, these citrus fruits make me feel like I actually taste them even when I am just thinking about them.
When I think or imagine of, let's say, lemons, the saliva gradually spreads in the mouth. I guess the eyes send the image of lemons, the brain reacts instantly towards the image and realizes that lemons are sour, the taste bud misunderstands that one eats lemons, and the the saliva spreads in the mouth.
As for this theory, we can't forget about umeboshi (pickled plums) which give us the exact same experience as citrus fruits do.