A study published in the Journal of Consumer Research (PDF) found that subjects presented with a picture of chocolate cake salivated equally when either actually handed something with chocolate cake smell, or encouraged to imagine the chocolate cake smell.
The researchers liken the effect to that of a visualization exercise, and call it “smellization.” (Yes, really.)
As the scientists found, the smellization effect cannot exist in a vacuum; it needs visual triggers. Like, say, a photo of delicious chocolate cake...
— Read more at Imagining The Smell Of Cake Will Actually Make You Buy More Cake, Researchers Find at Consumerist.