From physorg.com:

By directly manipulating the activity of individual neurons, scientists have given flies memories of a bad experience they never really had, according to a report in the October 16th issue of the journal Cell.

“Flies have the ability to learn, but the circuits that instruct memory formation were unknown,” said Gero Miesenböck of the University of Oxford. “We were able to pin the essential component down to 12 cells. It’s really remarkable resolution.” Those dozen cells are sufficient to manage what is a difficult cognitive problem: learning to associate a particular odor with something bad, like an electric shock. In essence, these cells create memories that the fly then uses to avoid that odor.