At the same time when unpleasant odor were being administered, the researchers matched each smell with a very specific tone. They performed the experiment repeatedly and long enough to provoke a Pavlovian response in the volunteers such that, while they were still sleeping, they could either elicit the shallow or deep breath response by simply playing the sound associated with either the pleasant or unpleasant odor.
The researchers then played the distinctive tones to the volunteers while they were awake. What they found was that each tone that had been matched to either a pleasant or unpleasant smell would cause the volunteer to take either a deep or shallow breath depending on which smell the sounds were associated with.
source : http://www.nature.com/neuro/journal/vaop/ncurrent/full/nn.3193.html
