Olfactometry in fMRI studies: odor presentation using nasal continuous positive airway pressure.

We describe a method for generating and presenting olfactory stimuli in functional magnetic resonance imaging (fMRI) studies for humans. The olfactometer is based on principles of air dilution olfactometry and consists of a nasal mask and a nasal Continuous Positive Airway Pressure device, both normally used for patients suffering from obstructive sleep apnea syndrome. The system allows online monitoring and recording of the subject's breathing pattern. Switching between different olfactory conditions can easily be synchronized with the inhalation phase and be controlled by a computer. Besides, switching occurs rapidly and without any optical, acoustic, thermal, or tactile cues for the subject. As an example of implementation we present a fMRI trial of olfaction using pleasant and unpleasant odor stimuli to induce different affective states in healthy subjects. The relatively inexpensive olfactometer is reliable and permits constant odor concentrations during the inherently prolonged imaging studies.