Brain response to odors presented inside the nose, directly in front of the nose or with ambient air

Functional magnetic resonance imaging (fMRI) allows the measurement of changes in blood flow in association with changes in brain activity. This technique has been used frequently to study brain activation in response to odorous stimuli. The aim of this study was to evaluate the effects of odor delivery conditions on brain responses obtained with fMRI.

Prospective cohort study SETTING: Academic institution.

Twenty healthy volunteers (mean age = 29.5 years; 9 women, 11 men) participated. Three odor delivery methods were used: "tube" (odor presented intranasally with separate tubing for each nostril), "mask" (odor presented in a face mask covering the subject's nose) and "vacuum" (odor presented into the ambient air). Presentation of the pleasant "peach" odor was performed using a computer-controlled olfactometer. Subjects were asked to evaluate the intensity of the odors after each fMRI run.

"Tube" showed higher self-rated odor intensity compared to "mask" and "vacuum" (F = 18.4, p < 0.001). Odor intensity had a positive correlation (r = 0.6, p < 0.05) with percent signal change extracted from the secondary olfactory cortex region in the mask condition. In the tube condition, several selected regions of interest (Amygdala, Insula, Thalamus) showed lower activations compared to the other two conditions (p_uncorrected < 0.001, mask > tube, vacuum > tube).

Activations of region of interests (ROIs) in response to the odorous stimuli showed differences under the three conditions (mask, tube, vacuum). In this passive fMRI paradigm, this may partly reflect the differences in odor intensity, but also in attention and contextual variables related to odor perception.