Unpleasant odors compared to pleasant ones cause higher cortical activations detectable by fNIRS and observable mostly in females.

Olfactory perception can be studied in deep brain regions at high spatial resolutions with functional magnetic resonance imaging (fMRI), but this is complex and expensive. Electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS) are limited to cortical responses and lower spatial resolutions but are easier and cheaper to use. Unlike EEG, available fNIRS studies on olfaction are few, limited in scope, and contradictory. Here, we investigated fNIRS efficacy in assessing the hedonic valence of pleasant and unpleasant odors, using ten channels on each hemisphere, covering the orbitofrontal cortex and adjacent areas involved in olfactory and cognitive tasks. Measurements on 22 subjects (11 males and 11 females) showed statistically significant higher increases in oxygenated hemoglobin concentration for the unpleasant odor, compared to the pleasant one (mean difference = 1.025 × 10^-1 μM). No difference in activation was found between the hemispheres. Conversely, differences were observed between the sexes: for the first time, we show that higher activations for the unpleasant odor relative to the pleasant one are detectable by fNIRS in females (mean difference = 1.704 × 10^-1 μM), but not in an equal-sized and equal-age group of males. Moreover, females had greater activations relative to males for the unpleasant odor (mean difference = 1.285 × 10^-1 μM). Therefore, fNIRS can capture peculiarities of olfactory activations, highlighting differences between odors with opposite valence and between sexes. This evidence positions fNIRS next to EEG as suitable technologies for cortical investigations of olfactory perception, providing complementary information (late and early response components, respectively), with lower costs and easier operation (albeit at lower resolutions) compared to fMRI.