Olfactory sensitivity is influenced by intranasal trigeminal sensation. For instance, sniffing is central to how humans and animals perceive odorants. Here, we investigated the influence of olfactory costimulation on the perception of intranasal somatosensory stimulation. In this study, 22 healthy human subjects, with normal olfactory function, performed a localization task for stimulation using weak air puffs, a pure odorant, phenyl ethyl alcohol (PEA; rose odor), or their combination. Visual

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... ues were used to inform participants to briefly hold their breath while weak, poorly localizable, air puffs and/or PEA were delivered to either nostril. Although PEA alone could not be localized to the correct nostril, when it accompanied a weak air puff in the ipsilateral nostril, localization accuracy significantly improved, relative to presentation of the air puff without the odorant. The enhancement of localization was absent when the air puff and PEA were presented to opposite nostrils. Since ipsilateral but not contralateral costimulation with PEA increased the accuracy of weak air puff localization, the results argue against a non-specific alerting effect of PEA. These findings suggest an interaction between olfactory and intranasal somatosensory stimuli leading to their integration.