Evaluation of human body malodor using functional near-infrared spectroscopy with regions of interest based on functional pathways in the olfactory system.

Introduction: Malodors in the human body can diminish comfort in living and working environments, potentially increasing stress and dissatisfaction levels. Therefore, there is a growing need to objectively characterize the properties of human body malodor. The brain extracts various types of information from odor stimuli. Although the central processing pathways of the human olfactory system are not fully understood, some suggest that it comprises distinct parallel functional pathways. This study aimed to clarify whether human body malodor can be objectively evaluated using functional near-infrared spectroscopy (fNIRS) by defining regions of interest in the cerebral cortices based on functional pathways in the olfactory system. Materials and methods: Eighteen healthy, right-handed Japanese adults in their 40s (nine women and nine men) participated in this study. During the three odor stimuli (middle-aged female scalp model odor (MFS model odor), isovaleric acid, and lavender), fNIRS measurements were performed on the cerebral cortical areas connected to the parallel functional pathways in the olfactory system. Odor intensity was adjusted to allow continuous inhalation without inducing marked changes in mood or stress, as confirmed by answering a questionnaire on temporary mood scales and measuring stress markers (heart rate, nose-tip temperature, fingertip blood flow, salivary cortisol, and salivary alpha amylase). Results: The MFS model odor elicited greater increases in oxygenated hemoglobin concentrations than lavender in the inferior frontal gyrus and left superior temporal gyrus, both of which are cortical regions that receive projections from the temporal piriform cortex. These differences in neural responses ceased to be significant after adjusting for perceived intensity, pleasure/displeasure valence, or subjective preference ratings. No significant odor-dependent differences were detected in physiological or endocrine stress markers. Conclusion: This study demonstrates the feasibility of objectively evaluating human body malodor using fNIRS in relation to cortical processing via olfactory pathways. The findings suggest that neural responses to human body odor, particularly in language-related cortical regions, do not simply reflect odor category, but are closely associated with subjective perceptual evaluations. This approach enables objective assessment of human body odor and holds potential for applications in sensory science and product development.