Effects of odor on cortical activity during gum chewing

Pleasant odors stimulate thalamic neural activity, enhance exercise motivation, and improve motor performance. Previous studies have shown that interactions between chewing and olfaction increase cerebral cortical activity; however, most focused on superficial areas, not deeper brain regions or olfactory-motor integration. This study investigated whether olfactory stimulation enhances cortical activity in motor-related regions (primary motor cortex, supplementary motor area, and dorsolateral prefrontal cortex), primarily in the left hemisphere. Thirty-six healthy right-handed individuals aged <35 years participated: Each chewed lemon-flavored (odorized) and odorless gum at a controlled rate (70 chews/min) in a randomized order. After each chewing session, the participants rated the taste, odor, and palatability on a visual analog scale. Changes in cerebral blood flow were monitored using functional near-infrared spectroscopy (fNIRS), focusing on the motor and prefrontal cortices. The masseter muscle and autonomic nervous system activities were also assessed. Data were analyzed using two-way analysis of variance (ANOVA) and paired t-tests with Bonferroni correction. Odorized gum rated higher than odorless gum in the sensory evaluation. Oxy-Hb levels increased during chewing, with odorized gum showing higher levels in the motor control and cognitive regions, particularly the left hemisphere. No differences in masseter activity were observed; however, parasympathetic activity was lower in the odor group, indicating autonomic modulation. Increased motor cortex activation showed no significant somatosensory cortex changes, indicating motor sensory dissociation during odor stimulation. Food odor affects neural activity in the primary motor, supplementary motor, and dorsolateral prefrontal cortices during chewing, enhancing left-hemispheric activity.