Using multi-channel near-infrared spectroscopy, the authors sought to monitor cortical activity during the sensory evaluation period to evaluate the effect of flavorings on taste caused by central integration of olfactory and gustatory modalities. They noted that the neocortical response to a test solution showed adaptation by the conditional sugar solution, which was administered 60 seconds before the test solution. Sugar-sugar self adaptation was greater than sugar-artificial sweetener cross adaptation recorded at specific regions of the frontal and temporal cortex. The magnitude of sugar-flavored artificial sweetener cross adaptation tended to approach that of sugar-sugar self adaptation. Therefore, the similarity of the adaptation of cortical responses might be an important indicator in the screening of effective flavorings in order to improve taste.
TopIntroduction
During food intake, flavor perception results from simultaneous stimulation of the gustatory, olfactory, and somatosensory systems. Flavor perception is one of the most complex human processes. It involves almost all of the senses, particularly the sense of smell, which is involved through odor images generated in the olfactory pathway. Olfactory stimulation occurs mainly through the retronasal pathway, and the resulting perception is often interpreted as a taste perception (Shephard, 2006; Verhagen & Engelen, 2006). Retronasal stimulation occurs during food ingestion, when volatile molecules released from the food in the mouth are forced up into the nasal passages. Because the molecules arise from food in the mouth, the sensing of these molecules is referred to the mouth; that is, these molecules are perceived as if they are within the mouth (Bartoshuk, et al., 2004). This retronasal food-molecule-laden air is judged to be part of the 'taste' of the food (Murphy, et al., 1977). Our attention has been focused on the importance of the olfactory modality in sensing flavor. Perceptual taste caused by central integration is thought to be affected by olfactory stimulation by particular flavorings. To evaluate the effect of added flavorings on taste, we sought to monitor objective neocortical responses when subjects sensorily evaluated food quality. Until now, the major human brain mapping techniques used for olfaction-related functions have been Positron Emission Tomography (PET) (Dade, et al., 1998; Zald & Pardo, 2000; Savic & Gulyas, 2000), functional Magnetic Resonance Imaging (fMRI) (Levy, et al., 1999; Weismann, et al., 2001; Wang, et al., 2005), Magnetoencephalography (MEG) (Tonoike, et al., 1998; Walla, et al., 2002; Miyanari, et al., 2006; Boesbeldt, et al., 2009), and Electroencephalography (EEG) (Tateyama, et al., 1998; Laudien, et al., 2006, 2008). Although these methods have made important contributions to mapping of cerebral olfactory processing, they require participants to be restricted in their movements, and this feature does not allow them to perform tasting in a natural manner. In the present study, we used Near Infrared Spectroscopy (NIRS) to evaluate the effect of added flavorings on taste. Although NIRS measurements are limited to the cortical surface, several studies using NIRS reported olfactory-related activations (Ishimaru, et al., 2004a, 2004b; Harada, et al., 2006; Aoyama, et al., 2011; Takakura, et al., 2011; Kokan, et al., 2011), and taste-related activations (Okamoto, et al., 2006, 2009, 2011; Bembich, et al., 2010). The recording of the hemodynamic signals with NIRS enabled us to test subjects in a normal, seated position with minimal restriction of movement during drinking. We therefore used NIRS to record hemodynamic signals when subjects sensory evaluate food quality and the effect of the odor on taste.