Chemosensory perception in human brain : the interaction of taste and smell as a simplified flavor model

Abstract

Flavor perception together with the sensation of taste and smell in a human brain have never been well studied until the establishment of different objective functional imaging methods such as Positron Emission Tomography and functional Magnetic Resonance Imaging (fMRI). While flavor perception is an everyday part of daily life, such as during consumption of food and drink, intimate contact with others, and oral intake of medicine, the full nature of the topic has yet to be clarified. In general, flavor is a food-related percept that requires integration of different sensory systems such as taste, smell, tactile and vision inside in the brain. As taste and smell constituted the two major elements of the flavor perception during food and drinks intake, this thesis set out to investigate the chemosensory perception of sour taste and mango smell and their possible neural interaction detectable by fMRI. Two analyses were conducted. In the first analysis, the brain responses detectable by fMRI subject to the stimulations of a sour taste, mango smell and their combined form as a flavor were investigated. The sour taste activated the taste- and food-related brain regions such as insula, prefrontal cortex and pallidum. Insula as the primary taste cortex detects and recognizes the taste quality whereas the other two regions play a role in higher level processing for taste. The mango smell delivered in retronasal route activated the olfactory tubercle and the hippocampus which are the primary and secondary olfactory cortices. Mixing the stated agents together, the chemosensory stimuli were proved to have a congruent pair of components from the sensory evaluation ratings. Activations due to the flavor stimulant were observed in the insula, frontal operculum and prefrontal cortex. These regions are commonly activated by different flavor stimuli and are suggested to take part in the multisensory integration. In the second analysis, the flavor-processing network in terms of the effective connectivity was investigated based on the fMRI result in the first analysis using dynamic causal modeling. The flavor-processing network was proved to be driven at the anterior insula and modulated by the taste and smell at the connection from frontal operculum to anterior insula. These clearly showed the causal functional relationship (effective connectivity with direction) between frontal operculum and anterior insula during flavor perception. The results in these studies provided more representable neuroimaging data for taste, smell and their interactions, which also provided some insights to the mechanism of flavor perception, and thereby, improved the understanding to food consumption process in the field of neuroimaging and chemosensory system.