Skinner, Martha
(2018)
Exploring individual variation in oral perception.
PhD thesis, University of Nottingham.
Abstract
Diet plays a pivotal role in preventing, managing, and reducing the risk of weight gain, diabetes and heart disease. Increasing pressure is directed towards food manufacturers to offer healthier options. The challenge is to develop products which are both nutritious and accepted by the consumer. Oral sensitivity, and therefore product perception, varies greatly amongst individuals, and likely affects food choice. Taste phenotype and genotype are frequently proposed as markers for overall oral sensitivity. This thesis performs fundamental research to further current understanding of the impact of taste phenotype and genotype on the response to oral stimuli.
The effect of 6-n-propylthiouracil (PROP) taster status (PTS), fungiform papillae density, TAS2R38 and gustin rs2274333 genotype on the perceived intensity of prototypical tastants and metallic stimuli is explored. Experiments were first conducted to develop oral stimuli for use in the subsequent fMRI studies, which interestingly identified that some metallic stimuli may have a gustatory component. Perceptually, few or no differences were identified across taste phenotypes or genotypes. Interestingly, functional Magnetic Resonance Imaging (fMRI) identified variation in cortical processing that was associated with PTS. PROP intensity ratings were found to correlate with cortical activation in the anterior insula, an area of the brain thought to be the primary gustatory cortex, in response to sweet and metallic stimuli, but not for sour, salt, bitter or umami stimuli. These limited differences observed may have been due to the occurrence of a concentration effect, where the increased gustatory sensitivity frequently associated with PROP tasters compared to PROP non-tasters was lost when administering strong supra-threshold stimuli used in the current study. These findings are of interest to food manufacturers and health professionals as they could indicate that taste phenotype and genotype has less impact on product perception, and therefore food choice, than previously proposed.
Thermal taster status (TTS) refers to a new taste phenotype in which individuals perceive phantom tastes when the tongue is thermally stimulated, whilst thermal non-tasters (TnTs) only perceive temperature. In this thesis, variation in the phantom tastes reported by thermal tasters (TTs) are explored, and for the first time the temporal phantom taste response is measured. Different categories of temporal taste responses are identified, and interestingly it is shown that phantom tastes are perceived at variable temperature ranges across both TTs and taste qualities. Importantly, the onset of sweet taste was found to occur as the temperature increased between 22-35°C, supporting the hypothesis that the TRPM5 may be involved in sweet phantom taste responses. This is the first study to assess the brain’s response when thermally stimulating the tongue of TTs to elicit a phantom taste response. Interestingly, when using fMRI it is shown that at the time when TTs perceive a phantom taste, cortical activation is induced in the anterior insula, which is thought to be the primary gustatory cortex. This indicates that thermal stimulation may activate temperature sensitive gustatory nerve fibres in TTs, and supports the hypothesis of cross wiring between gustatory and trigeminal nerves. When comparing the cortical response to thermal stimulation of the tongue across TTs and TnTs, greater activation is observed in oral somatosensory areas of the brain in TTs compared to TnTs. These findings show cortical processing differs across thermal taste phenotype, and supports evidence that thermal taster status may be a marker for oral sensitivity.
This original research provides a valuable contribution towards understanding the effect of taste phenotype and genotype on perception of prototypical taste, metallic, and thermal stimuli. The novel multidisciplinary approach of utilising sensory evaluation and fMRI techniques has provided valuable insights into the impact of taste phenotype on gustatory responses, and has suggested possible mechanisms that may be involved in thermal taste phenotype.
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