Neural and Psychological Mechanisms of Oral Sensation

This thesis set out to explore oral sensory processing. Oral sensory processing extends beyond taste perception, the nerves that innervate the mouth and carry taste information to the brain also carry chemosensations, thermal sensations and somatosensations. While a great deal is understood about oral chemo and thermal perception, this thesis focuses on the not fully recognised oral somatosensory processes. A substantial amount of movement occurs within the mouth, from movement while speaking to chewing food. As food moves around the mouth, different oral receptors are activated and the consumption experience changes. Taste perception varies between individuals in a way that has led to the identification of the taster status genetic polymorphism of taster status where three taster groups (hyper-taster, taster, tolerant taster) with differing sensitivity to bitter tastes were identified. This sensitivity is further represented in anatomical differences with differing densities of fungiform papillae on the tongue. Using psychophysical methods and the taster status phenotype, this thesis examined if different regions of the tongue and mouth experienced different chemostimulant intensity and if dynamic touch changed the intensity perception of chemostimulants in chapter 4. This identified that different regions of the oral cavity experience chemostimulant intensity differently with the tip of the tongue being the most sensitive and the vermillion of the lower lip the least sensitive to sensation. Furthermore, whilst there was no main effect of touch on sensation intensity an interaction between touch type, taster status and oral locations was found when using 10-ppm capsaicin and Sichuan pepper. A dynamic touch on the lip with mint oil was also considered more intense than a static touch. Chapter 5 investigated the possibility that C tactile (CT) afferents were present in the lower lip, the structure of the lip skin widely suggests that CTs are not present but their regular use in the affective behaviour of lip-to-lip contact between individuals suggests otherwise. By applying the standardised psychophysical stroking approach to the lip, cheek and mucosa the classic psychophysical inverted U associated with CT like behavioural responses to touch was found on the cheek where CTs are known to be present as well as on the lower lip. This CT like response on the lip warrants further detailed investigation. Serotonin (5-HT) is widely associated with hedonic experiences and reduced 5-HT levels are a linked with depression and anhedonia. 5-HT is also a candidate neurotransmitter associated with taste transduction. Chapter 6 describes an acute tryptophan depletion (ATD) study that examined the peripheral and central effect of reduced 5-HT levels on taste perception. The primary findings highlight that tryptophan levels do not effect sweet, sour, salt and bitter taste detection ability. A significant difference in bitter taste intensity and pleasantness was identified with tryptophan depletion increasing the taste intensity and decreasing bitter pleasantness at suprathreshold concentration. An effect of taster status was identified in bitter intensity ratings with tolerant-tasters reporting a greater intensity of sensation in the tryptophan depletion session than in the control. During the course of the experimental phase of this thesis, it became clear that describing oral sensations was a difficult task. When asked to describe how sensations felt within their mouth in chapter 4, participants were unable to find words to describe sensations. Therefore, the final study in chapter 7 describes the development of a candidate oral lexicon to aid in describing mouth feel and oral sensations highlighting that the approach to lexicon development previously used to develop the McGill Pain Questionnaire and the Touch Perception Task can successfully be applied to the development of an Oral Lexicon.