Exploring Gustatory Neural Coding and the Influence of Appetite and Expectancy

The purpose of this thesis was to explore human gustatory processing and how it is influenced by appetite and expectancy. The initial two years of the doctorate were dedicated to developing a gustometer mechanism and taste stimulus set to employ in the experimental investigations. Event-related potentials (ERPs), source-localised ERPs and event-related de-synchronisations and synchronisations (ERD/S) were then evaluated in response to taste characteristics under a variety of conditions. The first experiment assessed the ERP, source-localisation and ERD/S components associated with the processing of taste quality (sweet, salt, bitter, water), intensity (neutral, weak, medium, strong) and hedonicity (pleasant, unpleasant, neutral). Gustatory stimulation evoked activations within the primary gustatory cortex (PGC) and intensity was represented in early ERP epochs and by alpha- and beta-band ERD. Hedonicity was coded in late ERP epochs and by alpha-band ERD. Taste quality coding was difficult to determine from the EEG data. The second experiment compared the processing of pleasant sweet and unpleasant bitter tastes during states of hunger following overnight fasting and satiety induced by a standardised liquid meal. Hunger and satiety evoked maximal responses to tastes from limbic regions. Hunger greatly enhanced ERP and beta-band ERS responses to tastes in general. However, responses to sweet tastes were dependent on hunger state; with enhanced neural signals in response to sweet taste after satiating on a sweet meal - suggesting differential attentional and evaluative mechanisms employed under fasted and fed conditions. A final experiment examined the influence of cue-elicited expectancy on the processing of sweet tastes. Participants were validly or invalidly cued to expect a low- or high-concentration of sweet taste; both behavioural and neural responses to invalidly cued tastes assimilated to those that were produced by the taste the participants were cued to receive. These effects began ~100 ms after the onset of the tastes, suggesting that expectancy influences the early perceptual processing of taste. The overall findings of this thesis provide some of the first accounts of the temporal, source-localised and oscillatory dynamics of gustatory coding. The results also provide important implications for understanding how people’s experience of taste and food can be modified by appetite and expectancy