Taste processing in overweight and obesity: An EEG study testing the incentive sensitisation theory

Abstract

Two-thirds of adults in the United Kingdom currently suffer from overweight or obesity, making it one of the biggest contributors to health problems. Within the framework of the incentive sensitisation theory, it has been hypothesised that overweight people display a higher expected reward value, experiencing heightened reward anticipation when encountering food cues (e.g. food pictures and smells), but that they actually show a lower reward outcome value, experiencing less reward from consuming food than normal-weight people. It is hypothesised that this discrepancy between the reward that is expected and the reward that is experienced when eating causes overeating to make up for a lack of satisfaction. The goal of this thesis was to test both parts of this theory, and it was hypothesised that we would find a stronger attention to food cues in the overweight group compared to the normal-weight group, but blunted processing of the food stimuli following the cues. The first study in this thesis (see Chapter 2) set out to test the incentive sensitisation theory of obesity by using electroencephalography (EEG) to measure scalp activity when overweight and normal-weight people encountered cues signalling the imminent arrival of pleasant and neutral tastes and when they actually received these pleasant and neutral tastes. This was the first study to investigate both processes in tandem, and using EEG to do so. The behavioural data showed that the pleasantness ratings of the pleasant and neutral tastes were significantly more similar for the overweight group than for the normal-weight group. This potentially indicates blunted gustatory processing in the overweight group. However, the EEG data did not support this finding, as limited differences between the two groups were observed in these data. To the extent that ERPs index attention to reward-predicting cues and to reward consumption, the findings of this study do not support the incentive sensitisation theory of obesity. Interestingly, in this first study it was found that the liquid that was used as a neutral control condition ('artificial saliva,' distilled water with added KCl and NaHCO3) might not be perceived as neutral. Rather, the findings suggested that it is rated as an aversive taste. In a second study (see Chapter 3), this finding was explored further by contrasting this nominally-neutral solution to various control solutions. This is the first study to have tested these solutions on their valence and intensity, and it was found that previously-assumed neutral liquids may indeed be rated as aversive, as the same neutral control liquid was again rated as mildly aversive. Study two, therefore, has outlined a critical limitation of gustatory processing studies, which has hitherto gone unaddressed. More specifically, it has been shown in this thesis that the use of 'artificial saliva' as a neutral control condition in gustatory research might be misguided, as it was found that this solution was rated as mildly aversive in two separate samples. This finding is a significant addition to the literature, and should be taken into account in future studies attempting to study a gustatory stimulus in comparison to a neutral baseline.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Deborah Talmi (Supervisor)