Understanding the impact of food-associated stimuli on intake in humans

Environmental food-associated stimuli potentiate feeding in non-human animals and humans. However, there has been no investigation of this phenomena in human adults using novel stimuli which are then selectively associated with food through different learning processes. The aims of this thesis were twofold; firstly, to investigate whether Pavlovian cues (those that signal what and when an outcome will occur) and discriminative stimuli (those that signal whether an outcome will occur) which are associated with food will increase subsequent intake. Secondly, to investigate neural activity in response to these stimuli. The first set of studies examined Pavlovian cue-food associations. Study one utilised a new methodology to associate stimuli with specific tastes and demonstrated evidence for cue-potentiated feeding: people ate more in the presence of cues associated with a sweet taste (CS+). This potentiation was greater for foods which were sweet and so constonant with the trained taste. Study two utilised the same methodology but now contrasted sweet likers and sweet-dislikers. However, the cue-potentiation finding failed to replicate. The second set of studies examined cues associated with the chance to obtain food-rewards, interpreted as discriminative stimuli (DS). Study three trained participants to associate stimuli with obtaining food-rewards if the correct response was produced. The findings displayed a suppression of intake in the presence of a stimulus associated with not obtaining rewards compared to a stimulus associated with obtaining chocolate rewards. Study four extended Study three however sated half the participants prior to the intake test. However the previous cue-suppression finding did not replicate. Study five examined how these two different cue-food associations are encoded in the brain using fMRI. Analysis revealed that the stimuli modified activity in neural regions associated with reward, although whereas the DS enhanced striatal activation, the CS+ deactivated the striatum. The evidence for the lack of contingency awareness to affect behaviour throughout the thesis is discussed.