Say cheese! Brain imaging of texture perception
The aim of this research was to examine neural pathways of food texture perception during oral processing in the human brain. Oral processing presents technical challenges when analysed using functional magnetic resonance imaging (fMRI) and functional near-infrared spectroscopy (fNIRS). Thus, the work described within this thesis aimed to develop experimental procedures and implement advanced denoising and analysis techniques to make studying the brain during mastication feasible. Brain responses to stimuli that vary in food form, textural intensity or complexity were measured using multi-echo fMRI and fNIRS. Food stimuli were specifically designed to reduce masticatory movements. Further, multi-echo denoising and both univariate and multivariate analysis were implemented for fMRI; while for fNIRS, multiple motion-correction techniques were implemented and assessed on their ability to reduce masticatory-related noise. Multi-echo fMRI revealed neural selectivity to food texture within the right oral somatosensory and left orbitofrontal cortex using representational similarity analysis. Multi-echo denoising successfully removed head-motion related signal changes without concurrently removing brain activity; but only if steps were taken to ensure head motion during the task remained under 2mm. Thus, denoising was found to be essential, since motion patterns were correlated to both nondenoised brain activity patterns and stimulus conditions. Oral processing during fNIRS measurements resulted in haemodynamic responses originating from the temporal muscle, which masked brain activity in regions hypothesised to be involved in texture perception. In addition, this response is task-related and affected by oralprocessing behaviour and is therefore a serious confounder in fNIRS research. Further, current motion-correction techniques are not capable of separating masticatory-related noise from brain activity in regions underneath the temporal muscle. The outcomes reveal, for the first time, solid-food texture representation in the oral somatosensory and orbitofrontal cortex. This was achievable because experimental protocols were developed to mitigate the effects of oral processing and were used in combination with advanced analysis and denoising techniques to study texture perception with fMRI. Further development of denoising strategies for fNIRS should aim to remove muscle-related noise. The techniques developed during this research can be implemented in the future to broaden the possibilities of studying the human brain’s response to food.
Networked Digital Library of Theses and Dissertations