Human insular cortex : the hub for taste sensation and dental perception processing

Abstract

Taste processing is a complicated neurophysiological topic that is gaining impact over the years due to its relevance to dental, food and nutritional sciences. The advancement in the non-invasive functional magnetic resonance imaging (fMRI) has enabled scientists to better study the human brain responses to taste stimulations applied to the tongue in oral cavity. Existing literature focused on mapping out the brain regions responsible for processing taste information. Yet, the effect of taste on the communications between brain regions was largely unknown. To investigate this, several studies were performed. For the first two studies, the research publication and citation trends of the neuroscience and neuroimaging research fields were evaluated. Relevant bibliometric data were accessed to identify the most popular research topics and imaging modalities in the field. In the 2000s, these fields have been growing steadily. One research hotspot for neuroscience was brain connectivity while the aggregate impact factor of the journals indexed in Neuroimaging category has been increasing gradually. The most popular imaging modality was MRI. The third study was a meta-analysis conducted to identify the human brain regions responsible for taste processing. Pooling data from the literature by activation likelihood estimation (ALE) revealed that thalamus, insula and post-central gyrus are those brain regions consistently activated by taste stimulations on tongue. The fourth study investigated the effects of taste intensity information on brain connectivity during taste processing. Brain fMRI data were acquired from young healthy adults who tasted salty solutions in different concentrations inside a 3-Tesla MRI machine. The connectivity analyses focused on the taste-relevant regions namely the thalamus, the insula and the post-central gyrus. Numerous data models were constructed and data fitted with dynamic causal modeling. Results demonstrated that salty taste intensity had a stimulatory effect on the signal transmission from the insula to the thalamus during taste processing. The fifth study was a meta-analysis conducted to identify the human brain regions attributable to the fear of dentistry. Pooling data from the literature by ALE revealed that anterior cingulate cortex, insula and dorsolateral prefrontal cortex are those consistently activated by phobic dental stimuli. The sixth was an fMRI study that investigated the brain activity in insula triggered by dental anxiety-provoking stimuli. Finally, the neural validity of a dental anxiety trait questionnaire tool, short version of Dental Anxiety Inventory, was confirmed. Past literature has shown the insula is responsible for an integration of sensations and cognitions relevant to oral region. Within the limitations, the studies in this thesis demonstrated the importance of insula as a crucial hub for processing oral/dentally related perceptions; both neurophysiological processes are crucial for human general and dental behaviors.