(Kamer et al. 2015). In this study, the pathological feature of Aβ plaque was assessed using PET, and the association between brain pathology and periodontal health (e.g. clinical attachment loss) can be quantified (Kamer et al. 2015). The association between oral health and other pathological brain features, such as lacunar infarction, can also be investigated using neuroimaging methods (Taguchi et al. 2013). Neuroimaging is a valuable tool for investigating the sensory pathway of periodontal inputs and the association between periodontal health and systemic conditions.
1.3.3.3 Orthodontics
Just like prosthodontic treatment, the success of orthodontic treatment is associated with patients' adaptation to the oral appliance. Again, the neuroimaging findings revealed that the use of the oral appliance is associated with an extended area of brain activation, not just confined to the somatosensory cortex (Horinuki et al. 2015; Ozdiler et al. 2019) (Table 1.3). In rats, experimental tooth movement was associated with changes in brain activity of the secondary somatosensory cortex and the insula (Horinuki et al. 2015). Critically, the animal model revealed that during tooth movement, brain activity change was also associated with inflammation, as identified by the expression of the inflammatory factors and macrophage infiltration in the periodontal tissue (Horinuki et al. 2015). The findings have demonstrated the strength of combined neuroimaging and histological approaches, which help elucidate changes in clinical symptoms and signs related to dental treatment.
1.3.4 Summary
As an in vivo imaging method, neuroimaging has become the crucial method for studying the perceptual and psychosocial aspects of oral functions, which are difficult to approach with animal models.
Recent neuroimaging findings suggest that beyond sensory processing, the attentional and cognitive processing related to wearing a denture may play a vital role in the adaptation of prosthodontic treatment. Neuroimaging research provides new clues for prosthodontic treatment by highlighting the patient’s adaptation to dental devices.
Neuroimaging is a valuable tool for investigating the sensory pathway of periodontal inputs and the association between periodontal health and systemic conditions.
1.4 The Brain–Stomatognathic Axis
1.4.1 Introduction
As addressed in Section 1.1, our understanding of oral functions will not be completed if we overlook brain functions. On the one hand, the CNS plays a crucial role in sensorimotor control of the oral apparatus. On the other hand, all the behaviours related to oral functions, including feeding and oral healthcare, are closely associated with general mental functions, including cognitive and affective processing. Therefore, from the functional perceptive, the brain and the stomatognathic system are both essential for maintaining oral health.
However, the brain–stomatognathic connection stated above is descriptive and does not fully explain the underlying mechanisms. The critical and yet unanswered question is ‘how do the brain and the stomatognathic system work together to maintain our oral health?’. The question is difficult to answer because, before the advent of neuroimaging, researchers have had few tools to directly observe the brain mechanisms associated with oral functions in human subjects. In the following sections, we outline several theoretical frameworks for the brain–stomatognathic connection. The core concepts of the brain–stomatognathic connection are defined. Subsequently, three theoretical frameworks on the brain–stomatognathic connection are discussed. Finally, experimental design to test these theoretical frameworks are discussed.
1.4.2 Core Elements of the Brain–Stomatognathic Connection
Before discussing the brain–stomatognathic connection, we need to define the core elements of the connection. Particularly, we will clarify the functional element, i.e. the brain and the stomatognathic system, and human feeding behaviour.
1.4.2.1 Definition of the Functional Element
Based on the definition from Medical Subject Headings (MeSH) of the National Library of Medicine, USA, the stomatognathic system is defined as ‘the mouth, teeth, jaws, pharynx and related structures as they relate to mastication, deglutition and speech’ (MeSH 1986). By this definition, either intraoral structure (e.g. teeth and the tongue) or extraoral structure (e.g. the masseter) is part of the stomatognathic system since all the structure contributes to maintaining normal oral functions. Notably, from the functional perspective, our brain should also be considered as part of the functional element related to oral functions, even though the brain is not part of the stomatognathic system. Both cortical and subcortical regions are closely associated with oral functions (Figure 1.2).
1.4.2.2 Definition of the Behavioural Scope
Another core element to be defined is our behaviour that the brain–stomatognathic connection relates to. This book focuses on human eating and feeding behaviour, which is also the primary target for dental treatment. We define feeding behaviour as ‘behavioral responses or sequences associated with eating including modes of feeding, rhythmic patterns of eating and time intervals’ (MeSH 1969). However, we will need to revisit the definitions for several considerations. Firstly, our primary concern is the feeding behaviour of healthy adults. Eating disorders, such as anorexia and bulimia, are beyond the scope of our discussion. Secondly, the stomatognathic system is critical to speech (including the production of phonemes). The issues related to speech science and language production are not discussed in this book.
1.4.3 Theoretical Frameworks of the Brain–Stomatognathic Connection
There has been literature reporting the brain–stomatognathic connection for more than 100 years. However, most studies focused on the pathological mechanisms underlying diseases, such as the infectious routes between the brain and the head‐and‐neck regions. At present, very few frameworks have been proposed for the functional perspective of the brain–stomatognathic connection. By synthesizing recent clinical and experimental findings, in this section, we try to provide a ‘big picture’ regarding the brain–stomatognathic connection (Figure 1.3).
1.4.3.1 The Oral‐to‐Behaviour Framework
The most intuitive framework consists of the stomatognathic system as the only functional apparatus for feeding behaviour. According to the oral‐to‐behaviour (OB) framework, a sound stomatognathic apparatus directly links to good eating and feeding behaviour (Figure 1.3a). Based on this framework, the structural and functional parameters of the stomatognathic system determine how well one can eat, such that the more teeth and the greater biting force one has, the better mastication and swallowing one will achieve. Notably, the OB framework consists of a bi‐directional relationship: while improving the stomatognathic system will help better eating, a poor eating experience will motivate individuals to fix the deficits of the stomatognathic system. Empirically, the simple logic that ‘if you have got something wrong with eating, fix your teeth first’ goes well most of the time. It echoes the traditional view that dentists are trained as a surgeon who masters the surgical work in the oral cavity.
