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      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.

Figure 1.3 Theoretical frameworks of the association between the brain, oral functions and behaviour. (a) The oral‐to‐behaviour (OB) framework, (b) the oral‐brain‐behaviour (OBB) framework and (c) the brain–stomatognathic axis (BSA).

      1.4.3.2 Challenges from the OB Framework

However, the OB framework may not always provide a good prediction of the outcome of dental treatment. For example, temporomandibular disorders (TMD) are associated with various deficits of teeth, the temporomandibular joint and muscles. According to the OB framework, a primary step of treating TMD would be to fix the structural deficits, such as adjusting patients' occlusion by reshaping cusp morphology. However, cumulating evidence suggested that the relationship between occlusal adjustment and the improvement of patients' symptoms is controversial (Xie et al. 2013). In contrast, more evidence from the sensorimotor control of limb movement has revealed that human action is maintained by the corresponding motor program, shaped by learning and adaptation via a complex mechanism of the brain (Wolpert and Flanagan 2016). Notably, the brain would also play a major role in the stomatognathic functions since most of these functions related to feeding – either mastication or swallowing or pain, are highly associated with the integration between sensory feedback and motor commands, both mandated by the brain. Therefore, to strengthen the original OB framework, the link between the stomatognathic system and behaviour needs to be revisited.

      1.4.3.3 The Oral‐Brain‐Behaviour Framework

      According

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