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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 to the oral‐brain‐behaviour (OBB) framework, to maintain good eating ability, one needs (i) to restore structural deficits of the oral cavity/teeth and (ii) to maintain the sensorimotor control of oral functions. Traditionally, point (ii) is relatively ignored in dental treatment because dentists assume that sensorimotor processing works well. However, as shown in Chapter 7, the sensorimotor processing of the brain may alter as age increases or in patients with brain impairment (e.g. neurodegeneration or stroke). Therefore, oral dysfunctions and difficulty in feeding may be associated with deficits in brain functions (Figure 1.3b). According to the OBB framework, for elderly or special needs patients, both fixing structural deficits and maintaining brain functions are critical to improving patients' oral health.
1.4.4 The Brain–Stomatognathic Axis
While the OBB model emphasizes that the brain is critical to the stomatognathic functions, it does not directly account for the individual differences in feeding behaviour. The OBB framework suggests that a good stomatognathic condition (e.g. fully dentated) and the integrity of sensorimotor control of oral functions both contribute to good eating ability. However, the framework simplifies the association between the brain and oral health. In addition to sensorimotor control (which has been widely investigated via animal research), mastication and swallowing are also associated with cognitive, affective and motivational processing of the brain (Figure 1.3c). For example, as shown in Chapter 5, the tactile (e.g. ‘chewy’) and gustatory (e.g. ‘yummy’) experience from chewing is associated with an increased hedonic value and reward processing of food. Therefore, the brain–stomatognathic axis (BSA) framework highlights multiple associations between brain functions and feeding behaviour. Most importantly, the BSA framework highlights that all the functions participate in the adaptation of oral conditions. When dysfunction occurs (either due to structural deficits, aging or brain impairment), individuals also learn how to adapt to this new condition. For example, when having a meal, the patients with a new denture may keep on detecting if the denture is well fitted and judging if the food bolus is good to swallow. All the cases suggest that feeding behaviour is not a simple translation of oral sensory and motor functions. It is crucially associated with the attentional, cognitive, motivational and emotional processing related to eating.
In the BSA framework, the term ‘axis’ emphasizes a bi‐directional and dynamic relationship between the brain and the stomatognathic system (Lin 2018). In gastroenterology, the concept of ‘gut‐brain axis’ (GBA) has been proposed and widely distributed for many years. The GBA consists of ‘bidirectional communication between the central and the enteric nervous system, linking emotional and cognitive centres of the brain with peripheral intestinal functions’ (Carabotti et al. 2015). In parallel, the BSA emphasizes that the brain plays a more comprehensive role in sensorimotor and affective–cognitive processing on the stomatognathic functions and feeding behaviour.
1.4.5 How Can Neuroimaging Research Help Studying the Brain–Stomatognathic Connection?
One of the greatest advantages of neuroimaging is to explore the brain mechanisms associated with feeding behaviour directly on human subjects. For example, researchers can measure the signals related to brain activities associated with chewing and swallowing with different imaging approaches. The following sections focus on research design for investigating the brain–stomatognathic connection.
1.4.5.1 Investigation of the OBB Framework
The OBB framework highlights sensorimotor control of oral functions. The brain mechanisms associated with oral functions can be investigated using a task‐based study, in which brain activities are recorded concurrently when an individual is performing an oral function. The association between sensory or motor processing and oral functions can be modulated by different experimental conditions. For example, when chewing harder food, one should expect stronger sensory feedback from the periodontal tissue than soft food. In this case, changes in brain activation, as shown by functional MRI, would be identified at the somatosensory cortex, and this activity is known to reflect the intensity of sensory inputs (Onozuka et al. 2002; Takahashi et al. 2007). It is noteworthy that the interpretation of an association between task parameters (e.g. the hardness of food) and brain features (e.g. activation of the motor cortex) may be complicated. For example, when chewing a harder piece of food, one may pay more attention to the texture of the food. Therefore, the brain regions with activation may be associated with attention and cognitive control as well as sensory processing (Onozuka et al. 2002, Takahashi et al. 2007).
1.4.5.2 Investigation of the BSA Framework
In contrast to the OBB framework, the BSA framework additionally highlights the importance that the brain will actively adapt to the environment so that even though the functional apparatus is impaired (e.g. losing teeth), individuals can maintain feeding behaviour. A critical step to test this general hypothesis is to focus on the individuals who show structural deficits (e.g. with a higher number of missing teeth) but maintain a good oral function. In this group, the degree of sensorimotor adaptation and learning is assessed using specific tasks (see Chapter 8). The association between task performance and brain activation, particularly in the regions associated with cognitive, affective and motivational processing, is assessed using neuroimaging. Notably, the superiority of neuroimaging is that it can assess brain activation associated with complicated processing of learning and adaptation in human subjects – which may be challenging to perform on animal subjects.
1.4.6 Summary
The OB framework consists of the stomatognathic system as the only functional apparatus for
