Dental Neuroimaging. Chia-shu Lin

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 eating and feeding behaviour. According to the OB framework, a sound stomatognathic apparatus means good feeding behaviour.

       The OBB framework highlights the role of the exchange of sensorimotor information between the brain and the stomatognathic system in oral functions.

       In the BSA framework, the brain is not just a passive translator for the sensorimotor information but also a ‘moderator’ that actively engaged with one's feeding behaviour.

       Neuroimaging research on the BSA framework focuses on identifying individual differences in oral functions. The BSA emphasizes that the brain plays a more comprehensive role in sensorimotor and affective–cognitive processing on the stomatognathic functions and feeding behaviour.

      Further Readings

      1 Please see the Companion Website for Suggested Readings.

References

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      2 Anderson, T. (1790). Pathological observations on the brain. Lond. Med. J. 11: 182–190.

      3 Avivi‐Arber, L., Lee, J., Sood, C. et al. (2015). Long‐term neuroplasticity of the face primary motor cortex and adjacent somatosensory cortex induced by tooth loss can be reversed following dental implant replacement in rats. J. Comp. Neurol. 523: 2372–2389.

      4 Avivi‐Arber and Sessle (2018). Jaw sensorimotor control in healthy adults and effects of ageing. J. Oral Rehabil. 45: 50–80.

      5 Bandettini, P.A. (2012). Twenty years of functional MRI: the science and the stories. NeuroImage 62: 575–588.

      6 Brügger, M., Lutz, K., Brönnimann, B. et al. (2012). Tracing toothache intensity in the brain. J. Dent. Res. 91: 156–160.

      7 Carabotti,