Motor control of jaw movements: An fMRI study of parafunctional clench and grind behavior

Topology of the lateral visual system: The fundus of the superior temporal sulcus and parietal area H connect nonvisual cerebrum to the lateral occipital lobe

BACKGROUND AND PURPOSE

Mapping the topology of the visual system is critical for understanding how complex cognitive processes like reading can occur. We aim to describe the connectivity of the visual system to understand how the cerebrum accesses visual information in the lateral occipital lobe.

METHODS

Using meta-analytic software focused on task-based functional MRI studies, an activation likelihood estimation (ALE) of the visual network was created. Regions of interest corresponding to the cortical parcellation scheme previously published under the Human Connectome Project were co-registered onto the ALE to identify the hub-like regions of the visual network. Diffusion Spectrum Imaging-based fiber tractography was performed to determine the structural connectivity of these regions with extraoccipital cortices.

RESULTS

The fundus of the superior temporal sulcus (FST) and parietal area H (PH) were identified as hub-like regions for the visual network. FST and PH demonstrated several areas of coactivation beyond the occipital lobe and visual network. Furthermore, these parcellations were highly interconnected with other cortical regions throughout extraoccipital cortices related to their nonvisual functional roles. A cortical model demonstrating connections to these hub-like areas was created.

CONCLUSIONS

FST and PH are two hub-like areas that demonstrate extensive functional coactivation and structural connections to nonvisual cerebrum. Their structural interconnectedness with language cortices along with the abnormal activation of areas commonly located in the temporo-occipital region in dyslexic individuals suggests possible important roles of FST and PH in the integration of information related to language and reading. Future studies should refine our model by examining the functional roles of these hub areas and their clinical significance.

Topology of pain networks in patients with temporomandibular disorder and pain-free controls with and without concurrent experimental pain: A pilot study

Temporomandibular disorders (TMD) involve chronic pain in the masticatory muscles and jaw joints, but the mechanisms underlying the pain are heterogenous and vary across individuals. In some cases, structural, functional, and metabolic changes in the brain may underlie the condition. In the present study, we evaluated the functional connectivity between 86 regions of interest (ROIs), which were chosen based on previously reported neuroimaging studies of pain and differences in brain morphology identified in an initial surface-based morphometry analysis. Our main objectives were to investigate the topology of the network formed by these ROIs and how it differs between individuals with TMD and chronic pain (n = 16) and pain-free control participants (n = 12). In addition to a true resting state functional connectivity scan, we also measured functional connectivity during a 6-min application of a noxious cuff stimulus applied to the left leg. Our principal finding is individuals with TMD exhibit more suprathreshold correlations (higher nodal degree) among all ROIs but fewer "hub" nodes (i.e., decreased betweenness centrality) across conditions and across all pain pathways. These results suggest is this pain-related network of nodes may be "over-wired" in individuals with TMD and chronic pain compared to controls, both at rest and during experimental pain.

Is bruxism associated with changes in neural pathways? A systematic review and meta-analysis of clinical studies using neurophysiological techniques

This study aimed to systematically review the literature to identify clinical studies assessing neuroplasticity changes induced by or associated with bruxism or a tooth-clenching task using neurophysiological techniques. Searches were performed in five electronic databases (PubMed, EMBASE, Scopus, Web of Science, and Google Scholar) in April 2020. This review included clinical studies using neurophysiological techniques to assess neuroplasticity changes in healthy participants before and after a tooth-clenching task or comparing bruxers and non-bruxers. The quality assessment was performed with the Joanna Briggs Institute tool and Grading of Recommendations Assessment, Development, and Evaluation. Meta-analyses were conducted with studies reporting similar comparisons regarding masseter motor evoked potential amplitude and signal change outcomes. Of 151 articles identified in the searches, nine were included, and five proceeded to meta-analysis. Included studies presented moderate to very low methodological quality. From these included studies, eight evaluated bruxers and non-bruxers, of which five of them observed brain activity differences between groups, and three found no differences. Even so, all studies have suggested distinct difference in the central excitability between bruxers and non-bruxers, the meta-analysis revealed no statistically significant differences (P > 0.05). It appears that bruxism seems, indeed, to be associated with distinct differences in the neural pathways related to the control of the jaw-closing muscles, but that considerable variability in terms of classification of bruxism and assessment of neuroplasticity hamper a definite conclusion. Future research projects should take these concerns into consideration in order to further the understanding of bruxism physiology and pathophysiology.

Effects of preventing intercuspation on the precision of jaw movements

BACKGROUND

Closing movements are among the jaw's basic physiological motor actions. During functional movements, the jaw changes position continually, which requires appropriate proprioception. However, the significance of the various proprioceptive receptors involved and how they interact is not yet fully clear.

OBJECTIVES

This study's main objective was to test whether preventing intercuspation (IC) for 1 week would affect the precision of jaw-closing movements into IC and the functional space of habitual chewing movements (HCM). A secondary objective was to compare precision of jaw-closing movements into IC with the precision of movements into a target position (TP) far from IC.

METHODS

Fourteen participants' HCM and jaw-closing movements into IC were recorded on two sessions (T1 and T2) 1 week apart. Between sessions, participants wore posterior bite plates to prevent IC. They also received a 10-minute training session at T1 to guide their jaw-closing movements into TP. The precision of the closing movements into IC and TP was analysed. For HCM, the vertical amplitude, lateral width and area of chewing cycles were evaluated.

RESULTS

The precision of jaw movements into IC increased as the jaw gap decreased, but precision did not differ significantly between T1 and T2. For HCM, the vertical amplitude and area of chewing cycles increased significantly between T1 and T2. The precision of the closing trajectory into TP increased significantly during the training session.

CONCLUSION

Our results confirm the excellent adaptability of the craniomandibular system, controlled by stringent motor programmes that are supported by continuous peripheral sensory input.

Evolution of the speech-ready brain: The voice/jaw connection in the human motor cortex

A prominent model of the origins of speech, known as the "frame/content" theory, posits that oscillatory lowering and raising of the jaw provided an evolutionary scaffold for the development of syllable structure in speech. Because such oscillations are nonvocal in most nonhuman primates, the evolution of speech required the addition of vocalization onto this scaffold in order to turn such jaw oscillations into vocalized syllables. In the present functional MRI study, we demonstrate overlapping somatotopic representations between the larynx and the jaw muscles in the human primary motor cortex. This proximity between the larynx and jaw in the brain might support the coupling between vocalization and jaw oscillations to generate syllable structure. This model suggests that humans inherited voluntary control of jaw oscillations from ancestral species, but added voluntary control of vocalization onto this via the evolution of a new brain area that came to be situated near the jaw region in the human motor cortex.

Model-Based and Model-Free Analyses of the Neural Correlates of Tongue Movements

The tongue performs movements in all directions to subserve its diverse functions in chewing, swallowing, and speech production. Using task-based functional MRI in a group of 17 healthy young participants, we studied (1) potential differences in the cerebral control of frontal (protrusion), horizontal (side to side), and vertical (elevation) tongue movements and (2) inter-individual differences in tongue motor control. To investigate differences between different tongue movements, we performed voxel-wise multiple linear regressions. To investigate inter-individual differences, we applied a novel approach, spatio-temporal filtering of independent components. For this approach, individual functional data were decomposed into spatially independent components and corresponding time courses using independent component analysis. A temporal filter (correlation with the expected brain response) was used to identify independent components time-locked to the tongue motor tasks. A spatial filter (cross-correlation with established neurofunctional systems) was used to identify brain activity not time-locked to the tasks. Our results confirm the importance of an extended bilateral cortical and subcortical network for the control of tongue movements. Frontal (protrusion) tongue movements, highly overlearned movements related to speech production, showed less activity in the frontal and parietal lobes compared to horizontal (side to side) and vertical (elevation) movements and greater activity in the left frontal and temporal lobes compared to vertical movements (cluster-forming threshold of Z > 3.1, cluster significance threshold of p < 0.01, corrected for multiple comparisons). The investigation of inter-individual differences revealed a component representing the tongue primary sensorimotor cortex time-locked to the task in all participants. Using the spatial filter, we found the default mode network in 16 of 17 participants, the left fronto-parietal network in 16, the right fronto-parietal network in 8, and the executive control network in four participants (Pearson's r > 0.4 between neurofunctional systems and individual components). These results demonstrate that spatio-temporal filtering of independent components allows to identify individual brain activity related to a specific task and also structured spatiotemporal processes representing known neurofunctional systems on an individual basis. This novel approach may be useful for the assessment of individual patients and results may be related to individual clinical, behavioral, and genetic information.

Unbalanced Occlusion Modifies the Pattern of Brain Activity During Execution of a Finger to Thumb Motor Task

In order to assess possible influences of occlusion on motor performance, we studied by functional magnetic resonance imaging (fMRI) the changes in the blood oxygenation level dependent (BOLD) signal induced at brain level by a finger to thumb motor task in a population of subjects characterized by an asymmetric activation of jaw muscles during clenching (malocclusion). In these subjects, appropriate occlusal correction by an oral orthotic (bite) reduced the masticatory asymmetry. The finger to thumb task was performed while the subject's dental arches were touching, in two conditions: (a) with the teeth in direct contact (Bite OFF) and (b) with the bite interposed between the arches (Bite ON). Both conditions required only a very slight activation of masticatory muscles. Maps of the BOLD signal recorded during the movement were contrasted with the resting condition (activation maps). Between conditions comparison of the activation maps (Bite OFF/Bite ON) showed that, in Bite OFF, the BOLD signal was significantly higher in the trigeminal sensorimotor region, the premotor cortex, the cerebellum, the inferior temporal and occipital cortex, the calcarine cortex, the precuneus on both sides, as well as in the right posterior cingulate cortex. These data are consistent with the hypothesis that malocclusion makes movement performance more difficult, leading to a stronger activation of (a) sensorimotor areas not dealing with the control of the involved body part, (b) regions planning the motor sequence, and (c) the cerebellum, which is essential in motor coordination. Moreover, the findings of a higher activation of temporo-occipital cortex and precuneus/cingulus, respectively, suggest that, during malocclusion, the movement occurs with an increased visual imagery activity, and requires a stronger attentive effort.

Impact of sleep bruxism on training-induced cortical plasticity

PURPOSE

To investigate if sleep bruxism (SB) influences training-induced cortical plasticity and performance in terms of accuracy and precision of a tooth-clenching task (TCT).

METHODS

Thirty-eight participants were allocated into SB group (N=19) and control group (N=19) according to presence of SB based on a 2-week screening. The participants were instructed to perform a standardized TCT for 58min at three different force levels (10%, 20% and 40% of maximum voluntary contraction; MVC) in three series (first and third without visual-feedback and second with visual-feedback). Accuracy and precision of the TCT were calculated from actual bite force values. Transcranial magnetic stimulation was applied to elicit motor evoked potentials (MEPs) from the masseter and first dorsal interosseous muscle (FDI) before the TCT (pre-TCT-session) and 5-min after the TCT (post-TCT-session).

RESULTS

Accuracy was significantly dependent on the series and target force level (P<0.001), however, there was a significant decrease only in the control group at 10% MVC from first to third session (P<0.001). No significant differences between groups were observed for the precision of the TCT. Masseter MEPs in the SB group in the pre-TCT-session at 120% and 160% motor threshold were significantly lower than in the control group (P<0.05). Masseter MEPs of the control group in the post-TCT-session were significantly higher than the pre-TCT-session (P<0.05) but not SB. FDI MEPs were only dependent on stimulus intensity (P<0.001).

CONCLUSIONS

SB is associated with significant changes not only in excitability of corticomotor control but also motor learning of jaw movements and force control.

Reduced corticostriatal functional connectivity in temporomandibular disorders

Although temporomandibular disorders (TMD) have been associated with abnormal gray matter volumes in cortical areas and in the striatum, the corticostriatal functional connectivity (FC) of patients with TMD has not been studied. Here, we studied 30 patients with TMD and 20 healthy controls that underwent clinical evaluations, including Helkimo indices, pain assessments, and resting-state functional magnetic resonance imaging scans. The FCs of the striatal regions with the other brain areas were examined with a seed-based approach. As seeds, we used the dorsal caudate, ventral caudate/nucleus accumbens, dorsal caudal putamen, and ventral rostral putamen regions. Voxel-wise comparisons with controls revealed that the patients with TMD exhibited reduced FCs in the ventral corticostriatal circuitry, between the ventral striatum and ventral frontal cortices, including the anterior cingulate cortex and anterior insula; in the dorsal corticostriatal circuitry, between the dorsal striatum and the dorsal cortices, including the precentral gyrus and supramarginal gyrus; and also within the striatum. Additionally, we explored correlations between the reduced corticostriatal FCs and clinical measurements. These results directly supported the hypothesis that TMD is associated with reduced FCs in brain corticostriatal networks and that these reduced FCs may underlie the deficits in motor control, pain processing, and cognition in TMD. Our findings may contribute to the understanding of the etiologies and pathologies of TMD.

GABA and glutamate levels in occlusal splint-wearing males with possible bruxism

OBJECTIVE

The inhibitory neurotransmitter γ-aminobutyric acid (GABA) plays an important role in the pathophysiology of anxiety behavioural disorders such as panic disorder and post-traumatic stress disorder and is also implicated in the manifestation of tooth-grinding and clenching behaviours generally known as bruxism. In order to test whether the stress-related behaviours of tooth-grinding and clenching share similar underlying mechanisms involving GABA and other metabolites as do anxiety-related behavioural disorders, we performed a Magnetic Resonance Spectroscopy (MRS) study for accurate, in vivo metabolite quantification in anxiety-related brain regions.

DESIGN

MRS was performed in the right hippocampus and right thalamus involved in the hypothalamic-pituitary-adrenal axis system, together with a motor planning region (dorsal anterior cingulate cortex/pre-supplementary motor area) and right dorsolateral prefrontal cortex (DLPFC). Eight occlusal splint-wearing men (OCS) with possible tooth-grinding and clenching behaviours and nine male controls (CON) with no such behaviour were studied.

RESULTS

Repeated-measures ANOVA showed significant Group×Region interaction for GABA+ (p = 0.001) and glutamate (Glu) (p = 0.031). Between-group post hoc ANOVA showed significantly lower levels of GABA+ (p = 0.003) and higher levels of Glu (p = 0.002) in DLPFC of OCS subjects. These GABA+ and Glu group differences remained significant (GABA+, p = 0.049; Glu, p = 0.039) after the inclusion of anxiety as a covariate. Additionally, GABA and Glu levels in the DLPFC of all subjects were negatively related (Pearson's r = -0.75, p = 0.003).

CONCLUSIONS

These findings indicate that the oral behaviours of tooth-grinding and clenching, generally known as bruxism, may be associated with disturbances in brain GABAergic and glutamatergic systems.

To see bruxism: a functional MRI study

OBJECTIVE

Since the pathophysiology of bruxism is not clearly understood, there exists no possible treatment. The aim of this study is to investigate the cerebral activation differences between healthy subjects and patients with bruxism on behalf of possible aetiological factors.

METHODS

12 healthy subjects and 12 patients with bruxism, a total of 24 right-handed female subjects (aged 20-27 years) were examined using functional MRI during tooth-clenching and resting tasks. Imaging was performed with 3.0-T MRI scanner with a 32-channel head coil. Differences in regional brain activity between patients with bruxism and healthy subjects (control group) were observed with BrainVoyager QX 2.8 (Brain Innovation, Maastricht, Netherlands) statistical data analysis program. Activation maps were created using the general linear model: single study and multistudy multisubject for statistical group analysis. This protocol was approved by the ethics committee of medical faculty of Kirikkale University, Turkey (02/04), based on the guidelines set forth in the Declaration of Helsinki.

RESULTS

The group analysis revealed a statistically significant increase in blood oxygenation level-dependent signal of three clusters in the control group (p<0.005), which may indicate brain regions related with somatognosis, repetitive passive motion, proprioception and tactile perception. These areas coincide with Brodmann areas 7, 31, 39 and 40. It is conceivable that there are differences between healthy subjects and patients with bruxism.

CONCLUSIONS

Our findings indicate that there was a decrease of cortical activation pattern in patients with bruxism in clenching tasks. This indicates decreased blood flow and activation in regional neuronal activity. Bruxism, as an oral motor disorder concerns dentistry, neurology and psychiatry. These results might improve the understanding and physiological handling of sleep bruxism.

Precision of jaw-closing movements for different jaw gaps

Jaw-closing movements are basic components of physiological motor actions precisely achieving intercuspation without significant interference. The main purpose of this study was to test the hypothesis that, despite an imperfect intercuspal position, the precision of jaw-closing movements fluctuates within the range of physiological closing movements indispensable for meeting intercuspation without significant interference. For 35 healthy subjects, condylar and incisal point positions for fast and slow jaw-closing, interrupted at different jaw gaps by the use of frontal occlusal plateaus, were compared with uninterrupted physiological jaw closing, with identical jaw gaps, using a telemetric system for measuring jaw position. Examiner-guided centric relation served as a clinically relevant reference position. For jaw gaps ≤4 mm, no significant horizontal or vertical displacement differences were observed for the incisal or condylar points among physiological, fast, and slow jaw-closing. However, the jaw positions under these three closing conditions differed significantly from guided centric relation for nearly all experimental jaw gaps. The findings provide evidence of stringent neuromuscular control of jaw-closing movements in the vicinity of intercuspation. These results might be of clinical relevance to occlusal intervention with different objectives.

Differential responses to anticipation of reward after an acute dose of the designer drugs benzylpiperazine (BZP) and trifluoromethylphenylpiperazine (TFMPP) alone and in combination using functional magnetic resonance imaging (fMRI)

RATIONALE

Functional magnetic resonance imaging (fMRI) studies have reported increased activation of the mesolimbic system in response to anticipation of rewarding stimuli. The anticipation of uncertain outcomes evokes activation in the amygdala, orbitofrontal cortex, inferior frontal gyrus and insula. Drugs known to effect dopaminergic and serotonergic neurons also alter regional activation.

OBJECTIVES

Benzylpiperazine (BZP) and/or trifluoromethylphenylpiperazine (TFMPP) have been recreationally used worldwide for more than a decade. BZP affects mainly dopaminergic neurons, while TFMPP has serotonergic effects.

METHODS

We investigated the effects of an acute dose of BZP, TFMPP or a combination of BZP and TFMPP on the anticipation of reward in a double-blind, placebo-controlled, crossover study using fMRI. An event-related gambling paradigm was completed by healthy controls 90 min after taking an oral dose of either BZP (200 mg), TFMPP (either 50 or 60 mg), BZP + TFMPP (100 + 30 mg) or placebo.

RESULTS

After giving BZP, the anticipation of a $4 reward decreased the activation of the inferior frontal gyrus, insula and occipital regions in comparison to placebo. TFMPP increased the activation of the putamen but decreased the activity in the insula relative to placebo. When BZP and TFMPP were given in combination, activation of the rolandic operculum occurred. The magnitude of reward also affected neural correlates.

CONCLUSION

We propose that the effects of BZP and TFMPP on dopaminergic and serotonergic circuitry, respectively, reflect regional changes. The dopaminergic effects of BZP appear to increase positive arousal and subsequently reduce the response to uncertainty, while TFMPP appears to alter the response to uncertainty by increasing emotional responses.

Functional connectivity of human chewing: an fcMRI study

Mastication is one of the most important orofacial functions. The neurobiological mechanisms of masticatory control have been investigated in animal models, but less so in humans. This project used functional connectivity magnetic resonance imaging (fcMRI) to assess the positive temporal correlations among activated brain areas during a gum-chewing task. Twenty-nine healthy young-adults underwent an fcMRI scanning protocol while they chewed gum. Seed-based fcMRI analyses were performed with the motor cortex and cerebellum as regions of interest. Both left and right motor cortices were reciprocally functionally connected and functionally connected with the post-central gyrus, cerebellum, cingulate cortex, and precuneus. The cerebellar seeds showed functional connections with the contralateral cerebellar hemispheres, bilateral sensorimotor cortices, left superior temporal gyrus, and left cingulate cortex. These results are the first to identify functional central networks engaged during mastication.