Properties and plasticity of the primate somatosensory and motor cortex related to orofacial sensorimotor function

Pain-sensorimotor interactions: New perspectives and a new model

How pain and sensorimotor behavior interact has been the subject of research and debate for many decades. This article reviews theories bearing on pain-sensorimotor interactions and considers their strengths and limitations in the light of findings from experimental and clinical studies of pain-sensorimotor interactions in the spinal and craniofacial sensorimotor systems. A strength of recent theories is that they have incorporated concepts and features missing from earlier theories to account for the role of the sensory-discriminative, motivational-affective, and cognitive-evaluative dimensions of pain in pain-sensorimotor interactions. Findings acquired since the formulation of these recent theories indicate that additional features need to be considered to provide a more comprehensive conceptualization of pain-sensorimotor interactions. These features include biopsychosocial influences that range from biological factors such as genetics and epigenetics to psychological factors and social factors encompassing environmental and cultural influences. Also needing consideration is a mechanistic framework that includes other biological factors reflecting nociceptive processes and glioplastic and neuroplastic changes in sensorimotor and related brain and spinal cord circuits in acute or chronic pain conditions. The literature reviewed and the limitations of previous theories bearing on pain-sensorimotor interactions have led us to provide new perspectives on these interactions, and this has prompted our development of a new concept, the Theory of Pain-Sensorimotor Interactions (TOPSMI) that we suggest gives a more comprehensive framework to consider the interactions and their complexity. This theory states that pain is associated with plastic changes in the central nervous system (CNS) that lead to an activation pattern of motor units that contributes to the individual's adaptive sensorimotor behavior. This activation pattern takes account of the biological, psychological, and social influences on the musculoskeletal tissues involved in sensorimotor behavior and on the plastic changes and the experience of pain in that individual. The pattern is normally optimized in terms of biomechanical advantage and metabolic cost related to the features of the individual's musculoskeletal tissues and aims to minimize pain and any associated sensorimotor changes, and thereby maintain homeostasis. However, adverse biopsychosocial factors and their interactions may result in plastic CNS changes leading to less optimal, even maladaptive, sensorimotor changes producing motor unit activation patterns associated with the development of further pain. This more comprehensive theory points towards customized treatment strategies, in line with the management approaches to pain proposed in the biopsychosocial model of pain.

Multiple regions of sensorimotor cortex encode bite force and gape

The precise control of bite force and gape is vital for safe and effective breakdown and manipulation of food inside the oral cavity during feeding. Yet, the role of the orofacial sensorimotor cortex (OSMcx) in the control of bite force and gape is still largely unknown. The aim of this study was to elucidate how individual neurons and populations of neurons in multiple regions of OSMcx differentially encode bite force and static gape when subjects (Macaca mulatta) generated different levels of bite force at varying gapes. We examined neuronal activity recorded simultaneously from three microelectrode arrays implanted chronically in the primary motor (MIo), primary somatosensory (SIo), and cortical masticatory (CMA) areas of OSMcx. We used generalized linear models to evaluate encoding properties of individual neurons and utilized dimensionality reduction techniques to decompose population activity into components related to specific task parameters. Individual neurons encoded bite force more strongly than gape in all three OSMCx areas although bite force was a better predictor of spiking activity in MIo vs. SIo. Population activity differentiated between levels of bite force and gape while preserving task-independent temporal modulation across the behavioral trial. While activation patterns of neuronal populations were comparable across OSMCx areas, the total variance explained by task parameters was context-dependent and differed across areas. These findings suggest that the cortical control of static gape during biting may rely on computations at the population level whereas the strong encoding of bite force at the individual neuron level allows for the precise and rapid control of bite force.

Robust cortical encoding of 3D tongue shape during feeding in macaques

Dexterous tongue deformation underlies eating, drinking, and speaking. The orofacial sensorimotor cortex has been implicated in the control of coordinated tongue kinematics, but little is known about how the brain encodes-and ultimately drives-the tongue's 3D, soft-body deformation. Here we combine a biplanar x-ray video technology, multi-electrode cortical recordings, and machine-learning-based decoding to explore the cortical representation of lingual deformation. We trained long short-term memory (LSTM) neural networks to decode various aspects of intraoral tongue deformation from cortical activity during feeding in male Rhesus monkeys. We show that both lingual movements and complex lingual shapes across a range of feeding behaviors could be decoded with high accuracy, and that the distribution of deformation-related information across cortical regions was consistent with previous studies of the arm and hand.

Links Between Swallowing and Consciousness: A Narrative Review

This literature review explores a wide range of themes addressing the links between swallowing and consciousness. Signs of consciousness are historically based on the principle of differentiating reflexive from volitional behaviors. We show that the sequencing of the components of swallowing falls on a continuum of voluntary to reflex behaviors and we describe several types of volitional and non-volitional swallowing tasks. The frequency, speed of initiation of the swallowing reflex, efficacy of the pharyngeal phase of swallowing and coordination between respiration and swallowing are influenced by the level of consciousness during non-pathological modifications of consciousness such as sleep and general anesthesia. In patients with severe brain injury, the level of consciousness is associated with several components related to swallowing, such as the possibility of extubation, risk of pneumonia, type of feeding or components directly related to swallowing such as oral or pharyngeal abnormalities. Based on our theoretical and empirical analysis, the efficacy of the oral phase and the ability to receive exclusive oral feeding seem to be the most robust signs of consciousness related to swallowing in patients with disorders of consciousness. Components of the pharyngeal phase (in terms of abilities of saliva management) and evoked cough may be influenced by consciousness, but further studies are necessary to determine if they constitute signs of consciousness as such or only cortically mediated behaviors. This review also highlights the critical lack of tools and techniques to assess and treat dysphagia in patients with disorders of consciousness.

Neural Basis of Etiopathogenesis and Treatment of Cervicogenic Orofacial Pain

(1) Background and Objectives: The aim of this narrative review was to analyze the neuroanatomical and neurophysiological basis of cervicogenic pain in cervico-cranial pain syndromes, focusing particularly on cervico-orofacial syndromes as a background for the proper diagnosis and non-surgical treatment. Relevant literature on the topic from past 120 years has been surveyed. (2) Material and Methods: We surveyed all original papers, reviews, or short communications published in the English, Spanish, Czech or Slovak languages from 1900 to 2020 in major journals. (3) Results: The cervicogenic headache originates from the spinal trigeminal nucleus where axons from the C₁–C₃ cervical spinal nerves and three branches of the trigeminal nerve converge (trigeminocervical convergence) at the interneurons that mediate cranio-cervical nociceptive interactions. The role of the temporomandibular joint in the broad clinical picture is also important. Despite abundant available experimental and clinical data, cervicogenic orofacial pain may be challenging to diagnose and treat. Crucial non-surgical therapeutic approach is the orthopedic manual therapy focused on correction of body posture, proper alignment of cervical vertebra and restoration of normal function of temporomandibular joint and occlusion. In addition, two novel concepts for the functional synthesis of cervico-cranial interactions are the tricentric concept of mouth sensorimotor control and the concept of a cervicogenic origin of bruxism. (4) Conclusions: Understanding the basis of neuroanatomical and neurophysiological neuromuscular relations enables an effective therapeutic approach based principally on orthopedic manual and dental occlusal treatment.

Beyond language: The unspoken sensory-motor representation of the tongue in non-primates, non-human and human primates

The English idiom "on the tip of my tongue" commonly acknowledges that something is known, but it cannot be immediately brought to mind. This phrase accurately describes sensorimotor functions of the tongue, which are fundamental for many tongue-related behaviors (e.g., speech), but often neglected by scientific research. Here, we review a wide range of studies conducted on non-primates, non-human and human primates with the aim of providing a comprehensive description of the cortical representation of the tongue's somatosensory inputs and motor outputs across different phylogenetic domains. First, we summarize how the properties of passive non-noxious mechanical stimuli are encoded in the putative somatosensory tongue area, which has a conserved location in the ventral portion of the somatosensory cortex across mammals. Second, we review how complex self-generated actions involving the tongue are represented in more anterior regions of the putative somato-motor tongue area. Finally, we describe multisensory response properties of the primate and non-primate tongue area by also defining how the cytoarchitecture of this area is affected by experience and deafferentation.

Neurogenic Dysphagia and Nutrition in Disorder of Consciousness: An Overview with Practical Advices on an “Old” but Still Actual Clinical Problem

Neurogenic dysphagia is a difficulty in swallowing food caused by disease or impairment of the nervous system, including stroke and traumatic brain injury. The most clinically apparent complication of neurogenic dysphagia is pulmonary aspiration, which may manifest itself acutely as choking or coughing, respiratory distress, wheezing, gasping or gurgling, and tachycardia. However, chronic symptoms, including weight loss, production of excessive oral secretions and aspiration pneumonia, may be also present, especially in patients with a disorder of consciousness (DOC). Usually, patients with dysphagia after the acute phase need to be treated with enteral nutrition using a feeding tube. This avoids patient malnutrition and supports the rehabilitation program. This narrative review aims to investigate dysphagia and its complications and management in patients with DOC. Clinical indications and practical advice on how to assess and treat this complex problem are also provided.

Enhancement of Sensorimotor Cortical Adaptation after Dental Implantation in Comparison to the Conventional Denture - Demonstration by Functional MRI at 1-5T

Background and Purpose

Dental implantation is thought to be associated with enhancement of neuro-cortical sensorimotor activity which has been lost due to an edentulous state. Such changes are either feeble or absent after the placement of a conventional denture. In the present study, we test this hypothesis using blood oxygen level-dependent (BOLD) activity on functional MRI (fMRI) as a bio-surrogate.

Materials and Methods

fMRI was performed in 12 consecutive edentulous subjects (mean age = 59.2 years) after the placement of a conventional complete denture (CD) and subsequently after intraoral dental implantation (IOD). The semi-quantitative data of the BOLD activity was compiled to depict the activation seen in both scenarios in six anatomical regions. Statistical analysis was done to evaluate the significance of enhancement in BOLD activity in these regions in patients having an IOD as compared to those having a CD.

Results

The enhancement of BOLD activity on fMRI after placement of an IOD was much more significant as compared to that noted with CD. Using Wilcoxon's signed-rank test the nonparametric data showed a significant positive elevation in global and regional assigned mean ranks of BOLD activity.

Conclusion

Intraoral implantation leads to a significant elevation in the BOLD activity of the sensorimotor cortex as compared to the placement of a conventional CD.

Neural correlates of cricopharyngeal dysfunction after supratentorial stroke: A voxel-based lesion-symptom mapping with propensity score matched case-control

BACKGROUND

Poststroke cricopharyngeal dysfunction has been reported to occur in 50% of brainstem strokes; however, cricopharyngeal dysfunction also occurs commonly in patients with supratentorial stroke. The hemispheric neuroanatomical location of this dysfunction has not been clearly identified.

AIM

We aimed to analyze the relationship between cricopharyngeal dysfunction and supratentorial lesion location in poststroke patients through this retrospective case-control voxel-based lesion-symptom mapping study.

METHODS

Cricopharyngeal dysfunction was diagnosed when the residue after swallowing (pyriform sinus) accounted for more than 25% of volume of pyriform sinus. Medical records and the video fluoroscopic swallowing studies of first-ever stroke patients who were admitted to our hospital during acute to subacute phase from 2009 to 2019 were reviewed. After propensity score matching to reduce the likelihood of selection bias, 50 patients per group were included in the cricopharyngeal dysfunction and control groups. We used a P threshold of 0.01 corrected for multiple comparisons with permutation thresholding (5000 permutations). Dichotomized diagnosis of cricopharyngeal dysfunction and the magnitude of pyriform sinus were used as dependent variables.

RESULTS

Analysis using the Liebermeister statistics indicated that lesions of the right lentiform nucleus were associated with the development of cricopharyngeal dysfunction. After adjustment for age and total lesion volume, which are known effectors for the development of dysphagia, statistically significant correlations were found between pyriform sinus and lesions of the right lentiform nucleus and anterior corona radiata beneath the right middle frontal gyrus.

CONCLUSION

Thus, our study demonstrated for the first time that damages to the right lentiform nucleus, especially globus pallidus externa, and anterior corona radiata beneath the right middle frontal gyrus are associated with the development and severity of cricopharyngeal dysfunction.

Swallowing in individuals with disorders of consciousness: A cohort study

BACKGROUND

After a period of coma, a proportion of individuals with severe brain injury remain in an altered state of consciousness before regaining partial or complete recovery. Individuals with disorders of consciousness (DOC) classically receive hydration and nutrition through an enteral-feeding tube. However, the real impact of the level of consciousness on an individual's swallowing ability remains poorly investigated.

OBJECTIVE

We aimed to document the incidence and characteristics of dysphagia in DOC individuals and to evaluate the link between different components of swallowing and the level of consciousness.

METHODS

We analyzed clinical data on the respiratory status, oral feeding and otolaryngologic examination of swallowing in DOC individuals. We analyzed the association of components of swallowing and participant groups (i.e., unresponsive wakefulness syndrome [UWS] and minimally conscious state [MCS]).

RESULTS

We included 92 individuals with DOC (26 UWS and 66 MCS). Overall, 99% of the participants showed deficits in the oral and/or pharyngeal phase of swallowing. As compared with the MCS group, the UWS group more frequently had a tracheostomy (69% vs 24%), with diminished cough reflex (27% vs 54%) and no effective oral phase (0% vs 21%).

CONCLUSION

Almost all DOC participants had severe dysphagia. Some components of swallowing (i.e., tracheostomy, cough reflex and efficacy of the oral phase of swallowing) were related to consciousness. In particular, no UWS participant had an efficient oral phase, which suggests that its presence may be a sign of consciousness. In addition, no UWS participant could be fed entirely orally, whereas no MCS participant orally received ordinary food. Our study also confirms that objective swallowing assessment can be successfully completed in DOC individuals and that specific care is needed to treat severe dysphagia in DOC.

Functional Connectivity Evoked by Orofacial Tactile Perception of Velocity

The cortical representations of orofacial pneumotactile stimulation involve complex neuronal networks, which are still unknown. This study aims to identify the characteristics of functional connectivity (FC) evoked by three different saltatory velocities over the perioral and buccal surface of the lower face using functional magnetic resonance imaging in twenty neurotypical adults. Our results showed a velocity of 25 cm/s evoked stronger connection strength between the right dorsolateral prefrontal cortex and the right thalamus than a velocity of 5 cm/s. The decreased FC between the right secondary somatosensory cortex and right posterior parietal cortex for 5-cm/s velocity versus all three velocities delivered simultaneously (“All ON”) and the increased FC between the right thalamus and bilateral secondary somatosensory cortex for 65 cm/s vs “All ON” indicated that the right secondary somatosensory cortex might play a role in the orofacial tactile perception of velocity. Our results have also shown different patterns of FC for each seed (bilateral primary and secondary somatosensory cortex) at various velocity contrasts (5 vs 25 cm/s, 5 vs 65 cm/s, and 25 vs 65 cm/s). The similarities and differences of FC among three velocities shed light on the neuronal networks encoding the orofacial tactile perception of velocity.

Abnormal Emotional Processing and Emotional Experience in Patients with Peripheral Facial Nerve Paralysis: An MEG Study

Abnormal emotional reactions of the brain in patients with facial nerve paralysis have not yet been reported. This study aims to investigate this issue by applying a machine-learning algorithm that discriminates brain emotional activities that belong either to patients with facial nerve paralysis or to healthy controls. Beyond this, we assess an emotion rating task to determine whether there are differences in their experience of emotions. MEG signals of 17 healthy controls and 16 patients with facial nerve paralysis were recorded in response to picture stimuli in three different emotional categories (pleasant, unpleasant, and neutral). The selected machine learning technique in this study was the logistic regression with LASSO regularization. We demonstrated significant classification performances in all three emotional categories. The best classification performance was achieved considering features based on event-related fields in response to the pleasant category, with an accuracy of 0.79 (95% CI (0.70, 0.82)). We also found that patients with facial nerve paralysis rated pleasant stimuli significantly more positively than healthy controls. Our results indicate that the inability to express facial expressions due to peripheral motor paralysis of the face might cause abnormal brain emotional processing and experience of particular emotions.

Effects of mental and physical orofacial training on pressure pain sensitivity and tongue strength: A single-blind randomized controlled trial

OBJECTIVES

The main objective of this study was to analyze differences on pain pressure thresholds, tongue strength and perceived effort between various orofacial motor exercise training dosages of mental representation training through motor imagery (MI) and action observation (AO), first in isolation and then in combination with real exercise performance.

METHODS

A single-blind randomized controlled trial was designed. 48 asymptomatic individuals were randomized into two groups: Intensive training group (IG) and Moderate training group (MG). Both groups performed a first session of MI and AO of orofacial exercises training and a second session of actual orofacial exercises combined with mental representation training, but with different dosage in terms of series and repetitions. Pain pressure thresholds (PPTs) in the masseter and temporal muscles and tongue muscle strength were the main variables.

RESULTS

Regarding the PPT, ANOVA revealed significant between-group differences, where MG showed a significantly higher PPT than IG at post-day2, with a medium effect size. Both groups showed with-in group differences between pre and post intervention measures in the first session, but only the IG showed differences in the second. Regarding tongue muscle strength, ANOVA revealed significant within-group differences only in MG between the pre-day and post-day first intervention.

CONCLUSION

The results of the present study suggest that movement representation training performed in isolation may have a positive effect on PPTs and tongue muscle strength. In addition, the combination with the actual execution of the exercises could be considered effective, but it is necessary to take into account the training dosage to avoid fatigue responses.

The effectivity of pharyngeal strengthening exercise, hyolaryngeal complex range of motion exercise, and swallowing practice in swallowing function of ischemic stroke patients with neurogenic dysphagia

Dysphagia is associated with an increased risk of aspiration pneumonia which often results in death in stroke patients. Therefore, effective and efficient management is important. Behavioral therapy for swallowing rehabilitation based on the principles of neuroplasticity such as oropharyngeal strengthening and range of motion exercises are the alternative ones that often be used. This study aimed to assess the changes in swallowing function in ischemic stroke patients with neurogenic dysphagia after pharyngeal strengthening exercise, hyolaryngeal complex range of motion exercise and swallowing practice. Swallowing function was assessed using Penetration Aspiration Scale (PAS) and Functional Oral Intake Scale (FOIS) based on Flexible Endoscopic Evaluation of Swallowing before and after interventions. The interventions were given every day with a duration of 30–45 minutes for 4 weeks. There were six subjects who completed the study. The PAS before the interventions were 6.00±1.79 and after the interventions was 1.67±0.82 (P=0.003). Meanwhile, the FOIS score before the interventions was 3 (1–5) and after the interventions were 5.00±2.10 (P=0.041). There was an improvement of PAS and FOIS after the interventions. Therefore, the interventions can be suggested to be used as one of the treatments to improve swallowing function in ischemic stroke patients with neurogenic dysphagia.

Combination of jaw and tongue movement training influences neuroplasticity of corticomotor pathways in humans

Since humans in daily life perform multiple motor behaviors that often involve the simultaneous activation of both jaw and tongue muscles, it is essential to understand the effects of combined orofacial sensorimotor tasks on plasticity in corticomotor pathways. Moreover, to establish novel rehabilitation programs for patients, it is important to clarify the possible interrelationships in corticomotor excitability between jaw and tongue motor control. The aim of this study was to examine the effect of a combination of a repetitive tooth bite task (TBT) and a repetitive tongue lift task (TLT) on corticomotor excitability of the tongue and jaw muscles as assessed by transcranial magnetic stimulation (TMS). Sixteen healthy individuals participated in three kinds of training tasks consisting of 41-min TBT, 41-min TLT, and 82-min TBT + TLT. Motor-evoked potentials (MEPs) from the tongue muscle, masseter muscle, and first dorsal interosseous muscle were measured before and after the training tasks. The amplitude of tongue MEPs after training with TLT and TLT + TBT, and masseter MEPs after training with TBT and TLT + TBT, were significantly higher than before training (P < 0.05). Tongue MEPs and masseter MEPs were significantly higher after TLT + TBT than after TBT or TLT (P < 0.05). The present results suggest that a task combining both jaw and tongue movement training is associated with a greater degree of neuroplasticity in the corticomotor control of jaw and tongue muscles than either task alone.

The role of EEG and EMG combined virtual reality gaming system in facial palsy rehabilitation - A case report

BACKGROUND

The recovery rates for facial palsy are usually excellent; however, regularly patients present with problems with their fine facial movements that affect their emotional expressions.

OBJECTIVE

To discover the viability and ease of using an Electroencephalogram (EEG) and Electromyography (EMG) combined Virtual Reality (VR) gaming system - the 'Oculus Rift' device in the evaluation and rehabilitation of facial palsy.

DESIGN

Single case study.

PATIENT INFORMATION

A young 23-year-old female with facial palsy.

CLINICAL FINDINGS

Most of the patient's facial features were re-established within the recovery time frame, except for her right forehead and eyebrow movements.

INTERVENTION

A 10 day exercise program (Day 2-11) with an immersive virtual reality device, which randomly shoots virtually animated white balls in an unpredictable and testing pattern.

OUTCOME MEASURES

EEG and EMG patterns corresponding to the facial upper quadrant were taken at baseline, post-intervention, and at follow up.

RESULTS

EMG and EEG investigation revealed a progressive improvement in the muscle activation in response to the impulsive and unpredictable activities in the virtual environment provided through the immersive VR device.

CONCLUSION

The case report found a positive relationship between VR, facial upper quadrant EMG activation and EEG pattern changes following the intervention.

Motor imagery and swallowing: a systematic literature review

ABSTRACT Objetive: to identify, in the literature, studies that address the use of motor imagery of swallowing. Methods: a systematic review in SCOPUS databases, Science Direct and Medline, with descriptors and free terms "Motor Imagery"; "Swallow"; "Feeding"; "Stomatognathic System"; "mastication ", "Chew "; "Deglutition "; "Deglutition Disorders "; and "Mental Practice". Original articles using the motor imagery of swallowing were included, while reviews were excluded. For data analysis, at the first and second steps, the reading of titles and abstracts of the studies was carried out. In the third step, all studies that were not excluded were read in full. Results: four manuscripts were selected. The use of motor imagery in the rehabilitation of swallowing shows to be a recent proposal (2014-2015). The sample was reduced and comprised mainly healthy individuals. The EMG of the supra-hyoid muscles was used in two manuscripts. The most used neuroimaging technique was the Near-Infrared Spectroscopy, demonstrating the occurrence of hemodynamic changes during motor imagery and motor execution of swallowing. Conclusion: the motor imagery produces brain response in the motor area of the brain, suggesting that mentalization of actions related to swallowing is effective. However, further studies are needed for the application of this approach in the swallowing rehabilitation.

Effects of pharyngeal electrical stimulation on swallowing performance

Pharyngeal electrical stimulation (PEStim) has been found to facilitate voluntary swallowing. This study investigated how PEStim contributed to modulation of swallowing function in 15 healthy humans. In the involuntary swallowing test, water was injected onto the pharynx at 0.05 ml/s and the onset latency of the first swallow was measured. In the voluntary swallowing test, subjects swallowed their own saliva as quickly as possible for 30 s and the number of swallows was counted. Voluntary and involuntary swallowing was evaluated before (baseline), immediately after, and every 10 min after 10-min PEStim for 60 min. A voluntary swallowing test with simultaneous 30-s PEStim was also conducted before and 60 min after 10-min PEStim. The number of voluntary swallows with simultaneous PEStim significantly increased over 60 min after 10-min PEStim compared with the baseline. The onset latency of the first swallow in the involuntary swallowing test was not affected by 10-min PEStim. The results suggest that PEStim may have a long-term facilitatory effect on the initiation of voluntary swallowing in healthy humans, but not on peripherally-evoked swallowing. The physiological implications of this modulation are discussed.

"Closed Reduction" Principles Can Manage Diverse Conditions of Temporomandibular Joint Vertical Height Loss: From Displaced Condylar Fractures to Idiopathic Condylar Resorption

PURPOSE

The aim of this case series was to describe a modification of the classic "closed reduction" technique to manage unilateral or anterior open bite owing to a loss in vertical height (LVH) caused by several disorders and pathologies other than displaced condylar fractures.

MATERIALS AND METHODS

The protocol included insertion of an occlusal appliance to increase the height of the premature contact and the width of the open bite, stabilization of the dental arches by rigid arches, and the use, during sleep, of rubber bands in the open bite region to pull the mandible cranially. In addition, when awake, the patient performed physiotherapy exercises to guide the mandible into maximum intercuspation. The increased open bite enhanced the effect of the rubber bands in guiding the mandible into the original habitual occlusion and the rigid arches served to minimize tooth eruption.

RESULTS

The present cases showed the favorable outcome of this low-risk treatment in the re-establishment of the original habitual occlusion within 1 to 4 weeks and without reconstruction of the LVH.

CONCLUSION

The efficacy of this complication-free approach to correct occlusion in various conditions of LVH suggests that this protocol should be applied before venturing into surgical intervention.

Fine motor control of the jaw following alteration of orofacial afferent inputs

Objective

The study was designed to investigate if alteration of different orofacial afferent inputs would have different effects on oral fine motor control and to test the hypothesis that reduced afferent inputs will increase the variability of bite force values and jaw muscle activity, and repeated training with splitting of food morsel in conditions with reduced afferent inputs would decrease the variability and lead to optimization of bite force values and jaw muscle activity.

Material methods

Forty-five healthy volunteers participated in a single experimental session and were equally divided into incisal, mucosal, and block anesthesia groups. The participants performed six series (with ten trials) of a standardized hold and split task after the intervention with local anesthesia was made in the respective groups. The hold and split forces along with the corresponding jaw muscle activity were recorded and compared to a reference group.

Results

The hold force and the electromyographic (EMG) activity of the masseter muscles during the hold phase were significantly higher in the incisal and block anesthesia group, as compared to the reference group (P < 0.001). However, there was no significant effect of groups on the split force (P = 0.975) but a significant decrease in the EMG activity of right masseter in mucosal anesthesia group as compared to the reference group (P = 0.006). The results also revealed that there was no significant effect of local anesthesia on the variability of the hold and split force (P < 0.677). However, there was a significant decrease in the variability of EMG activity of the jaw closing muscles in the block anesthesia group as compared to the reference group (P < 0.041), during the hold phase and a significant increase in the variability of EMG activity of right masseter in the mucosal anesthesia group (P = 0.021) along with a significant increase in the EMG activity of anterior temporalis muscle in the incisal anesthesia group, compared to the reference group (P = 0.018), during the split phase.

Conclusions

The results of the present study indicated that altering different orofacial afferent inputs may have different effects on some aspects of oral fine motor control. Further, inhibition of afferent inputs from the orofacial or periodontal mechanoreceptors did not increase the variability of bite force values and jaw muscle activity; indicating that the relative precision of the oral fine motor task was not compromised inspite of the anesthesia. The results also suggest the propensity of optimization of bite force values and jaw muscle activity due to repeated splitting of the food morsels, inspite of alteration of sensory inputs.

Clinical relevance

Skill acquisition following a change in oral sensory environment is crucial for understanding how humans learn and re-learn oral motor behaviors and the kind of adaptation that takes place after successful oral rehabilitation procedures.

Bilateral sensory deprivation of trigeminal afferent fibres on corticomotor control of human tongue musculature: a preliminary study

Transcranial magnetic stimulation (TMS) has demonstrated changes in motor evoked potentials (MEPs) in human limb muscles following modulation of sensory afferent inputs. The aim of this study was to determine whether bilateral local anaesthesia (LA) of the lingual nerve affects the excitability of the tongue motor cortex (MI) as measured by TMS. The effect on MEPs after bilateral LA of the lingual nerve was studied, while the first dorsal interosseous (FDI) muscle served as a control in ten healthy participants. MEPs were measured on the right side of the tongue dorsum in four different conditions: (i) immediately prior to anaesthesia (baseline), (ii) during bilateral LA block of the lingual nerve, (iii) after anaesthesia had subjectively subsided (recovery) and (iv) 3 h after bilateral lingual block injection. MEPs were assessed using stimulus-response curves in steps of 10% of motor threshold (T). Eight stimuli were given at each stimulus level. The amplitudes of the tongue MEPs were significantly influenced by the stimulus intensity (P < 0·001) but not by condition (P = 0·186). However, post hoc tests showed that MEPS were statistically significantly higher during bilateral LA block condition compared with baseline at T + 40%, T + 50% and T + 60% (P < 0·028) and also compared with recovery at T + 60% (P = 0·010) as well as at 3 h after injection at T + 50% and T + 60% (P < 0·029). Bilateral LA block of the lingual nerve seems to be associated with a facilitation of the corticomotor pathways related to the tongue musculature.

Osseoperception in Dental Implants: A Systematic Review

PURPOSE

Replacement of lost teeth has significant functional and psychosocial effects. The capability of osseointegrated dental implants to transmit a certain amount of sensibility is still unclear. The phenomenon of developing a certain amount of tactile sensibility through osseointegrated dental implants is called osseoperception. The aim of this article is to evaluate the available literature to find osseoperception associated with dental implants.

MATERIALS AND METHODS

To identify suitable literature, an electronic search was performed using Medline and PubMed database. Articles published in English and articles whose abstract is available in English were included. The articles included in the review were based on osseoperception, tactile sensation, and neurophysiological mechanoreceptors in relation to dental implants. Articles on peri-implantitis and infection-related sensitivity were not included. Review articles without the original data were excluded, although references to potentially pertinent articles were noted for further follow-up. The phenomenon of osseoperception remains a matter of debate, so the search strategy mainly focused on articles on osseoperception and tactile sensibility of dental implants. This review presents the histological, neurophysiological, and psychophysical evidence of osseoperception and also the role of mechanoreceptors in osseoperception.

RESULTS

The literature on osseoperception in dental implants is very scarce. The initial literature search resulted in 90 articles, of which 81 articles that fulfilled the inclusion criteria were included in this systematic review.

CONCLUSION

Patients restored with implant-supported prostheses reported improved tactile and motor function when compared with patients wearing complete dentures.

Time-course of Changes in Activation Among Facial Nerve Injury: A Functional Imaging Study

Patients suffering different intervals of facial nerve injury were investigated by functional magnetic resonance imaging to study changes in activation within cortex.Forty-five patients were divided into 3 groups based on intervals of facial nerve injury. Another 16 age and sex-matched healthy participants were included as a control group. Patients and healthy participants underwent task functional magnetic resonance imaging (eye blinking and lip pursing) examination.Functional reorganization after facial nerve injury is dynamic and time-dependent. Correlation between activation in sensorimotor area and intervals of facial nerve injury was significant, with a Pearson correlation coefficient of -0.951 (P < 0.001) in the left sensorimotor area and a Pearson correlation coefficient of 0.333 (P = 0.025) in the right sensorimotor area.Increased activation in integration areas, such as supramarginal gyrus and precunes lobe, could be detected in the early-middle stage of facial dysfunction compared with normal individuals. Decreased activation in sensorimotor area contralateral to facial nerve injury could be found in late stage of facial dysfunction compared with normal individuals. Dysfunction in the facial nerve has devastating effects on the activity of sensorimotor areas, whereas enhanced intensity in the sensorimotor area ipsilateral to the facial nerve injury in middle stage of facial dysfunction suggests the possible involvement of interhemispheric reorganization. Behavioral or brain stimulation technique treatment in this stage could be applied to alter reorganization within sensorimotor area in the rehabilitation of facial function, monitoring of therapeutic efficacy, and improvement in therapeutic intervention along the course of recovery.

Investigating complex basal ganglia circuitry in the regulation of motor behaviour, with particular focus on orofacial movement

Current concepts of basal ganglia function have evolved from the essentially motoric, to include a range of extramotoric functions that involve not only dopaminergic but also cholinergic, γ-aminobutyric acid (GABA)ergic and glutamatergic mechanisms. We consider these mechanisms and their efferent systems, including spiralling, feed-forward striato-nigro-striatal circuitry, involving the dorsal and ventral striatum and the nucleus accumbens (NAc) core and shell. These processes are illustrated using three behavioural models: turning-pivoting, orofacial movements in rats and orofacial movements in genetically modified mice. Turning-pivoting indicates that dopamine-dependent behaviour elicited from the NAc shell is funnelled through the NAc-nigro-striato-nigro-pedunculopontine pathway, whereas acetylcholine-dependent behaviour elicited from the NAc shell is funnelled through the NAc-ventral pallidum-mediodorsal thalamus pathway. Cooperative/synergistic interactions between striatal D1-like and D2-like dopamine receptors regulate individual topographies of orofacial movements that are funnelled through striatal projection pathways and involve interactions with GABAergic and glutamatergic receptor subtypes. This application of concerted behavioural, neurochemical and neurophysiological techniques implicates a network that is yet broader and interacts with other neurotransmitters and neuropeptides within subcortical, cortical and brainstem regions to 'sculpt' aspects of behaviour into its topographical collective.

Effects of short-term training on behavioral learning and skill acquisition during intraoral fine motor task

Sensory information from the orofacial mechanoreceptors are used by the nervous system to optimize the positioning of food, determine the force levels, and force vectors involved in biting of food morsels. Moreover, practice resulting from repetition could be a key to learning and acquiring a motor skill. Hence, the aim of the experiment was to test the hypothesis that repeated splitting of a food morsel during a short-term training with an oral fine motor task would result in increased performance and optimization of jaw movements, in terms of reduction in duration of various phases of the jaw movements. Thirty healthy volunteers were asked to intraorally manipulate and split a chocolate candy, into two equal halves. The participants performed three series (with 10 trials) of the task before and after a short-term (approximately 30 min) training. The accuracy of the split and vertical jaw movement during the task were recorded. The precision of task performance improved significantly after training (22% mean deviation from ideal split after vs. 31% before; P<0.001). There was a significant decrease in the total duration of jaw movements during the task after the training (1.21 s total duration after vs. 1.56 s before; P<0.001). Further, when the jaw movements were divided into different phases, the jaw opening phase and contact phase were significantly shorter after training than before training (P=0.001, P=0.002). The results indicate that short-term training of an oral fine motor task induces behavior learning, skill acquisition and optimization of jaw movements in terms of better performance and reduction in the duration of jaw movements, during the task. The finding of the present study provides insights into how humans learn oral motor behaviors or the kind of adaptation that takes place after a successful prosthetic rehabilitation.

Modulation dynamics in the orofacial sensorimotor cortex during motor skill acquisition

The orofacial sensorimotor cortex is known to play a role in motor learning. However, how motor learning changes the dynamics of neuronal activity and whether these changes differ between orofacial primary motor (MIo) and somatosensory (SIo) cortices remain unknown. To address these questions, we used chronically implanted microelectrode arrays to track learning-induced changes in the activity of simultaneously recorded neurons in MIo and SIo as two naive monkeys (Macaca mulatta) were trained in a novel tongue-protrusion task. Over a period of 8-12 d, the monkeys showed behavioral improvements in task performance that were accompanied by rapid and long-lasting changes in neuronal responses in MIo and SIo occurring in parallel: (1) increases in the proportion of task-modulated neurons, (2) increases in the mutual information between tongue-protrusive force and spiking activity, (3) reductions in the across-trial firing rate variability, and (4) transient increases in coherent firing of neuronal pairs. More importantly, the time-resolved mutual information in MIo and SIo exhibited temporal alignment. While showing parallel changes, MIo neurons exhibited a bimodal distribution of peak correlation lag times between spiking activity and force, whereas SIo neurons showed a unimodal distribution. Moreover, coherent activity between pairs of MIo neurons was higher and centered around force onset compared with pairwise coherence of SIo neurons. Overall, the results suggest that the neuroplasticity in MIo and SIo occurring in parallel serves as a substrate for linking sensation and movement during sensorimotor learning, whereas the differing dynamic organizations reflect specific ways to control movement parameters as learning progresses.

Influence of position and stimulation parameters on intracortical inhibition and facilitation in human tongue motor cortex

Paired-pulse transcranial magnetic stimulation (ppTMS) can be used to assess short-interval intracortical inhibitory (SICI) and facilitatory (ICF) networks. Many methodological parameters may however influence the outcome. The aim of the study was to examine the influence of body positions (recline and supine), inter-stimulus intervals (ISI) between the test stimulus (TS) and conditioning stimulus (CS) and intensities of the TS and CS on the degree of SICI and ICF. In studies 1 and 2, fourteen and seventeen healthy volunteers participated respectively. ppTMS was applied over the "hot-spot" of the tongue motor cortex and motor evoked potentials (MEPs) were recorded from contralateral tongue muscles. In study 1, single pulse and three ppTMS ISIs, 2, 10, and 15ms, were applied 8 times each in three blocks (TS: 120%, 140% and 160% of resting motor threshold (rMT); CS: 80% of rMT) in two different body positions (recline and supine) randomly. In study 2, single pulse and four ppTMS ISIs, 2, 2.5, 3, and 3.5ms, were applied 8 times each in randomized order in two blocks (CS: 70% and 80% of rMT; TS: 120% of rMT). There was a significant effect of body position (P=0.049), TS intensities (P<0.001) and ISIs (P<0.001) and interaction between intensity and ISIs (P=0.042) in study 1. In study 2, there was a significant effect of ISI (P<0.001) but not CS intensity (P=0.984) on MEP amplitude. These results may be applied in future studies on the mechanisms of cortical plasticity in the tongue motor pathways using ppTMS and SICI and ICF.

Rethinking Rehab: Skill-Based Training for Swallowing Impairment

Current rehabilitation approaches for swallowing impairment are limited by a general lack of specificity to associated pathophysiology, with many of our practices focusing on increasing strength of muscle activation. However, alternative rehabilitative options are emerging. One of these speculates on the concept of “skill training” for swallowing rehabilitation in neurogenic dysphagia. The presumed intent of this approach is to modulate neural substrates and refine motor planning for swallowing using intrinsically generated cortical modulation and adaptive practice. This manuscript provides a discussion of skill training in the context of physical rehabilitation, illustrating how this information may translate to the diagnosis and remediation of swallowing impairment. This information prompts the consideration of more diverse swallowing pathophysiologies, beyond peripheral muscle weakness. The focus on skill training approaches provides a pathway by which greater specificity of diagnosis and treatment can occur.

Directional information from neuronal ensembles in the primate orofacial sensorimotor cortex

Neurons in the arm and orofacial regions of the sensorimotor cortex in behaving monkeys display directional tuning of their activity during arm reaching and tongue protrusion, respectively. While studies on population activity abound for the arm motor cortex, how populations of neurons from the orofacial sensorimotor cortex represent direction has never been described. We therefore examined and compared the directional information contained in the spiking activity of populations of single neurons recorded simultaneously from chronically implanted microelectrode arrays in the orofacial primary motor (MIo, N = 345) and somatosensory (SIo, N = 336) cortices of monkeys (Macaca mulatta) as they protruded their tongue in different directions. Differential modulation to the direction of tongue protrusion was found in >60% of task-modulated neurons in MIo and SIo and was stronger in SIo (P < 0.05). Moreover, mutual information between direction and spiking was significantly higher in SIo compared with MIo at force onset and force offset (P < 0.01). Finally, the direction of tongue protrusion was accurately predicted on a trial-by-trial basis from the spiking activity of populations of MIo or SIo neurons by using a discrete decoder (P < 0.01). The highly reliable decoding was comparable between MIo and SIo neurons. However, the temporal evolution of the decoding performance differed between these two areas: MIo showed late-onset, fast-rising, and phasic performance, whereas SIo exhibited early-onset, slow-rising, and sustained performance. Overall, the results suggest that both MIo and SIo are highly involved in representing the direction of tongue protrusion but they differ in the amplitude and temporal processing of the directional information distributed across populations of neurons.

Generation of the masticatory central pattern and its modulation by sensory feedback

The basic pattern of rhythmic jaw movements produced during mastication is generated by a neuronal network located in the brainstem and referred to as the masticatory central pattern generator (CPG). This network composed of neurons mostly associated to the trigeminal system is found between the rostral borders of the trigeminal motor nucleus and facial nucleus. This review summarizes current knowledge on the anatomical organization, the development, the connectivity and the cellular properties of these trigeminal circuits in relation to mastication. Emphasis is put on a population of rhythmogenic neurons in the dorsal part of the trigeminal sensory nucleus. These neurons have intrinsic bursting capabilities, supported by a persistent Na(+) current (I(NaP)), which are enhanced when the extracellular concentration of Ca(2+) diminishes. Presented evidence suggest that the Ca(2+) dependency of this current combined with its voltage-dependency could provide a mechanism for cortical and sensory afferent inputs to the nucleus to interact with the rhythmogenic properties of its neurons to adjust and adapt the rhythmic output. Astrocytes are postulated to contribute to this process by modulating the extracellular Ca(2+) concentration and a model is proposed to explain how functional microdomains defined by the boundaries of astrocytic syncitia may form under the influence of incoming inputs.

Increased occlusal vertical dimension induces cortical plasticity in the rat face primary motor cortex

Previous studies have demonstrated that functional plasticity in the primary motor cortex (M1) is related to motor-skill learning and changes in the environment. Increased occlusal vertical dimension (iOVD) may modulate mastication, such as in the masticatory cycle, and the firing properties of jaw-muscle spindles. However, little is known about the changes in motor representation within the face primary motor cortex (face-M1) after iOVD. The purpose of the present study was to determine the effect of iOVD on the face-M1 using intracortical microstimulation (ICMS). In an iOVD group, the maxillary molars were built-up by 2mm with acrylic. The electromyographic (EMG) activities from the left (LAD) and right (RAD) anterior digastric (AD), masseter and genioglossus (GG) muscles elicited by ICMS within the right face-M1 were recorded 1, 2 and 8 weeks after iOVD. IOVD was associated with a significant increase in the number of sites within the face-M1 from which ICMS evoked LAD and/or GG EMG activities, as well as a lateral shift in the center of gravity of the RAD and LAD muscles at 1 and 2 weeks, but not at 8 weeks. These findings suggest that a time-dependent neuroplastic change within the rat face-M1 occurs in association with iOVD. This may be related to the animal's ability to adapt to a change in the oral environment.

Time Course of Cortical Plasticity After Facial Nerve Palsy

Background. Functional connectivity is defined as the temporal correlation between spatially remote neurophysiological events. This method has become particularly useful for studying neuroplasticity to detect changes in the collaboration of brain areas during cortical reorganization. Methods. In this article, the authors longitudinally studied voxel-based morphometry and resting state functional magnetic resonance imaging 10 times in 1 patient during the course of Bell palsy (idiopathic facial nerve palsy) up to complete clinical recovery. Results. Morphometric analysis revealed a significant alteration in the face area of the primary motor cortex (M1) contralateral to the paretic face, with an initial increase in gray matter concentration. Functional connectivity analysis between the M1 and other parts of the facial motor network revealed acutely disrupted intrahemispheric connectivity but unaltered interhemispheric connectivity. The disrupted functional connectivity was most pronounced on the day of the onset of symptoms, with a subsequent return toward normal during the course of recovery. This time course was found to differ between the selected parts of the facial motor network. However, the increase in functional connectivity strength preceded clinical recovery in all areas and reached a stable level before the patient fully recovered. Conclusion. These results demonstrate that recovery from facial nerve palsy is complemented by cortical reorganization, with pronounced changes of functional connectivity that precede clinical recovery.

Corticomotor plasticity induced by tongue-task training in humans: a longitudinal fMRI study

Corticomotor pathways may undergo neuroplastic changes in response to acquisition of new motor skills. Little is known about the motor control strategies for learning new tongue tasks. The aim of this study was to investigate the longitudinal effect of novel tongue-task training on corticomotor neuroplasticity. Thirteen healthy, right-handed men, aged 24-35 years (mean age ± SD: 27.3 ± 0.3 years), performed a training task consisting of standardized tongue protrusion onto a force transducer. The tongue task consisted of a relax-protrude-hold-relax cycle with 1.0 N as the target at the hold phase lasting for 1.5 s. Subjects repeated this task for 1 h. Functional magnetic resonance imaging was carried out before the tongue-task training (baseline), 1-h after the training, and one-day and one-week follow-up. During scanning, the subjects performed tongue protrusion in blocks interspersed with rest. A region-of-interest (ROI) approach and an explorative search were implemented for the analysis of corticomotor activity across conditions. All subjects completed the tongue-task training (mean success rate 43.0 ± 13.2%). In the baseline condition, tongue protrusion resulted in bilateral activity in regions most typically associated with a motor task including medial frontal gyrus (supplementary motor area [SMA]), precentral gyrus (tongue motor cortex), putamen, thalamus, and cerebellum. The ROI analysis revealed increased activity in the precentral gyrus already 1 h post-training. One day after the training, increased activity was observed in the precentral gyrus, SMA, putamen, and cerebellum. No increase was found 1 week after training. Correlation analyses between changes in success rates and changes in the numbers of voxels showed robust associations for left Area 4a in primary motor cortex 1 h, 1 day, and 1 week after the tongue-task training and for the left Area 4p in primary motor cortex and the left lateral premotor cortex 1 day after the training. In the unrestricted analysis, increased activity was found in the parahippocampal gyrus 1 h after the tongue-task training and remained for a week. Decreased activity was found in right post-central and middle frontal gyri 1 h and 1 week post-training. The results verified the involvement of specific corticomotor areas in response to tongue protrusion. Short-term tongue-task training was associated with longer-lasting (up to 1 week) changes in motor-related brain activity. The results suggested that primary motor areas are involved in the early and late stages, while other motor areas mainly are engaged in the later stage of corticomotor neuroplasticity of the tongue.

Cutaneous stimulation of the digits and lips evokes responses with different adaptation patterns in primary somatosensory cortex

Neuromagnetic evoked fields were recorded to compare the adaptation of the primary somatosensory cortex (SI) response to tactile stimuli delivered to the glabrous skin at the fingertips of the first three digits (condition 1) and between midline upper and lower lips (condition 2). The stimulation paradigm allowed to characterize the response adaptation in the presence of functional integration of tactile stimuli from adjacent skin areas in each condition. At each stimulation site, cutaneous stimuli (50 ms duration) were delivered in three runs, using trains of 6 pulses with regular stimulus onset asynchrony (SOA). The pulses were separated by SOAs of 500 ms, 250 ms or 125 ms in each run, respectively, while the inter-train interval was fixed (5s) across runs. The evoked activity in SI (contralateral to the stimulated hand, and bilaterally for lips stimulation) was characterized from the best-fit dipoles of the response component peaking around 70 ms for the hand stimulation, and 8 ms earlier (on average) for the lips stimulation. The SOA-dependent long-term adaptation effects were assessed from the change in the amplitude of the responses to the first stimulus in each train. The short-term adaptation was characterized by the lifetime of an exponentially saturating model function fitted to the set of suppression ratios of the second relative to the first SI response in each train. Our results indicate: 1) the presence of a rate-dependent long-term adaptation effect induced only by the tactile stimulation of the digits; and 2) shorter recovery lifetimes for the digits compared with the lips stimulation.