Modulation dynamics in the orofacial sensorimotor cortex during motor skill acquisition

Biomechanical and Cortical Control of Tongue Movements During Chewing and Swallowing

Tongue function is vital for chewing and swallowing and lingual dysfunction is often associated with dysphagia. Better treatment of dysphagia depends on a better understanding of hyolingual morphology, biomechanics, and neural control in humans and animal models. Recent research has revealed significant variation among animal models in morphology of the hyoid chain and suprahyoid muscles which may be associated with variation in swallowing mechanisms. The recent deployment of XROMM (X-ray Reconstruction of Moving Morphology) to quantify 3D hyolingual kinematics has revealed new details on flexion and roll of the tongue during chewing in animal models, movements similar to those used by humans. XROMM-based studies of swallowing in macaques have falsified traditional hypotheses of mechanisms of tongue base retraction during swallowing, and literature review suggests that other animal models may employ a diversity of mechanisms of tongue base retraction. There is variation among animal models in distribution of hyolingual proprioceptors but how that might be related to lingual mechanics is unknown. In macaque monkeys, tongue kinematics-shape and movement-are strongly encoded in neural activity in orofacial primary motor cortex, giving optimism for development of brain-machine interfaces for assisting recovery of lingual function after stroke. However, more research on hyolingual biomechanics and control is needed for technologies interfacing the nervous system with the hyolingual apparatus to become a reality.

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.

Propagating motor cortical patterns of excitability are ubiquitous across human and non-human primate movement initiation

A spatiotemporal pattern of excitability propagates across the primary motor cortex prior to the onset of a reaching movement in non-human primates. If this pattern is a necessary component of voluntary movement initiation, it should be present across a variety of motor tasks, end-effectors, and even species. Here, we show that propagating patterns of excitability occur during the initiation of precision grip force and tongue protrusion in non-human primates, and even isometric wrist extension in a human participant. In all tasks, the directions of propagation across the cortical sheet were bimodally distributed across trials with modes oriented roughly opposite to one another. Propagation speed was unimodally distributed with similar mean speeds across tasks and species. Additionally, propagation direction and speed did not vary systematically with any behavioral measures except response times indicating that this propagating pattern is invariant to kinematic or kinetic details and may be a generic movement initiation signal.

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.

Cortex-dependent corrections as the tongue reaches for and misses targets

Precise tongue control is necessary for drinking, eating and vocalizing^1-3. However, because tongue movements are fast and difficult to resolve, neural control of lingual kinematics remains poorly understood. Here we combine kilohertz-frame-rate imaging and a deep-learning-based neural network to resolve 3D tongue kinematics in mice drinking from a water spout. Successful licks required corrective submovements that-similar to online corrections during primate reaches^4-11-occurred after the tongue missed unseen, distant or displaced targets. Photoinhibition of anterolateral motor cortex impaired corrections, which resulted in hypometric licks that missed the spout. Neural activity in anterolateral motor cortex reflected upcoming, ongoing and past corrective submovements, as well as errors in predicted spout contact. Although less than a tenth of a second in duration, a single mouse lick exhibits the hallmarks of online motor control associated with a primate reach, including cortex-dependent corrections after misses.

The Association between Age-Related Changes in Oral Neuromechanics and Alzheimer's Disease

The global population of 80 years and older is predicted to reach 437 million by 2050. As overall brain structure and function progressively degrades, older and younger adults show differences in sensorimotor performance and brain activity in the sensorimotor regions. Oral sensorimotor functions are an important area of focus in natural aging and Alzheimer’s Disease (AD) because oral health issues are commonly found in both elderly and AD populations. While human behavioral studies on changes in oral sensorimotor functions abound, very little is known about their neuronal correlates in normal and pathological aging.

Visual detection is locked to the internal dynamics of cortico-motor control

Movements overtly sample sensory information, making sensory analysis an active-sensing process. In this study, we show that visual information sampling is not just locked to the (overt) movement dynamics but to the internal (covert) dynamics of cortico-motor control. We asked human participants to perform continuous isometric contraction while detecting unrelated and unpredictable near-threshold visual stimuli. The motor output (force) shows zero-lag coherence with brain activity (recorded via electroencephalography) in the beta-band, as previously reported. In contrast, cortical rhythms in the alpha-band systematically forerun the motor output by 200 milliseconds. Importantly, visual detection is facilitated when cortico-motor alpha (not beta) synchronization is enhanced immediately before stimulus onset, namely, at the optimal phase relationship for sensorimotor communication. These findings demonstrate an ongoing coupling between visual sampling and motor control, suggesting the operation of an internal and alpha-cycling visuomotor loop.

The Evolution of Neuroscience as a Research Field Relevant to Dentistry

The field of neuroscience did not exist as such when the Journal of Dental Research was founded 100 y ago. It has emerged as an important scientific field relevant to dentistry in view of the many neurally based functions manifested in the orofacial area (e.g., pain, taste, chewing, swallowing, salivation). This article reviews many of the novel insights that have been gained through neuroscience research into the neural basis of these functions and their clinical relevance to the diagnosis and management of pain and sensorimotor disorders. These include the neural pathways and brain circuitry underlying each of these functions and the role of nonneural as well as neural processes and their "plasticity" in modulating these functions and allowing for adaptation to tissue injury and pain and for learning or rehabilitation of orofacial functions.

Wireless Sensing of Lower Lip and Thumb-Index Finger 'Ramp-and-Hold' Isometric Force Dynamics in a Small Cohort of Unilateral MCA Stroke: Discussion of Preliminary Findings

Automated wireless sensing of force dynamics during a visuomotor control task was used to rapidly assess residual motor function during finger pinch (right and left hand) and lower lip compression in a cohort of seven adult males with chronic, unilateral middle cerebral artery (MCA) stroke with infarct confirmed by anatomic magnetic resonance imaging (MRI). A matched cohort of 25 neurotypical adult males served as controls. Dependent variables were extracted from digitized records of ‘ramp-and-hold’ isometric contractions to target levels (0.25, 0.5, 1, and 2 Newtons) presented in a randomized block design; and included force reaction time, peak force, and dF/dt_max associated with force recruitment, and end-point accuracy and variability metrics during the contraction hold-phase (mean, SD, criterion percentage ‘on-target’). Maximum voluntary contraction force (MVCF) was also assessed to establish the force operating range. Results based on linear mixed modeling (LMM, adjusted for age and handedness) revealed significant patterns of dissolution in fine force regulation among MCA stroke participants, especially for the contralesional thumb-index finger followed by the ipsilesional digits, and the lower lip. For example, the contralesional thumb-index finger manifest increased reaction time, and greater overshoot in peak force during recruitment compared to controls. Impaired force regulation among MCA stroke participants during the contraction hold-phase was associated with significant increases in force SD, and dramatic reduction in the ability to regulate force output within prescribed target force window (±5% of target). Impaired force regulation during contraction hold-phase was greatest in the contralesional hand muscle group, followed by significant dissolution in ipsilateral digits, with smaller effects found for lower lip. These changes in fine force dynamics were accompanied by large reductions in the MVCF with the LMM marginal means for contralesional and ipsilesional pinch forces at just 34.77% (15.93 N vs. 45.82 N) and 66.45% (27.23 N vs. 40.98 N) of control performance, respectively. Biomechanical measures of fine force and MVCF performance in adult stroke survivors provide valuable information on the profile of residual motor function which can help inform clinical treatment strategies and quantitatively monitor the efficacy of rehabilitation or neuroprotection strategies.

Neural Efficiency and Acquired Motor Skills: An fMRI Study of Expert Athletes

The neural efficiency hypothesis was investigated. Functional magnetic resonance imaging was used to study the differences in brain activity between athletes imagining performing different movements: basketball athletes imagined throwing and volleyball athletes imagined serving. These comparisons of brain activity among athletes imagining movements from their self-sport (e.g., a basketball throw in basketball athletes) versus movements from other sport (e.g., a volleyball serve in basketball athletes) revealed the neural energy consumption each task costs. The results showed better temporal congruence between motor execution and motor imagery and vividness of motor imagery, but lower levels of activation in the left putamen, inferior parietal lobule, supplementary motor area, postcentral gyrus, and the right insula when both groups of athletes imagined movements from their self-sport compared with when they imagined movements from the other-sport. Athletes were more effective in the representation of the motor sequences and the interoception of the motor sequences for their self-sport. The findings of present study suggest that elite athletes achieved superior behavioral performance with minimal neural energy consumption, thus confirming the neural efficiency hypotheses.

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.

Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke

Integration of sensory and motor information is one-step, among others, that underlies the successful production of goal-directed hand movements necessary for interacting with our environment. Disruption of sensorimotor integration is prevalent in many neurologic disorders, including stroke. In most stroke survivors, persistent paresis of the hand reduces function and overall quality of life. Current rehabilitative methods are based on neuroplastic principles to promote motor learning that focuses on regaining motor function lost due to paresis, but the sensory contributions to motor control and learning are often overlooked and currently understudied. There is a need to evaluate and understand the contribution of both sensory and motor function in the rehabilitation of skilled hand movements after stroke. Here, we will highlight the importance of integration of sensory and motor information to produce skilled hand movements in healthy individuals and individuals after stroke. We will then discuss how compromised sensorimotor integration influences relearning of skilled hand movements after stroke. Finally, we will propose an approach to target sensorimotor integration through manipulation of sensory input and motor output that may have therapeutic implications.

Movements following force-field adaptation are aligned with altered sense of limb position

Previous work has shown that motor learning is associated with changes to both movements and to the somatosensory perception of limb position. In an earlier study that motivates the current work, it appeared that following washout trials, movements did not return to baseline but rather were aligned with associated changes to sensed limb position. Here, we provide a systematic test of this relationship, examining the idea that adaptation-related changes to sensed limb position and to the path of the limb are linked, not only after washout trials but at all stages of the adaptation process. We used a force-field adaptation paradigm followed by washout trials in which subjects performed movements without visual feedback of the limb. Tests of sensed limb position were conducted at each phase of adaptation, specifically before and after baseline movements in a null field, after force-field adaptation, and following washout trials in a null field. As in previous work, sensed limb position changed in association with force-field adaptation. At each stage of adaptation, we observed a correlation between the sensed limb position and associated path of the limb. At a group level, there were differences between the clockwise and counter-clockwise conditions. However, whenever there were changes in sensed limb position, movements following washout did not return to baseline. This suggests that adaptation in sensory and motor systems is not independent processes but rather sensorimotor adaptation is linked to sensory change. Sensory change and limb movement remain in alignment throughout adaptation such that the path of the limb is aligned with the altered sense of limb position.

Dynamics of motor cortical activity during naturalistic feeding behavior

OBJECTIVE

The orofacial primary motor cortex (MIo) plays a critical role in controlling tongue and jaw movements during oral motor functions, such as chewing, swallowing and speech. However, the neural mechanisms of MIo during naturalistic feeding are still poorly understood. There is a strong need for a systematic study of motor cortical dynamics during feeding behavior.

APPROACH

To investigate the neural dynamics and variability of MIo neuronal activity during naturalistic feeding, we used chronically implanted micro-electrode arrays to simultaneously recorded ensembles of neuronal activity in the MIo of two monkeys (Macaca mulatta) while eating various types of food. We developed a Bayesian nonparametric latent variable model to reveal latent structures of neuronal population activity of the MIo and identify the complex mapping between MIo ensemble spike activity and high-dimensional kinematics.

MAIN RESULTS

Rhythmic neuronal firing patterns and oscillatory dynamics are evident in single-unit activity. At the population level, we uncovered the neural dynamics of rhythmic chewing, and quantified the neural variability at multiple timescales (complete feeding sequences, chewing sequence stages, chewing gape cycle phases) across food types. Our approach accommodates time-warping of chewing sequences and automatic model selection, and maps the latent states to chewing behaviors at fine timescales.

SIGNIFICANCE

Our work shows that neural representations of MIo ensembles display spatiotemporal patterns in chewing gape cycles at different chew sequence stages, and these patterns vary in a stage-dependent manner. Unsupervised learning and decoding analysis may reveal the link between complex MIo spatiotemporal patterns and chewing kinematics.

The impact of motor impairment on the processing of sensory information

Sensorimotor adaptation is driven by mismatch errors between desired movements and actual movement outcomes. A mismatch error can be minimized by adjusting movements or by altering the interpretation of sensory information. While the effect of mismatch errors on the motor system has received much attention, the contribution of somatosensory feedback, particularly the sensory-motor interplay in the process of adaptation, remains poorly understood. Our study analyzes the impact of peripheral deefferentation on the plasticity of the brain networks responsible for sensory-motor adaptation, focusing particularly on changes in the processing of somatosensory information. For this aim, task-based and resting-state functional MRI was performed on 24 patients in the acute state of a left-sided idiopathic peripheral facial nerve palsy. The functional connectivity of cortical and subcortical networks was analyzed and compared to a healthy control group. We found a strong involvement of the somatosensory system and the thalamus in the adaptation process following an acute peripheral deefferentation. The investigated network shows the principal pattern of a reduced connectivity between cortical areas, while the connectivity to subcortical areas (the basal ganglia and the thalamus) is increased. We suggest that the increased connectivity between the subcortical and cortical structures indicates an active sensory-motor adaptation process. We further hypothesize that the decreased functional connectivity at the cortical level reflects an unsuccessful sensorimotor adaptation process due to the inability to solve the somatosensory-motor mismatch. These results extend our understanding of the somatosensory-motor interaction in response to a mismatch signal and highlight the importance of the thalamus in this process.

Interhemispheric connectivity of primary sensory cortex is associated with motor impairment after stroke

Neuroimaging-derived markers are used to model post-stroke impairment. Among these, lesion size, corticospinal-tract lesion-load (CST-LL) and resting-state functional-connectivity (rs-FC) have been correlated with impairment. It has been shown that the sensory cortex (S1) is associated with motor learning and is essential for post-stroke recovery; yet stroke-induced changes in S1 connectivity alone are yet to be investigated. We aim to determine whether interhemispheric rs-FC could be used to refine imaging models of stroke-related impairment. Subjects’ post-stroke and age-matched controls underwent rs-fMRI. Stroke-related disability was correlated with lesion size, CST-LL and interhemispheric S1 and M1 rs-FC as independent seeds. Regression analyses were performed to assess the contribution of these markers in stroke-related deficits. Post-stroke subjects showed an asymmetrical pattern of rs-FC in which affected hemisphere S1 and M1 were mostly connected with ipsi-lesional regions. Correlations between rs-FC and stroke-severity were found. Adding rs-FC of S1 to the regression model of impairment decreased the variance 31% compared to lesion size only. After a stroke, S1 interhemispheric connectivity is decreased, with S1 only connected with ipsi-lesional regions. This asymmetry correlates with neurological and motor impairment. Furthermore, when combined with lesion anatomical measures, S1 connectivity might be an important marker in explaining stroke outcome.

Primary sensorimotor cortex exhibits complex dependencies of spike-field coherence on neuronal firing rates, field power, and behavior

Spike-field coherence (SFC) is widely used to assess cortico-cortical interactions during sensorimotor behavioral tasks by measuring the consistency of the relative phases between the spike train of a neuron and the concurrent local field potentials (LFPs). Interpretations of SFC as a measure of functional connectivity are complicated by theoretical work suggesting that estimates of SFC depend on overall neuronal activity. We evaluated the dependence of SFC on neuronal firing rates, LFP power, and behavior in the primary motor (MIo) and primary somatosensory (SIo) areas of the orofacial sensorimotor cortex of monkeys ( Macaca mulatta) during performance of a tongue-protrusion task. Although we occasionally observed monotonically increasing linear relationships between coherence and firing rate, we most often found highly complex, nonmonotonic relationships in both SIo and MIo and sometimes even found that coherence decreased with increasing firing rate. The lack of linear relationships was also true for both LFP power and tongue-protrusive force. Moreover, the ratio between maximal firing rate and the firing rate at peak coherence deviated significantly from unity, indicating that MIo and SIo neurons achieved maximal SFC at a submaximal level of spiking. Overall, these results point to complex relationships of SFC to firing rates, LFP power, and behavior during sensorimotor cortico-cortical interactions: coherence is a measure of functional connectivity whose magnitude is not a mere monotonic reflection of changes in firing rate, LFP power, or the relevantly controlled behavioral parameter. NEW & NOTEWORTHY The concern that estimates of spike-field coherence depend on the firing rates of single neurons has influenced analytical methods employed by experimental studies investigating the functional interactions between cortical areas. Our study shows that the overwhelming majority of the estimated spike-field coherence exhibited complex relations with firing rates of neurons in the orofacial sensorimotor cortex. The lack of monotonic relations was also evident after testing the influence of local field potential power and force on spike-field coherence.

Local cortical activity of distant brain areas can phase-lock to the olfactory bulb's respiratory rhythm in the freely behaving rat

An important unresolved question about neural processing is the mechanism by which distant brain areas coordinate their activities and relate their local processing to global neural events. A potential candidate for the local-global integration are slow rhythms such as respiration. In this study, we asked if there are modulations of local cortical processing that are phase-locked to (peripheral) sensory-motor exploratory rhythms. We studied rats on an elevated platform where they would spontaneously display exploratory and rest behaviors. Concurrent with behavior, we monitored whisking through electromyography and the respiratory rhythm from the olfactory bulb (OB) local field potential (LFP). We also recorded LFPs from dorsal hippocampus, primary motor cortex, primary somatosensory cortex, and primary visual cortex. We defined exploration as simultaneous whisking and sniffing above 5 Hz and found that this activity peaked at ~8 Hz. We considered rest as the absence of whisking and sniffing, and in this case, respiration occurred at ~3 Hz. We found a consistent shift across all areas toward these rhythm peaks accompanying behavioral changes. We also found, across areas, that LFP gamma (70-100 Hz) amplitude could phase-lock to the animal's OB respiratory rhythm, a finding indicative of respiration-locked changes in local processing. In a subset of animals, we also recorded the hippocampal theta activity and found that occurred at frequencies overlapped with respiration but was not spectrally coherent with it, suggesting a different oscillator. Our results are consistent with the notion of respiration as a binder or integrator of activity between brain regions.

Orofacial and thumb-index finger ramp-and-hold isometric force dynamics in young neurotypical adults

The relation among several parameters of the ramp-and-hold isometric force contraction (peak force and dF/dt_max during the initial phase of force recruitment, and the proportion of hold-phase at target) was quantified for the right and left thumb-index finger pinch, and lower lip midline compression in 40 neurotypical right-handed young adults (20 female/20 males) using wireless force sensors and data acquisition technology developed in our laboratory. In this visuomotor control task, participants produced ramp-and-hold isometric forces as 'rapidly and accurately' as possible to end-point target levels at 0.25, 0.5, 1 and 2 Newtons presented to a computer monitor in a randomized block design. Significant relations were found between the parameters of the ramp-and-hold lip force task and target force level, including the peak rate of force change (dF/dt_max), peak force, and the criterion percentage of force within ±5% of target during the contraction hold phase. A significant performance advantage was found among these force variables for the thumb-index finger over the lower lip. The maximum voluntary compression force (MVCF) task revealed highly significant differences in force output between the thumb-index fingers and lower lip (∼4.47-4.70 times greater for the digits versus lower lip), a significant advantage of the right thumb-index finger over the non-dominant left thumb-index finger (12% and 25% right hand advantage for males and females, respectively), and a significant sex difference (∼1.65-1.73 times greater among males).

Expertise-Level-Dependent Functionally Plastic Changes During Motor Imagery in Basketball Players

Motor imagery is the mental process of rehearsing or simulating a given action without overt movements. The aim of the present study is to examine plastic changes in relevant brain areas during motor imagery with increasing expertise level. Subjects (novices, intermediate and elite players) performed motor imagery of basketball throws under two experimental conditions (with-ball and without-ball). We found that all basketball players exhibited better temporal congruence (between motor imagery and motor execution) and higher vividness of motor imagery than novices. The vividness of motor imagery was higher for the with-ball than for the without-ball conditions in all three subject groups. The results from functional magnetic resonance imaging (fMRI) showed three different patterns of cortical activation. Activation in the left middle frontal gyrus increased and that in the left supplementary motor area decreased with increasing levels of motor expertise. Importantly, brain activation in the left postcentral gyrus was the highest in the intermediate players compared to both novices and elite players. For the elite group, these three areas showed higher activation in the without-ball condition than the with-ball condition, while the opposite trend was found in intermediate players. Our findings suggest that the level of motor expertise may be related to high-order brain functions that are linked to different activation patterns in different brain areas.

Face sensorimotor cortex undergoes neuroplastic changes in a rat model of trigeminal neuropathic pain

Trigeminal nerve injury can result in neuropathic pain behavior and alterations in motor function, but it is unclear if such injury produces neuroplastic alterations in face sensorimotor cortex that could contribute to the alterations in motor function. Therefore, this study aimed to determine if trigeminal nerve injury in a rat neuropathic pain model induces neuroplastic changes in jaw and tongue motor representations in face sensorimotor cortex in association with facial nociceptive behavior. Right infraorbital nerve transection was performed in adult male Sprague-Dawley rats; sham-operated rats served as controls. Nociceptive behavior was assessed by testing facial mechanical sensitivity pre-operatively and post-operatively (1-28 days). Intracortical microstimulation was also applied post-operatively in a series of microelectrode penetrations to map jaw and tongue motor representations in the face sensorimotor cortex by analyzing anterior digastric and genioglossus electromyographic activities evoked by microstimulation at histologically verified sites in face primary somatosensory cortex (face-SI) as well as face primary motor cortex (face-MI). Compared to sham, infraorbital nerve injury induced a significant (2-way repeated-measures analysis of variance, P < 0.001) bilateral decrease in facial mechanical threshold that lasted up to 28 days post-operatively. Nerve injury also induced a significant bilateral decrease compared to sham (P < 0.05) in the number of anterior digastric and/or genioglossus sites in face-MI and in face-SI. These findings indicate that trigeminal nerve injury induces neuroplastic alterations in jaw and tongue motor representations in face sensorimotor cortex that are associated with facial nociceptive behavior and that may contribute to sensorimotor changes following trigeminal nerve injury.

Functional and Quantitative MRI Mapping of Somatomotor Representations of Human Supralaryngeal Vocal Tract

Speech articulation requires precise control of and coordination between the effectors of the vocal tract (e.g., lips, tongue, soft palate, and larynx). However, it is unclear how the cortex represents movements of and contact between these effectors during speech, or how these cortical responses relate to inter-regional anatomical borders. Here, we used phase-encoded fMRI to map somatomotor representations of speech articulations. Phonetically trained participants produced speech phones, progressing from front (bilabial) to back (glottal) place of articulation. Maps of cortical myelin proxies (R₁ = 1/T₁) further allowed us to situate functional maps with respect to anatomical borders of motor and somatosensory regions. Across participants, we found a consistent topological map of place of articulation, spanning the central sulcus and primary motor and somatosensory areas, that moved from lateral to inferior as place of articulation progressed from front to back. Phones produced at velar and glottal places of articulation activated the inferior aspect of the central sulcus, but with considerable across-subject variability. R₁ maps for a subset of participants revealed that articulator maps extended posteriorly into secondary somatosensory regions. These results show consistent topological organization of cortical representations of the vocal apparatus in the context of speech behavior.

In vivo bone strain and finite element modeling of a rhesus macaque mandible during mastication

Finite element analysis (FEA) is a commonly used tool in musculoskeletal biomechanics and vertebrate paleontology. The accuracy and precision of finite element models (FEMs) are reliant on accurate data on bone geometry, muscle forces, boundary conditions and tissue material properties. Simplified modeling assumptions, due to lack of in vivo experimental data on material properties and muscle activation patterns, may introduce analytical errors in analyses where quantitative accuracy is critical for obtaining rigorous results. A subject-specific FEM of a rhesus macaque mandible was constructed, loaded and validated using in vivo data from the same animal. In developing the model, we assessed the impact on model behavior of variation in (i) material properties of the mandibular trabecular bone tissue and teeth; (ii) constraints at the temporomandibular joint and bite point; and (iii) the timing of the muscle activity used to estimate the external forces acting on the model. The best match between the FEA simulation and the in vivo experimental data resulted from modeling the trabecular tissue with an isotropic and homogeneous Young's modulus and Poisson's value of 10GPa and 0.3, respectively; constraining translations along X,Y, Z axes in the chewing (left) side temporomandibular joint, the premolars and the m₁; constraining the balancing (right) side temporomandibular joint in the anterior-posterior and superior-inferior axes, and using the muscle force estimated at time of maximum strain magnitude in the lower lateral gauge. The relative strain magnitudes in this model were similar to those recorded in vivo for all strain locations. More detailed analyses of mandibular strain patterns during the power stroke at different times in the chewing cycle are needed.

Animal Models for Dysphagia Studies: What Have We Learnt So Far

Research using animal models has contributed significantly to realizing the goal of understanding dysfunction and improving the care of patients who suffer from dysphagia. But why should other researchers and the clinicians who see patients day in and day out care about this work? Results from studies of animal models have the potential to change and grow how we think about dysphagia research and practice in general, well beyond applying specific results to human studies. Animal research provides two key contributions to our understanding of dysphagia. The first is a more complete characterization of the physiology of both normal and pathological swallow than is possible in human subjects. The second is suggesting of specific, physiological, targets for development and testing of treatment interventions to improve dysphagia outcomes.

Exploring the effects of synchronous pharyngeal electrical stimulation with swallowing carbonated water on cortical excitability in the human pharyngeal motor system

BACKGROUND

Previous reports have revealed that excitation of human pharyngeal motor cortex can be induced by pharyngeal electrical stimulation (PES) and swallowing carbonated water (CW). This study investigated whether combining PES with swallowing (of still water, SW or CW) can potentiate this excitation in either cortical and/or brain stem areas assessed with transcranial and transcutaneous magnetic stimulation (TMS).

METHODS

Fourteen healthy volunteers participated and were intubated with an intraluminal catheter to record pharyngeal electromyography and deliver PES. Each participant underwent baseline corticopharyngeal, hand and craniobulbar motor-evoked potential (MEP) measurements. Subjects were then randomized to receive each of four 10-min interventions (PES only, ShamPES+CW, PES+CW, and PES+SW). Corticobulbar, craniobulbar and hand MEPs were then remeasured for up to 60 min and data analyzed using anova and post hoc t-tests.

KEY RESULTS

A two-way rmanova for Interventions × Time-point showed a significant corticopharyngeal interaction (p = 0.010). One-way anova with post hoc t-tests indicated significant cortical changes with PES only at 45 (p = 0.038) and 60 min (p = 0.023) and ShamPES+CW immediately (p = 0.008) but not with PES+CW or PES+SW. By contrast, there were immediate craniobulbar amplitude changes only with PES+CW (p = 0.020) which were not sustained.

CONCLUSIONS & INFERENCES

We conclude that only PES produced long-term changes in corticopharyngeal excitability whereas combination stimuli were less effective. Our data suggest that PES alone rather than in combination, may be better for the patients who have difficulty in performing voluntary swallows.

Primary motor and sensory cortical areas communicate via spatiotemporally coordinated networks at multiple frequencies

Skilled movements rely on sensory information to shape optimal motor responses, for which the sensory and motor cortical areas are critical. How these areas interact to mediate sensorimotor integration is largely unknown. Here, we measure intercortical coherence between the orofacial motor (MIo) and somatosensory (SIo) areas of cortex as monkeys learn to generate tongue-protrusive force. We report that coherence between MIo and SIo is reciprocal and that neuroplastic changes in coherence gradually emerge over a few days. These functional networks of coherent spiking and local field potentials exhibit frequency-specific spatiotemporal properties. During force generation, theta coherence (2-6 Hz) is prominent and exhibited by numerous paired signals; before or after force generation, coherence is evident in alpha (6-13 Hz), beta (15-30 Hz), and gamma (30-50 Hz) bands, but the functional networks are smaller and weaker. Unlike coherence in the higher frequency bands, the distribution of the phase at peak theta coherence is bimodal with peaks near 0° and ±180°, suggesting that communication between somatosensory and motor areas is coordinated temporally by the phase of theta coherence. Time-sensitive sensorimotor integration and plasticity may rely on coherence of local and large-scale functional networks for cortical processes to operate at multiple temporal and spatial scales.

High-Resolution, Non-Invasive Imaging of Upper Vocal Tract Articulators Compatible with Human Brain Recordings

A complete neurobiological understanding of speech motor control requires determination of the relationship between simultaneously recorded neural activity and the kinematics of the lips, jaw, tongue, and larynx. Many speech articulators are internal to the vocal tract, and therefore simultaneously tracking the kinematics of all articulators is nontrivial—especially in the context of human electrophysiology recordings. Here, we describe a noninvasive, multi-modal imaging system to monitor vocal tract kinematics, demonstrate this system in six speakers during production of nine American English vowels, and provide new analysis of such data. Classification and regression analysis revealed considerable variability in the articulator-to-acoustic relationship across speakers. Non-negative matrix factorization extracted basis sets capturing vocal tract shapes allowing for higher vowel classification accuracy than traditional methods. Statistical speech synthesis generated speech from vocal tract measurements, and we demonstrate perceptual identification. We demonstrate the capacity to predict lip kinematics from ventral sensorimotor cortical activity. These results demonstrate a multi-modal system to non-invasively monitor articulator kinematics during speech production, describe novel analytic methods for relating kinematic data to speech acoustics, and provide the first decoding of speech kinematics from electrocorticography. These advances will be critical for understanding the cortical basis of speech production and the creation of vocal prosthetics.

Chronic recordings reveal tactile stimuli can suppress spontaneous activity of neurons in somatosensory cortex of awake and anesthetized primates

In somatosensory cortex, tactile stimulation within the neuronal receptive field (RF) typically evokes a transient excitatory response with or without postexcitatory inhibition. Here, we describe neuronal responses in which stimulation on the hand is followed by suppression of the ongoing discharge. With the use of 16-channel microelectrode arrays implanted in the hand representation of primary somatosensory cortex of New World monkeys and prosimian galagos, we recorded neuronal responses from single units and neuron clusters. In 66% of our sample, neuron activity tended to display suppression of firing when regions of skin outside of the excitatory RF were stimulated. In a small proportion of neurons, single-site indentations suppressed firing without initial increases in response to any of the tested sites on the hand. Latencies of suppressive responses to skin indentation (usually 12-34 ms) were similar to excitatory response latencies. The duration of inhibition varied across neurons. Although most observations were from anesthetized animals, we also found similar neuron response properties in one awake galago. Notably, suppression of ongoing neuronal activity did not require conditioning stimuli or multi-site stimulation. The suppressive effects were generally seen following single-site skin indentations outside of the neuron's minimal RF and typically on different digits and palm pads, which have not often been studied in this context. Overall, the characteristics of widespread suppressive or inhibitory response properties with and without initial facilitative or excitatory responses add to the growing evidence that neurons in primary somatosensory cortex provide essential processing for integrating sensory stimulation from across the hand.

Sensory Plasticity in Human Motor Learning

There is accumulating evidence from behavioral, neurophysiological, and neuroimaging studies that the acquisition of motor skills involves both perceptual and motor learning. Perceptual learning alters movements, motor learning, and motor networks of the brain. Motor learning changes perceptual function and the sensory circuits of the brain. Here, we review studies of both human limb movement and speech that indicate that plasticity in sensory and motor systems is reciprocally linked. Taken together, this points to an approach to motor learning in which perceptual learning and sensory plasticity have a fundamental role.

Dental Occlusal Changes Induce Motor Cortex Neuroplasticity

Modification to the dental occlusion may alter oral sensorimotor functions. Restorative treatments aim to restore sensorimotor functions; however, it is unclear why some patients fail to adapt to the restoration and remain with sensorimotor complaints. The face primary motor cortex (face-M1) is involved in the generation and control of orofacial movements. Altered sensory inputs or motor function can induce face-M1 neuroplasticity. We took advantage of the continuous eruption of the incisors in Sprague-Dawley rats and used intracortical microstimulation (ICMS) to map the jaw and tongue motor representations in face-M1. Specifically, we tested the hypothesis that multiple trimming of the right mandibular incisor, to keep it out of occlusal contacts for 7 d, and subsequent incisor eruption and restoration of occlusal contacts, can alter the ICMS-defined features of jaw and tongue motor representations (i.e., neuroplasticity). On days 1, 3, 5, and 7, the trim and trim-recovered groups had 1 to 2 mm of incisal trimming of the incisor; a sham trim group had buccal surface trimming with no occlusal changes; and a naive group had no treatment. Systematic mapping was performed on day 8 in the naive, trim, and sham trim groups and on day 14 in the trim-recovered group. In the trim group, the tongue onset latency was shorter in the left face-M1 than in the right face-M1 ( P < .001). In the trim-recovered group, the number of tongue sites and jaw/tongue overlapping sites was greater in the left face-M1 than in the right face-M1 ( P = 0.0032, 0.0016, respectively), and the center of gravity was deeper in the left than in the right face-M1 ( P = 0.026). Therefore, incisor trimming and subsequent restoration of occlusal contacts induced face-M1 neuroplasticity, reflected in significant disparities between the left and right face-M1 in some ICMS-defined features of the tongue motor representations. Such neuroplasticity may reflect or contribute to subjects’ ability to adapt their oral sensorimotor functions to an altered dental occlusion.

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.