Converging patterns of inputs from oral structures in the postcentral somatosensory cortex of conscious macaque monkeys

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.

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.

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.

Somatosensory evoked magnetic fields caused by mechanical stimulation of the periodontal ligaments

The periodontal ligaments are very important sensory organ for our daily life such as perception of food size or hardness, determination of jaw position, and adjustment of masticatory strength. The sensory properties of the periodontal ligament, especially those of the maxillary and mandibular molars, have not yet been fully investigated. Somatosensory evoked magnetic fields (SEFs) can be measured and evaluated for latency and intensity to determine the sensory transmission characteristics of each body parts. However, previous reports on SEFs in the oral region have only reported differences in upper and lower gingival and lip sensations. In this study, the aim was to clarify these sensory characteristics by measuring SEFs during mechanical stimulation of the periodontal ligament in the maxillary and mandibular first molars. Somatosensory evoked magnetic fields were measured in the contralateral hemispheres of 33 healthy volunteers. Mechanical stimulation of the maxillary and mandibular right first molars, and the left wrist was performed with a specific handmade tool. The first peak latency for the mandibular first molars was 41.7 ± 5.70 ms (mean ± SD), significantly shorter than that for the maxillary first molars at 47.7 ± 7.36 ms. The peak intensity for the mandibular first molars was 13.9 ± 6.06 nAm, significantly larger than that for the maxillary first molars at 7.63 ± 3.55 nAm. The locations in the contralateral hemispheres showed no significant difference between the maxillary first molars and mandibular first molars. These locations were more anteroinferior and exterior than that of the wrist, as suggested by the brain homunculus. Neural signals from the mandibular periodontal ligaments pass faster and more intensely to the central nervous system than those from the maxillary periodontal ligaments, and may preferentially participate in adjustment of the occlusal force and the occlusal position.

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.

Somatosensory evoked magnetic fields of periodontal mechanoreceptors

To evaluate the localization of responses to stimulation of the periodontal mechanoreceptors in the primary somatosensory cortex, somatosensory evoked fields (SEFs) were measured for stimulation of the left mandibular canine and first molar using magnetoencephalography in 25 healthy subjects. Tactile stimulation used a handmade stimulus device which recorded the trigger at the moment of touching the teeth.SEFs for the canine and first molar were detected in 20 and 19 subjects, respectively. Both responses were detected in the bilateral hemispheres. The latency for the canine was 62.1 ± 12.9 ms in the ipsilateral hemisphere and 65.9 ± 14.8 ms in the contralateral hemisphere. The latency for the first molar was 47.4 ± 6.6 ms in the ipsilateral hemisphere and 47.8 ± 9.1 ms in the contralateral hemisphere. The latency for the first molar was significantly shorter than that for the canine. The equivalent current dipoles were estimated in the central sulcus and localized anteroinferiorly compared to the locations for the SEFs for the median nerve. No significant differences in three-dimensional coordinates were found between the canine and first molar. These findings demonstrate the precise location of the teeth within the orofacial representation area in the primary somatosensory cortex.

Population response characteristics of intrinsic signals in the cat somatosensory cortex following canine mechanical stimulation

Intrinsic signal optical imaging has been widely used to measure functional maps in various sensory cortices due to better spatial resolution and sensitivity for detecting cortical neuroplasticity. However, application of this technique in dentistry has not been reported. In this study, intrinsic signal optical imaging was used to investigate mechanically driven responses in the cat somatosensory cortex, when punctate mechanical stimuli were applied to maxillary canines. The global signal and its spatial organization pattern were obtained. Global signal strength gradually increased with stimulus strength. There was no significant difference in response strength between contralateral and ipsilateral mechanical stimulation. A slightly greater response was recorded in the sigmoidal gyrus than in the coronal gyrus. The cat somatosensory cortex activated by sensory inputs from mechanical stimulation of canines lacks both topographical and functional organization. It is not organized into columns that represent sensory input from each tooth or direction of stimulation. These results demonstrate that intrinsic signal optical imaging is a valid tool for investigating neural responses and neuroplasticity in the somatosensory cortex that represents teeth.

Corticocortical projections to representations of the teeth, tongue, and face in somatosensory area 3b of macaques

We placed injections of anatomical tracers into representations of the tongue, teeth, and face in the primary somatosensory cortex (area 3b) of macaque monkeys. Our injections revealed strong projections to representations of the tongue and teeth from other parts of the oral cavity responsive region in 3b. The 3b face also provided input to the representations of the intraoral structures. The primary representation of the face showed a pattern of intrinsic connections similar to that of the mouth. The area 3b hand representation provided little to no input to either the mouth or the face representations. The mouth and face representations of area 3b received projections from the presumptive oral cavity and face regions of other somatosensory areas in the anterior parietal cortex and the lateral sulcus, including areas 3a, 1, 2, the second somatosensory area (S2), the parietal ventral area (PV), and cortex that may include the parietal rostral (PR) and ventral somatosensory (VS) areas. Additional inputs came from primary motor (M1) and ventral premotor (PMv) areas. This areal pattern of projections is similar to the well-studied pattern revealed by tracer injections in regions of 3b representing the hand. The tongue representation appeared to be unique in area 3b in that it also received inputs from areas in the anterior upper bank of the lateral sulcus and anterior insula that may include the primary gustatory area (area G) and other cortical taste-processing areas, as well as a region of lateral prefrontal cortex (LPFC) lining the principal sulcus.

Amplitude-integrated EEG and range-EEG modulation associated with pneumatic orocutaneous stimulation in preterm infants

OBJECTIVE

Controlled somatosensory stimulation strategies have demonstrated merit in developing oral feeding skills in premature infants who lack a functional suck, however, the effects of orosensory entrainment stimulation on electrocortical dynamics is unknown. The objective of the study was to determine the effects of servo-controlled pneumatic orocutaneous stimulation presented during gavage feedings on the modulation of amplitude-integrated electroencephalogram (aEEG) and range electroencephalogram (rEEG) activity.

STUDY DESIGN

Two-channel EEG recordings were collected during 180 sessions that included orocutaneous stimulation and non-stimulation epochs among 22 preterm infants (mean gestational age=28.56 weeks) who were randomized to treatment and control 'sham' conditions. The study was initiated at around 32 weeks post-menstrual age. The raw EEG was transformed into aEEG margins, and rEEG amplitude bands measured at 1-min intervals and subjected to a mixed models statistical analysis.

RESULT

Multiple significant effects were observed in the processed EEG during and immediately following 3-min periods of orocutaneous stimulation, including modulation of the upper and lower margins of the aEEG, and a reorganization of rEEG with an apparent shift from amplitude bands D and E to band C throughout the 23-min recording period that followed the first stimulus block when compared with the sham condition. Cortical asymmetry also was apparent in both EEG measures.

CONCLUSION

Orocutaneous stimulation represents a salient trigeminal input, which has both short- and long-term effects in modulating electrocortical activity, and thus is hypothesized to represent a form of neural adaptation or plasticity that may benefit the preterm infant during this critical period of brain maturation.

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.

Putative lateral inhibition in sensory processing for directional turns

Computing targeted responses is a general problem in goal-directed behaviors. We sought the sensory template for directional turning in the predatory sea slug Pleurobranchaea californica, which calculates precise turn angles by averaging multiple stimulus sites on its chemotactile oral veil (Yafremava LS, Anthony CW, Lane L, Campbell JK, Gillette R. J Exp Biol 210: 561-569, 2007). Spiking responses to appetitive chemotactile stimulation were recorded in the two bilateral pairs of oral veil nerves, the large oral veil nerve (LOVN) and the tentacle nerve (TN). The integrative abilities of the peripheral nervous system were significant. Nerve spiking responses to punctate, one-site stimulation of the oral veil followed sigmoid relations as stimuli moved between lateral tentacle and the midline. Receptive fields of LOVN and TN were unilateral, overlapping, and oppositely weighted for responsiveness across the length of oral veil. Simultaneous two-site stimulation caused responses of amplitudes markedly smaller than the sum of corresponding one-site responses. Plots of two-site nerve responses against the summed approximate distances from midline of each site were markedly linear. Thus the sensory paths in the peripheral nervous system show reciprocal occlusion similar to lateral inhibition. This outcome suggests a novel neural function for lateral inhibitory mechanisms, distinct from simple contrast enhancement, in computation of both sensory maps and targeted motor actions.

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.

Motor cortex neuroplasticity associated with lingual nerve injury in rats

The aim of this study was to determine if lingual nerve trauma affects the features of face primary motor cortex (MI) defined by intracortical microstimulation (ICMS). The left lingual nerve was transected in adult male rats by an oral surgical procedure; sham rats (oral surgery but no nerve transection) as well as naive intact rats served as control groups. ICMS was applied at post-operative days 0, 7, 14, 21, and 28 to map the jaw and tongue motor representations in face MI by analyzing ICMS-evoked movements and electromyographic activity recorded in the genioglossus (GG) and anterior digastric (AD) muscles. There were no statistically significant effects of acute (day 0) nerve transection or sham procedure (p > 0.05). The surgery in the sham animals was associated with limited post-operative change; this was reflected in a significant (p < 0.05) increase in the number of GG sites in left MI at post-operative day 14 compared to day 0. However, nerve transection was associated with significant increases in the total number of AD and GG sites in left or right MI or specifically the number of GG sites in rats at post-operative days 21 or 28 compared to earlier time periods. There were also significant differences between nerve-transected and sham groups at post-operative days 7, 14, or 21. These findings suggest that lingual nerve transection is associated with significant time-dependent neuroplastic changes in the tongue motor representations in face MI.

Postcentral neurons with covert receptive fields in conscious macaque monkeys: their selective responsiveness to simultaneous two-point stimuli applied to discrete oral portions

The representation of the oral structures in the postcentral somatosensory cortex was studied in conscious macaque monkeys by recording the activity of single neurons. A total of 2,807 neurons were isolated in the oral regions of three hemispheres in two animals. Of these, 375 neurons (area 3a, 3; area 3b, 123; area 1, 99; area 2, 150) lacked an apparent receptive field (RF), and their relative frequency was significantly higher in area 2 (19%) than in more rostral areas (area 3a, 8%; area 3b, 10%; area 1, 12%). We tested the responsiveness of these neurons to stimuli applied simultaneously to two discrete, but functionally related, oral structures (interstructural two-point stimuli: iTPS). Neurons in areas 3a, 3b, and 1 that lacked an apparent RF were not responsive to iTPS. However, 35 neurons in area 2 responded stably to iTPS applied to either of the following sets of oral structures: the tongue and incisors (n=18), incisors and lip (n=9), lip and tongue (n=12), or upper and lower lips (n=8). Of them, 19 neurons were activated during self-movements such as tongue protrusion, lip licking, and food manipulation. The neurons selectively responsive to iTPS might detect converging inputs from different oral structures and play a pivotal role in detecting objects straddling different oral structures and the mutual contact of oral structures.

The Representation of the Human Oral Area in the Somatosensory Cortex: a Functional MRI Study

The tactile sensation of the teeth is involved in various oral functions, such as mastication and speech. Using functional magnetic resonance imaging, we investigated the cortical sensory representation of the oral area, including the teeth. First, we identified the somatotopic representation of the lips, teeth and tongue in the postcentral gyrus (GpoC). Tactile stimuli were applied to the lower lip, tongue and teeth. The foci activated by each stimulus were characterized by the center of gravity (COG) of activated areas. Secondly, we examined the rostro-caudal changes in the somatotopic organization in the GPoC in terms of the overlap between each sensory representation. In the rostral portion of the GPoC, the COG of the representation of teeth was located significantly superior to that of the tongue and inferior to that of the lip, consistent with the classical 'sensory homunculus' proposed by Penfield; however, this somatotopic representation became unclear in the middle and caudal portions of the GPoC. The overlap between each representation in the middle and caudal portions of the GPoC was significantly greater than that in the rostral portion of the GPoC. These findings support the theory that the input from oral structures converges hierarchically across the primary somatosensory cortex.