Functional properties of single neurons in the primate face primary somatosensory cortex. I. Relations with trained orofacial motor behaviors

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.

Cerebral control of swallowing: An update on neurobehavioral evidence

This review aims to update the current knowledge on the cerebral control of swallowing. We review data from both animal and human studies spanning across the fields of neuroanatomy, neurophysiology and neuroimaging to evaluate advancements in our understanding in the brain's role in swallowing. Studies have collectively shown that swallowing is mediated by multiple distinct cortical and subcortical regions and that lesions to these regions can result in dysphagia. These regions are functionally connected in separate groups within and between the two hemispheres. While hemispheric dominance for swallowing has been reported in most human studies, the laterality is inconsistent across individuals. Moreover, there is a shift in activation location and laterality between swallowing preparation and execution, although such activation changes are less well-defined than that for limb motor control. Finally, we discussed recent neurostimulation treatments that may be beneficial for dysphagia after brain injury through promoting the reorganization of the swallowing neural network.

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.

Effect of mandibular advancement device on plasticity in corticomotor control of tongue and jaw muscles

STUDY OBJECTIVES

This study aims to investigate if the use of a mandibular advancement device (MAD) is associated with neuroplasticity in corticomotor control of tongue and jaw muscles.

METHODS

Eighteen healthy individuals participated in a randomized crossover study with 3 conditions for 2 weeks each: baseline, wearing an oral appliance (OA: sham MAD) or MAD during sleep. The custom-made MAD was constructed by positioning the mandible to 50% of its maximal protrusion limit. Transcranial magnetic stimulation (TMS) was applied to elicit motor evoked potentials (MEPs). The MEPs were assessed by constructing stimulus-response curves at four stimulus intensities: 90%, 100%, 120%, and 160% of the motor threshold (MT) from the right tongue and right masseter, and the first dorsal interosseous muscles (FDI, control) at baseline, after the first and the second intervention.

RESULTS

There was a significant effect of condition and stimulus intensity both on the tongue and as well as on masseter MEPs (P < 0.01). Tongue and masseter MEPs were significantly higher at 120% and 160% following the MAD compared to the OA (P < 0.05). There were no effects of condition on FDI MEPs (P = 0.855).

CONCLUSIONS

The finding suggests that MAD induces neuroplasticity in the corticomotor pathway of the tongue and jaw muscles associated with the new jaw position. Further investigations are required in patients with obstructive sleep apnea (OSA) to see if this cortical neuroplasticity may contribute or perhaps predict treatment effects with MADs in OSA.

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.

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.

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.

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.

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.

Somatosensory gating is dependent on the rate of force recruitment in the human orofacial system

PURPOSE

Functional orofacial behaviors vary in their force endpoint and rate of recruitment. This study assessed the gating of orofacial cutaneous somatosensation during different cyclic lip force recruitment rates. Understanding how differences in the rate of force recruitment influences trigeminal system function is an important step toward furthering the knowledge of orofacial sensorimotor control.

METHOD

Lower lip vibrotactile detection thresholds (LL-VDTs) were sampled in response to sinusoidal inputs delivered to the lip vermilion at 5, 10, 50, and 150 Hz while adult participants engaged in a baseline condition (no force), 2 low-level lip force recruitment tasks differing by rate (0.1 Hz or 2 Hz), and passive displacement of the lip as a control to approximate the mechanosensory consequences of voluntary movement.

RESULTS

LL-VDTs increased significantly for test frequencies at or below 50 Hz during voluntary lip force recruitment. LL-VDT shifts were positively related to changes in the rate of lip force recruitment, whereas passively imposed displacements of the lip were ineffective in shifting LL-VDTs.

CONCLUSIONS

These findings are considered in relation to published reports of force-related sensory gating in orofacial and limb systems and the potential role of somatosensory gating along the trigeminal system during orofacial behaviors.

Functional brain imaging of swallowing: an activation likelihood estimation meta-analysis

A quantitative, voxel-wise meta-analysis was performed to investigate the cortical control of water and saliva swallowing. Studies that were included in the meta-analysis (1) examined water swallowing, saliva swallowing, or both, and (2) reported brain activation as coordinates in standard space. Using these criteria, a systematic literature search identified seven studies that examined water swallowing and five studies of saliva swallowing. An activation likelihood estimation (ALE) meta-analysis of these studies was performed with GingerALE. For water swallowing, clusters with high activation likelihood were found in the bilateral sensorimotor cortex, right inferior parietal lobule, and right anterior insula. For saliva swallowing, clusters with high activation likelihood were found in the left sensorimotor cortex, right motor cortex, and bilateral cingulate gyrus. A between-condition meta-analysis revealed clusters with higher activation likelihood for water than for saliva swallowing in the right inferior parietal lobule, right postcentral gyrus, and right anterior insula. Clusters with higher activation likelihood for saliva than for water swallowing were found in the bilateral supplementary motor area, bilateral anterior cingulate gyrus, and bilateral precentral gyrus. This meta-analysis emphasizes the distributed and partly overlapping cortical networks involved in the control of water and saliva swallowing. Water swallowing is associated with right inferior parietal activation, likely reflecting the sensory processing of intraoral water stimulation. Saliva swallowing more strongly involves premotor areas, which are crucial for the initiation and control of movements.

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.

Organization of cortical processing for facial movements during licking in cats

We proposed that cortical organization for the execution of adequate licking in cats was processed under the control of two kinds of affiliated groups for face and jaw & tongue movements (Hiraba H, Sato T. 2005A. Cerebral control of face, jaw, and tongue movements in awake cats: Changes in regional cerebral blood flow during lateral feeding Somatosens Mot Res 22:307-317). We assumed the cortical organization for face movements from changes in MRN (mastication-related neuron) activities recorded at area M (motor cortex) and orofacial behaviors after the lesion in the facial SI (facial region in the primary somatosensory cortex). Although we showed the relationship between facial SI (area 3b) and area M (area 4delta), the property of area C (area 3a) was not fully described. The aim of this present study is to investigate the functional role of area C (the anterior part of the coronal sulcus) that transfers somatosensory information in facial SI to area M, as shown in a previous paper (Hiraba H. 2004. The function of sensory information from the first somatosensory cortex for facial movements during ingestion in cats Somatosens Mot Res 21:87-97). We examined the properties of MRNs in area C and changes in orofacial behaviors after the area C or area M lesion. MRNs in area C had in common RFs in the lingual, perioral, and mandibular parts, and activity patterns of MRNs showed both post- and pre-movement types. Furthermore, cats with the area C lesion showed similar disorders to cats with the area M lesion, such as the dropping of food from the contralateral mouth, prolongation of the period of ingestion and mastication, and so on. From these results, we believe firmly the organization of unilateral cortical processing in facial SI, area C, and area M for face movements during licking.

Cerebral cortical processing of swallowing in older adults

While brain-imaging studies in young adults have implicated multiple cortical regions in swallowing, investigations in older subjects are lacking. This study examined the neural representations of voluntary saliva swallowing and water swallowing in older adults. Nine healthy females were examined with event-related functional magnetic resonance imaging (fMRI) while laryngeal swallow-related movements were recorded. Swallowing in the older adults, like young adults, activated multiple cortical regions, most prominently the lateral pericentral, perisylvian, and anterior cingulate cortex. Activation of the postcentral gyrus was lateralized to the left hemisphere for saliva and water swallowing, consistent with our findings in young female subjects. Comparison of saliva and water swallowing revealed a fourfold increase in the brain volume activated by the water swallow compared to the saliva swallow, particularly within the right premotor and prefrontal cortex. This task-specific activation pattern may represent a compensatory response to the demands of the water swallow in the face of age-related diminution of oral sensorimotor function.

Perceptual distortion of face deletion by local anaesthesia of the human lips and teeth

As visual guidance of facial movements is impossible, accurate movements for speech and mastication require an established body image that is formed via the information from mechanoreceptors in the skin, mucosa, periodontium, and proprioceptors in the facial and masticatory muscles and in the jaw joints. In this study we aimed to investigate how the acute deafferentation of lips and teeth alters the established image of lips, teeth and the thumb. We used a psychophysical method to determine whether the perceived sizes of the upper lip and front teeth change when the sensory input from the lips and front teeth is fully blocked. We also examined the perceived size of the thumb to test for acute interactions between the thumb and facial structures. Local anaesthetic blocking of upper lip and upper front teeth significantly increased the perceived size of the upper lip by as much as 100% (range 21-100%) in ten out of eleven subjects tested (overall mean 52%; p=0.001). The perceived size of the upper teeth also significantly increased by as much as 155% (range 30-155%) in eight of the eleven subjects during anaesthesia (overall mean 41%; p=0.035). When the region of anaesthesia was increased and both upper and lower teeth and lips were anaesthetised, the perceived size of the upper lip again increased, by 53% (p=0.040), but the change in perceived size of the upper front teeth (18%) was not significant (p=0.206). In both studies there was no change in perceived size of the thumb. The results illustrate the labile central interaction between sensory inputs and the importance of feedback from peripheral afferents in generating the subjective facial image. The timing, level, and area of anaesthesia may be important modifiers of these interactions.

Afferent cortical connections of the motor cortical larynx area in the rhesus monkey

The present study describes the cortical input into the motor cortical larynx area. The retrograde tracer horseradish peroxidase-conjugated wheat germ agglutinin was injected into the electrophysiologically identified motor cortical larynx area in three rhesus monkeys (Macaca mulatta). Retrogradely labeled cells were found in the surrounding premotor cortex (areas 6V and 6D), primary motor cortex (area 4), primary somatosensory cortex (areas 3, 1 and 2), anterior and posterior secondary somatosensory cortex and the probable homologue of Broca's area (areas 44 and 45); furthermore, labeling was found in the supplementary motor area, anterior and posterior cingulate cortex (areas 24 and 23), prefrontal and orbital frontal cortex (areas 8A, 46V, 47/12L, 47/12O, 13), agranular, dysgranular and granular insula as well as in the cortex within the upper bank of the middle third of the superior temporal sulcus (area TPO). The majority of these regions are reciprocally connected with the motor cortical larynx area [Brain Res 949 (2000) 23]. The laryngeal motor cortical input is discussed in relation to the connections of other motor cortical areas and its role in vocal control.

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

1. The lateral pericentral region of the cerebral cortex has been well documented in primates to be important in sensorimotor integration and control and in the learning of new motor skills. 2. The present article provides, first, an overview of limb sensorimotor cortical mechanisms and, second, outlines recent evidence pointing to an important role for the face sensorimotor cortex in semi-automatic, as well as trained, orofacial motor behaviour and to its propensity for neuroplastic changes in association with orofacial motor skill acquisition or an altered oral environment.

Cerebral areas processing swallowing and tongue movement are overlapping but distinct: a functional magnetic resonance imaging study

Although multiple regions of the cerebral cortex have been implicated in swallowing, the functional contributions of each brain area remain unclear. The present study sought to clarify the roles of these cortical foci in swallowing by comparing brain activation associated with voluntary saliva swallowing and voluntary tongue elevation. Fourteen healthy right-handed subjects were examined with single-event-related functional magnetic resonance imaging (fMRI) while laryngeal movements associated with swallowing and tongue movement were simultaneously recorded. Both swallowing and tongue elevation activated 1) the left lateral pericentral and anterior parietal cortex, and 2) the anterior cingulate cortex (ACC) and adjacent supplementary motor area (SMA), suggesting that these brain regions mediate processes shared by swallowing and tongue movement. Tongue elevation activated a larger total volume of cortex than swallowing, with significantly greater activation within the ACC, SMA, right precentral and postcentral gyri, premotor cortex, right putamen, and thalamus. Although a contrast analysis failed to identify activation foci specific to swallowing, superimposed activation maps suggested that the most lateral extent of the left pericentral and anterior parietal cortex, rostral ACC, precuneus, and right parietal operculum/insula were preferentially activated by swallowing. This finding suggests that these brain areas may mediate processes specific to swallowing. Approximately 60% of the subjects showed a strong functional lateralization of the postcentral gyrus toward the left hemisphere for swallowing, whereas 40% showed a similar activation bias for the tongue elevation task. This finding supports the view that the oral sensorimotor cortices within the left and right hemispheres are functionally nonequivalent.

Integration of the upper and lower lips in the postcentral area 2 of conscious macaque monkeys (Macaca fuscata)

The representation of the lip in area 2 of the postcentral somatosensory cortex was studied in conscious macaque monkeys by recording single-neurone activities. Seventy penetrations were made in the oral region of six hemispheres of four animals and 1157 neurones were isolated. The receptive field characteristics of 839 neurones were identified. Among them, 363 neurones along 47 penetrations responded to mechanical lip stimulation (lip neurones). A substantial number of lip neurones (17%, 62/363) had composite receptive fields that included not only the lip but also other oral structures. Although, the majority of lip neurones had receptive fields on either the upper or the lower lip (unilabial neurones), about 20% had receptive fields including both the upper and lower lips (bilabial neurones). Receptive field features of bilabial neurones were summarized as follows: (1) the receptive fields always included the corresponding sites of the upper and lower lips that would come into contact when the jaw closed; (2) the submodality preferences of the upper and lower portions of the receptive fields were identical in all cases; (3) if a light stroking stimulus in a specific direction was adequate, portions of the receptive field on the upper and lower lips responded with a common directional preference. Furthermore, bilabial receptive fields were unlikely to be the simple 'dimer' of unilabial receptive fields: the relative incidence of neurones with bilateral or composite receptive fields was much higher in bilabial than in unilabial neurones. That is, bilabial integration was accompanied by the integration of both sides of the lips, and of the lip and other adjacent oral structures. These features of bilabial neurones appear to be suitable for the form discrimination of objects held in the anterior part of the mouth. These neurones may be the prerequisite neural basis for the oral stereognosis that would take place in the neighbouring association cortices.

Perceptual distortions of the human body image produced by local anaesthesia, pain and cutaneous stimulation

1. Knowledge of the size and orientation of the hand is essential if it is to be moved accurately in space. We used two psychophysical methods to determine whether the perceived size of a body part changes when its sensory input is changed: first, the selection of scaled drawings which matched the apparent size of a body part, and second, a motor task in which the subject drew the body part to depict its perceived size. 2. Complete anaesthesia of the thumb (with a digital nerve block) significantly increased its perceived size by 60-70% when assessed with both psychophysical methods. During this anaesthesia, the perceived size of the adjacent index finger or digits on the contralateral side was unaltered. However, the size of the unanaesthetized lips increased (by approximately 50%). 3. Marked sensory loss for the lips (produced by topical anaesthetics) significantly increased their perceived size when assessed with both methods of measurement. There was a small increase in apparent size of the thumb. 4. To determine whether changes in perceived size could also be produced by an elevation of peripheral inputs, innocuous electrical stimulation of the digital nerves and also painful cooling of the digit were used. Both procedures produced small but significant increases in perceived size of the stimulated part. 5. The results highlight lability in the perceived size of parts of the body and how this affects motor output. The data may reveal perceptual consequences of acute changes in central somatosensory maps, changes which are known to occur with deafferentation.

Differences in fine control of forces generated by the tongue, lips and fingers in humans

This study compared the fine control of forces generated by the tongue, lips and fingers in middle-aged adults. The aims were to determine whether (1) the articulatory organs (tongue, lips) and fingers differed in the manner of motor control, (2) force control of the various articulatory organs was similar, and (3) control of forces generated by males was different from that of forces generated by females. The relation among several variables of the ramp-and-hold force contraction and target force level was quantified for the articulatory organs and the fingers in 14 normal individuals (7 males and 7 females). Using visual feedback, participants produced ramp-and-hold compression forces as rapidly and accurately as possible to targets ranging from 0.25 to 2 N. The results showed differences in the profiles of forces generated by the articulatory organs and fingers. In particular, the forefingers were characterized by a greater accuracy of force control and precision of movement, a greater stability of the hold phase, but by slower velocities than the articulatory organs. Motor control of the lower lip differed from that of the upper lip and tongue. Mostly, the lower lip was characterized by a greater precision of contraction, faster development of the force, and greater stability of the hold phase than the upper lip and tongue. Gender was a distinguishing factor in the force task; males were able to produce forces with higher velocities and greater precision than females.

Effects on non-human primate mastication of reversible inactivation by cooling of the face primary somatosensory cortex

Rhythmical jaw movements can be evoked by intracortical microstimulation within four physiologically defined regions, one of which is the primary face somatosensory cortex (face SI). It has been proposed that these regions may be involved in the selection and/or control of masticatory patterns generated at the brainstem level. The aim here was to determine if mastication is affected by reversible, cooling-induced inactivation of the face SI. Two cranial chambers were chronically implanted in two monkeys (Macaca fascicularis) to allow access bilaterally to the face SI. A thermode was placed on the dura or pia overlying each SI that had been shown with micro-electrode recordings to receive intraoral inputs. A hot or cold alcohol-water solution was pumped through the thermodes while the monkey chewed a small piece of apple or a sultana during precool (thermode temperature, 37 degree C), cool (2-4 degrees C), and postcool (37 degrees C) conditions. Electromyographic (EMG) activity was recorded intramuscularly from the masseter, genioglossus, and anterior digastric. Cooling of SI impaired rhythmical jaw and tongue movements and EMG activity associated with mastication in one monkey (H5), and modified the pattern of EMG activity in the other (H6). The total masticatory time (i.e., time taken for chewing and manipulation of the bolus before swallowing) was increased. This was due principally to an increase in the oral transport time (i.e., time taken for manipulation of bolus after chewing and before swallowing: monkey H6, control, 2.7 sec; cool, 5.2 sec, p < 0.05); the bolus was manipulated by the tongue during this period before swallowing. Within the chewing time (i.e., time during which chewing occurred), cooling resulted in a significant increase in anterior digastric muscle duration, a significant delay in the onset of masseter EMG activity, and a significant increase in the variance of genioglossus EMG duration. The data support the view that the face SI plays a part in modulating the central pattern generator for mastication.

Discharge frequencies and intervals between pulses: are they always the same?

Neuronal discharge recorded as action potentials with intervening interpulse intervals can be converted to a frequency format. The ease of this interconversion masks two important problems in this process. For the same data set, the dynamics of change with the measured variable will be different for each of these two methods of characterizing discharge and each of these forms of rate indexing will have a different mean value. In all neuronal discharge studies, it may be initially necessary to determine whether normal population statistics apply to the primary data expressed in the IPI-mode or in the Hz-mode.