Purpose: Orofacial somatosensory inputs play an important role in speech motor control and speech learning. Since receiving specific auditory-somatosensory inputs during speech perceptual training alters speech perception, similar perceptual training could also alter speech production. We examined whether the production performance was changed by perceptual training with orofacial somatosensory inputs.
Methods: We focused on the French vowels /e/ and /ø/, contrasted in their articulation by horizontal gestures. Perceptual training consisted of a vowel identification task contrasting /e/ and /ø/. Along with training, for the first group of participants, somatosensory stimulation was applied as facial skin stretch in backward direction. We recorded the target vowels uttered by the participants before and after the perceptual training and compared their F1, F2 and F3 formants. We also tested a control group with no somatosensory stimulation and another somatosensory group with a different vowel continuum (/e/-/i/) for perceptual training.
Results: Perceptual training with somatosensory stimulation induced changes in F2 and F3 in the produced vowel sounds. F2 decreased consistently in the two somatosensory groups. F3 increased following the /e/-/ø/ training and decreased following the /e/-/i/ training. This change was significantly correlated with the perceptual shift between the first and second half of the training phase in the somatosensory group with the /e/-/ø/ training, but not with the /e/-/i/ training. The control group displayed no effect on F2 and F3, and just a tendency of F1 increase.
Conclusions: The results suggest that somatosensory inputs associated to speech sound inputs can play a role in speech training and learning in both production and perception.

Although there is no doubt from an empirical viewpoint that reflex mechanisms can contribute to tongue motor control in humans, there is limited neurophysiological evidence to support this idea. Previous results failing to observe any tonic stretch reflex in the tongue had reduced the likelihood of a reflex contribution in tongue motor control. The current study presents experimental evidence of a human tongue reflex in response to a sudden stretch while holding a posture for speech. The latency was relatively long (50 ms), which is possibly mediated through cortical-arc. The activation peak in a speech task was greater than in a non-speech task while background activation levels were similar in both tasks, and the peak amplitude in a speech task was not modulated by the additional task to react voluntarily to the perturbation. Computer simulations with a simplified linear mass-spring-damper model showed that the recorded muscle activation response is suited for the generation of tongue movement responses that were observed in a previous study with the appropriate timing when taking into account a possible physiological delay between reflex muscle activation and the corresponding force. Our results evidenced clearly that reflex mechanisms contribute to tongue posture stabilization for speech production.

Orofacial somatosensory inputs modify the perception of speech sounds. Such auditory-somatosensory integration likely develops alongside speech production acquisition. We examined whether the somatosensory effect in speech perception varies depending on individual characteristics of speech production. The somatosensory effect in speech perception was assessed by changes in category boundary between /e/ and /ø/ in a vowel identification test resulting from somatosensory stimulation providing facial skin deformation in the rearward direction corresponding to articulatory movement for /e/ applied together with the auditory input. Speech production performance was quantified by the acoustic distances between the average first, second and third formants of /e/ and /ø/ utterances recorded in a separate test. The category boundary between /e/ and /ø/ was significantly shifted towards /ø/ due to the somatosensory stimulation which is consistent with previous research. The amplitude of the category boundary shift was significantly correlated with the acoustic distance between the mean second – and marginally third – formants of /e/ and /ø/ productions, with no correlation with the first formant distance. Greater acoustic distances can be related to larger contrasts between the articulatory targets of vowels in speech production. These results suggest that the somatosensory effect in speech perception can be linked to speech production performance.

Orofacial somatosensory inputs may play a role in the link between speech perception and production. Given the fact that speech motor learning, which involves paired auditory and somatosensory inputs, results in changes to speech perceptual representations, somatosensory inputs may also be involved in learning or adaptive processes of speech perception. Here we show that repetitive pairing of somatosensory inputs and sounds, such as occurs during speech production and motor learning, can also induce a change of speech perception. We examined whether the category boundary between /ε/ and /a/ was changed as a result of perceptual training with orofacial somatosensory inputs. The experiment consisted of three phases: Baseline, Training, and Aftereffect. In all phases, a vowel identification test was used to identify the perceptual boundary between /ε/ and /a/. In the Baseline and the Aftereffect phase, an adaptive method based on the maximum-likelihood procedure was applied to detect the category boundary using a small number of trials. In the Training phase, we used the method of constant stimuli in order to expose participants to stimulus variants which covered the range between /ε/ and /a/ evenly. In this phase, to mimic the sensory input that accompanies speech production and learning in an experimental group, somatosensory stimulation was applied in the upward direction when the stimulus sound was presented. A control group followed the same training procedure in the absence of somatosensory stimulation. When we compared category boundaries prior to and following paired auditory-somatosensory training, the boundary for participants in the experimental group reliably changed in the direction of /ε/, indicating that the participants perceived /a/ more than /ε/ as a consequence of training. In contrast, the control group did not show any change. Although a limited number of participants were tested, the perceptual shift was reduced and almost eliminated one week later. Our data suggest that repetitive exposure of somatosensory inputs in a task that simulates the sensory pairing which occurs during speech production, changes perceptual system and supports the idea that somatosensory inputs play a role in speech perceptual adaptation, probably contributing to the formation of sound representations for speech perception.

Musicians tend to have better auditory and motor performance than non-musicians because of their extensive musical experience. In a previous study, we established that loudness discrimination acuity is enhanced when sound is produced by a precise force generation task. In this study, we compared the enhancement effect between experienced pianists and non-musicians. Without the force generation task, loudness discrimination acuity was better in pianists than non-musicians in the condition. However, the force generation task enhanced loudness discrimination acuity similarly in both pianists and non-musicians. The reaction time was also reduced with the force control task, but only in the non-musician group. The results suggest that the enhancement of loudness discrimination acuity with the precise force generation task is independent of musical experience and is, therefore, a fundamental function in auditory-motor interaction.

Recent studies have demonstrated that the auditory speech perception of a listener can be modulated by somatosensory input applied to the facial skin suggesting that perception is an embodied process. However, speech perception is a multisensory process involving both the auditory and visual modalities. It is unknown whether and to what extent somatosensory stimulation to the facial skin modulates audio-visual speech perception. If speech perception is an embodied process, then somatosensory stimulation applied to the perceiver should influence audio-visual speech processing. Using the McGurk effect (the perceptual illusion that occurs when a sound is paired with the visual representation of a different sound, resulting in the perception of a third sound) we tested the prediction using a simple behavioral paradigm and at the neural level using event-related potentials (ERPs) and their cortical sources. We recorded ERPs from 64 scalp sites in response to congruent and incongruent audio-visual speech randomly presented with and without somatosensory stimulation associated with facial skin deformation. Subjects judged whether the production was /ba/ or not under all stimulus conditions. In the congruent audio-visual condition subjects identifying the sound as /ba/, but not in the incongruent condition consistent with the McGurk effect. Concurrent somatosensory stimulation improved the ability of participants to more correctly identify the production as /ba/ relative to the non-somatosensory condition in both congruent and incongruent conditions. ERP in response to the somatosensory stimulation for the incongruent condition reliably diverged 220 ms after stimulation onset. Cortical sources were estimated around the left anterior temporal gyrus, the right middle temporal gyrus, the right posterior superior temporal lobe and the right occipital region. The results demonstrate a clear multisensory convergence of somatosensory and audio-visual processing in both behavioral and neural processing consistent with the perspective that speech perception is a self-referenced, sensorimotor process.

Motor executions alter sensory processes. Studies have shown that loudness perception changes when a sound is generated by active movement. However, it is still unknown where and how the motor-related changes in loudness perception depend on the task demand of motor execution. We examined whether different levels of precision demands in motor control affects loudness perception. We carried out a loudness discrimination test, in which the sound stimulus was produced in conjunction with the force generation task. We tested three target force amplitude levels. The force target was presented on a monitor as a fixed visual target. The generated force was also presented on the same monitor as a movement of the visual cursor. Participants adjusted their force amplitude in a predetermined range without overshooting using these visual targets and moving cursor. In the control condition, the sound and visual stimuli were generated externally (without a force generation task). We found that the discrimination performance was significantly improved when the sound was produced by the force generation task compared to the control condition, in which the sound was produced externally, although we did not find that this improvement in discrimination performance changed depending on the different target force amplitude levels. The results suggest that the demand for precise control to produce a fixed amount of force may be key to obtaining the facilitatory effect of motor execution in auditory processes.

Somatosensory stimulation associated with facial skin deformation has been developed and efficiently applied in the study of speech production and speech perception. However, the technique is limited to a simplified unidirectional pattern of stimulation, and cannot adapt to realistic stimulation patterns related to multidimensional orofacial gestures. To overcome this issue, we developed a new multi-actuator system enabling to synchronously deform the facial skin in multiple directions. The first prototype involves stimulation in two directions and we demonstrate its efficiency through a temporal order judgement test involving vertical and horizontal facial skin stretches at the sides of the mouth.

Speech learning requires precise motor control but it likewise requires transient storage of information to enable the adjustment of upcoming movements based on the success or failure of previous attempts. The contribution of somatic sensory memory for limb position has been documented in work on arm movement, however, in speech, the sensory support for speech production comes both from somatosensory and auditory inputs and accordingly sensory memory for either or both of sounds and somatic inputs might contribute to learning. In the present study, adaptation to altered auditory feedback was used as an experimental model of speech motor learning. Participants also underwent tests of both auditory and somatic sensory memory. We found that although auditory memory for speech sounds is better than somatic memory for speech-like facial skin deformations, somatic sensory memory predicts adaptation, whereas auditory sensory memory does not. Thus, even though speech relies substantially on auditory inputs and in the present manipulation adaptation requires the minimization of auditory error, it is somatic inputs that provide the memory support for learning.

The human tongue is atypical as a motor system since its movement is determined by deforming its soft tissues via muscles that are in large part embedded in it (muscular hydrostats). However, the neurophysiological mechanisms enabling fine tongue motor control are not well understood. We investigated sensorimotor control mechanisms of the tongue through a perturbation experiment. A mechanical perturbation was applied to the tongue during the articulation of three vowels (/i/,/e/, /ε/) under conditions of voicing, whispering and posturing. Tongue movements were measured at three surface locations in the sagittal plane using electromagnetic articulography. We found that the displacement induced by the external force was quickly compensated for. Individual sensors did not return to their original positions but went towards a position on the original tongue contour for that vowel. The amplitude of compensatory response at each tongue site varied systematically according to the articulatory condition. A mathematical simulation that included reflex mechanisms suggested that the observed compensatory response can be attributed to a reflex mechanism, rather than passive tissue properties. The results provide evidence for the existence of quick compensatory mechanisms in the tongue that may be dependent on tunable reflexes. The tongue posture for vowels could be regulated in relation to the shape of the tongue contour, rather than to specific positions for individual tissue points.

Modulation of auditory activity occurs before and during voluntary speech movement. However, it is unknown whether orofacial somatosensory input is modulated in the same manner. The current study examined whether or not the somatosensory event-related potentials (ERPs) in response to facial skin stretch are changed during speech and nonspeech production tasks. Specifically, we compared ERP changes to somatosensory stimulation for different orofacial postures and speech utterances. Participants produced three different vowel sounds (voicing) or non-speech oral tasks in which participants maintained a similar posture without voicing. ERP's were recorded from 64 scalp sites in response to the somatosensory stimulation under six task conditions (three vowels × voicing/posture) and compared to a resting baseline condition. The first negative peak for the vowel /u/ was reliably reduced from the baseline in both the voicing and posturing tasks, but the other conditions did not differ. The second positive peak was reduced for all voicing tasks compared to the posturing tasks. The results suggest that the sensitivity of somatosensory ERP to facial skin deformation is modulated by the task and that somatosensory processing during speaking may be modulated differently relative to phonetic identity.

There is accumulating evidence that articulatory/motor knowledge plays a role in phonetic processing, such as the recent finding that orofacial somatosensory inputs may influence phoneme categorization. We here show that somatosensory inputs also contribute at a higher level of the speech perception chain, that is, in the context of word segmentation and lexical decision. We carried out an auditory identification test using a set of French phrases consisting of a definite article “la” followed by a noun, which may be segmented differently according to the placement of accents within the phrase. Somatosensory stimulation was applied to the facial skin at various positions within the acoustic utterances corresponding to these phrases, which had been recorded with neutral accent, that is, with all syllables given similar emphasis. We found that lexical decisions reflecting word segmentation were significantly and systematically biased depending on the timing of somatosensory stimulation. This bias was not induced when somatosensory stimulation was applied to the skin other than on the face. These results provide evidence that the orofacial somatosensory system contributes to lexical perception in situations that would be disambiguated by different articulatory movements, and suggests that articulatory/motor knowledge might be involved in speech segmentation.

Multisensory integration allows us to link sensory cues from multiple sources and plays a crucial role in speech development. However, it is not clear whether humans have an innate ability or whether repeated sensory input while the brain is maturing leads to efficient integration of sensory information in speech. We investigated the integration of auditory and somatosensory information in speech processing in a bimodal perceptual task in 15 young adults (age 19 to 30) and 14 children (age 5 to 6). The participants were asked to identify if the perceived target was the sound /e/ or /ø/. Half of the stimuli were presented under a unimodal condition with only auditory input. The other stimuli were presented under a bimodal condition with both auditory input and somatosensory input consisting of facial skin stretches provided by a robotic device, which mimics the articulation of the vowel /e/. The results indicate that the effect of somatosensory information on sound categorization was larger in adults than in children. This suggests that integration of auditory and somatosensory information evolves throughout the course of development.

Speech motor control and learning rely both on somatosensory and auditory inputs. Somatosensory inputs associated with speech production can also affect the process of auditory perception of speech, and the somatosensory-auditory interaction may play a fundamental role in auditory perception of speech. Here, we show that the somatosensory system contributes to perceptual recalibration, separate from its role in motor function. Subjects participated in speech motor adaptation to altered auditory feedback. Auditory perception of speech was assessed in phonemic identification tests prior to and following speech adaptation. To investigate a role of the somatosensory system in motor adaptation and subsequent perceptual change, we applied orofacial skin stretch in either a backward or forward direction during the auditory feedback alteration as a somatosensory modulation. We found that the somatosensory modulation did not affect the amount of adaptation at the end of training although it changed the rate of adaptation. However, the perception following speech adaptation was altered depending on the direction of the somatosensory modulation. Somatosensory inflow rather than motor outflow thus drives changes to auditory perception of speech following speech adaptation, suggesting that somatosensory inputs play an important role in tuning of perceptual system.

Purpose: The purpose of this study was to examine whether developmental dyslexia (DD) is characterized by deficiencies in speech sensory and motor feedforward and feedback mechanisms, which are involved in the modulation of phonological representations.
Method: A total of 42 adult native speakers of Dutch (22 adults with DD; 20 participants who were typically reading controls) were asked to produce /bep/ while the first formant (F1) of the /e/ was not altered (baseline), increased (ramp), held at maximal perturbation (hold), and not altered again (after-effect). The F1 of the produced utterance was measured for each trial and used for statistical analyses. The measured F1s produced during each phase were entered in a linear mixed-effects model.
Results: Participants with DD adapted more strongly during the ramp phase and returned to baseline to a lesser extent when feedback was back to normal (after-effect phase) when compared with the typically reading group. In this study, a faster deviation from baseline during the ramp phase, a stronger adaptation response during the hold phase, and a slower return to baseline during the after-effect phase were associated with poorer reading and phonological abilities.
Conclusion: The data of the current study are consistent with the notion that the phonological deficit in DD is associated with a weaker sensorimotor magnet for phonological representations.

In the present paper, we present evidence for the idea that speech motor learning is accompanied by changes to the neural coding of both auditory and somatosensory stimuli. Participants in our experiments undergo adaptation to altered auditory feedback, an experimental model of speech motor learning which like visuo-motor adaptation in limb movement, requires that participants change their speech movements and associated somatosensory inputs to correct for systematic real-time changes to auditory feedback. We measure the sensory e ects of adaptation by examining changes to auditory and somatosensory event-related responses. We nd that adaptation results in progressive changes to speech acoustical outputs that serve to correct for the perturbation. We also observe changes in both auditory and somatosensory event-related responses that are correlated with the magnitude of adaptation. These results indicate that sensory change occurs in conjunction with the processes involved in speech motor adaptation.

Cortical processing associated with orofacial somatosensory function in speech has received limited experimental attention due to the difficulty of providing precise and controlled stimulation. This article introduces a technique for recording somatosensory event-related potentials (ERP) that uses a novel mechanical stimulation method involving skin deformation using a robotic device. Controlled deformation of the facial skin is used to modulate kinesthetic inputs through excitation of cutaneous mechanoreceptors. By combining somatosensory stimulation with electroencephalographic recording, somatosensory evoked responses can be successfully measured at the level of the cortex. Somatosensory stimulation can be combined with the stimulation of other sensory modalities to assess multisensory interactions. For speech orofacial stimulation is combined with speech sound stimulation to assess the contribution of multi-sensory processing including the effects of timing differences to examine the manner in which the two sensory signals combine. The ability to precisely control orofacial somatosensory stimulation during speech perception and speech production with ERP recording is an important tool providing new insights into the neural organization and neural representations for speech.

Articulatory information can support learning or remediating pronunciation of a second language (L2). This paper describes an electromagnetic articulometer-based visual-feedback approach using an articulatory target presented in real-time to facilitate L2 pronunciation learning. This approach trains learners to adjust articulatory positions to match targets for a novel L2 vowel estimated from productions of vowels that overlap in both L1 and L2. Training of Japanese learners for the American English vowel 􏰀􏰁􏰀 that included visual training improved its pronunciation regardless of whether audio training was also included. Articulatory visual feedback is shown to be an effective method for facilitating L2 pronunciation learning.

Speech perception is known to rely on both auditory and visual information. However, sound-specific somatosensory input has been shown also to influence speech perceptual processing (Ito et al., 2009). In the present study, we addressed further the relationship between somatosensory information and speech perceptual processing by addressing the hypothesis that the temporal relationship between orofacial movement and sound processing contributes to somatosensory–auditory interaction in speech perception. We examined the changes in event-related potentials (ERPs) in response to multisensory synchronous (simultaneous) and asynchronous (90 ms lag and lead) somatosensory and auditory stimulation compared to individual unisensory auditory and somatosensory stimulation alone. We used a robotic device to apply facial skin somatosensory deformations that were similar in timing and duration to those experienced in speech production. Following synchronous multisensory stimulation the amplitude of the ERP was reliably different from the two unisensory potentials. More importantly, the magnitude of the ERP difference varied as a function of the relative timing of the somatosensory– auditory stimulation. Event-related activity change due to stimulus timing was seen between 160 and 220 ms following somatosensory onset, mostly around the parietal area. The results demonstrate a dynamic modulation of somatosensory–auditory convergence and suggest the contribution of somatosensory information for speech processing process is dependent on the specific temporal order of sensory inputs in speech production.

Purpose: Somatosensory information associated with speech articulatory movements affects the perception of speech sounds and vice versa, suggesting an intimate linkage between speech production and perception systems. However, it is unclear which cortical processes are involved in the interaction between speech sounds and orofacial somatosensory inputs. The authors examined whether speech sounds modify orofacial somatosensory cortical potentials that were elicited using facial skin perturbations.
Method: Somatosensory event-related potentials in EEG were recorded in 3 background sound conditions (pink noise, speech sounds, and nonspeech sounds) and also in a silent condition. Facial skin deformations that are similar in timing and duration to those experienced in speech production were used for somatosensory stimulation.
Results: The authors found that speech sounds reliably enhanced the first negative peak of the somatosensory event-related potential when compared with the other 3 sound conditions. The enhancement was evident at electrode locations above the left motor and premotor area of the orofacial system. The result indicates that speech sounds interact with somatosensory cortical processes that are produced by speech-production-like patterns of facial skin stretch.
Conclusion: Neural circuits in the left hemisphere, presumably in left motor and premotor cortex, may play a prominent role in the interaction between auditory inputs and speech-relevant somatosensory processing.

Interactions between auditory and somatosensory information are relevant to the neural processing of speech since speech processes and certainly speech production in- volves both auditory information and inputs that arise from the muscles and tissues of the vocal tract. We previously demonstrated that somatosensory inputs associated with facial skin deformation alter the perceptual processing of speech sounds. We show here that the reverse is also true, that speech sounds alter the perception of facial somatosensory inputs. As a somatosensory task, we used a robotic device to create patterns of facial skin deformation that would normally accompany speech production. We found that the perception of the facial skin deformation was altered by speech sounds in a manner that reflects the way in which auditory and somatosensory effects are linked in speech production. The modulation of orofacial somatosensory processing by auditory inputs was specific to speech and likewise to facial skin deformation. Somatosensory judgments were not affected when the skin deformation was delivered to the forearm or palm or when the facial skin deformation accompanied nonspeech sounds. The perceptual modulation that we observed in conjunction with speech sounds shows that speech sounds specifically affect neural processing in the facial somatosensory system and suggest the involvement of the somatosensory system in both the production and perceptual processing of speech.

Motor learning is dependent on kinesthetic information that is obtained both from cutaneous afferents and from muscle receptors. In human arm move- ment, information from these two kinds of afferents is largely corre- lated. The facial skin offers a unique situation in which there are plentiful cutaneous afferents and essentially no muscle receptors and, accordingly, experimental manipulations involving the facial skin may be used to assess the possible role of cutaneous afferents in motor learning. We focus here on the information for motor learning pro- vided by the deformation of the facial skin and the motion of the lips in the context of speech. We used a robotic device to slightly stretch the facial skin lateral to the side of the mouth in the period immedi- ately preceding movement. We found that facial skin stretch increased lip protrusion in a progressive manner over the course of a series of training trials. The learning was manifest in a changed pattern of lip movement, when measured after learning in the absence of load. The newly acquired motor plan generalized partially to another speech task that involved a lip movement of different amplitude. Control tests indicated that the primary source of the observed adaptation was sensory input from cutaneous afferents. The progressive increase in lip protrusion over the course of training fits with the basic idea that change in sensory input is attributed to motor performance error. Sensory input, which in the present study precedes the target move- ment, is credited to the target-related motion, even though the skin stretch is released prior to movement initiation. This supports the idea that the nervous system generates motor commands on the assumption that sensory input and kinematic error are in register.

Somatosensory signals from the facial skin and muscles of the vocal tract provide a rich source of sensory input in speech production. We show here that the somatosensory system is also involved in the perception of speech. We use a robotic device to create patterns of facial skin deformation that would normally accompany speech production. We find that when we stretch the facial skin while people listen to words, it alters the sounds they hear. The systematic perceptual variation we observe in conjunction with speech-like patterns of skin stretch indicates that somatosen- sory inputs affect the neural processing of speech sounds and shows the involvement of the somatosensory system in the perceptual processing in speech.

Owing to the lack of muscle spindles and tendon organs in the perioral system, cutaneous receptors may contribute to speech sensorimotor processes. We have investigated this possibility in the context of upper lip reflexes, which we have induced by unexpectedly stretching the facial skin lateral to the oral angle. Skin stretch at this location resulted in long latency reflex responses that were similar to the cortical re£exes observed previously. This location reliably elicited the re£ex response, whereas the skin above the oral angle and the skin on the cheek did not. The data suggest that cutaneous mechanoreceptors are narrowly tuned to deformation of the facial skin and provide kinesthetic information for rapid sensorimotor processing in speech.

Although speech articulation relies heavily on the sensorimotor processing, little is known about its brain control mechanisms. Here, we investigate, using transcranial magnetic stimulation, whether the motor cortex contributes to the generation of quick sensorimotor responses involved in speech motor coordination. By applying a jaw-lowering perturbation, we induced a reflexive compensatory upper-lip response, which assists in maintaining the intact labial aperture in the production of bilabial fricative consonants. This reflex response was significantly facilitated by subthreshold transcranial magnetic stimulation over the motor cortex, whereas a simple perioral reflex that is mediated only within the brainstem was not. This suggests that the motor cortex is involved in generating this functional reflexive articulatory compensation.

Different kinds of articulators, such as the upper and lower lips, jaw, and tongue, are precisely coordinated in speech production. Based on a perturbation study of the production of a fricative consonant using the upper and lower lips, it has been suggested that increasing the stiffness in the muscle linkage between the upper lip and jaw is beneficial for maintaining the constriction area between the lips (Gomi et al. 2002). This hypothesis is crucial for examining the mechanism of speech motor control, that is, whether mechanical impedance is controlled for the speech motor coordination. To test this hypothesis, in the current study we performed a dynamical simulation of lip compensatory movements based on a muscle linkage model and then evaluated the performance of compensatory movements. The temporal pattern of stiffness of muscle linkage was obtained from the electromyogram (EMG) of the orbicularis oris superior (OOS) muscle by using the temporal transformation (second-order dynamics with time delay) from EMG to stiffness, whose parameters were experimentally determined. The dynamical simulation using stiffness estimated from empirical EMG successfully reproduced the temporal profile of the upper lip com- pensatory articulations. Moreover, the estimated stiffness variation significantly contributed to reproduce a functional modulation of the compensatory response. This result supports the idea that the mechanical impedance highly contributes to organizing coordination among the lips and jaw. The motor command would be programmed not only to generate movement in each articulator but also to regulate mechanical impedance among articulators for robust coordination of speech motor control.

To explore the mechanisms of speech articulation, which is one of the most sophisticated human motor skills controlled by the central nervous system, we investigated the force-generation dynamics of the human speech articulator muscles [orbicularis oris superior (OOS) and inferior (OOI) muscles of the lips]. Short-pulse electrical stimulation (300 􏰂s) with approximately three or four times the sensation threshold intensity of each subject induced the muscle response. The responses of these muscles were modeled as second-order dynamics with a time delay (TD), and the model parameters [natural frequency (NF), damping ratio (DR), and TD] were identified with a nonlinear least mean squares method. The OOS (NF: 6.1 Hz, DR: 0.71, TD: 14.5 ms) and OOI (NF: 6.1 Hz, DR: 0.68, TD: 15.6 ms) showed roughly similar characteristics in eight subjects. The dynamics in the tongue (generated by combined muscles) also showed similar characteristics (NF: 6.1 Hz, DR: 0.68, TD: 17.4 ms) in two subjects. The NF was higher, and the DR was lower than results measured for arm muscles (NF: 4.25 Hz, DR: 1.05, TD: 23.8 ms for triceps long head), indicating that articulatory organs adapt for more rapid movement. In contrast, slower response dynamics was estimated when muscle force data by voluntarily contraction task were used for force-generation dynamics modeling. We discuss methodological problems in estimating muscle dynamics when different kinds of muscle contraction methods are used.

Previous studies showed that the upper lip compensatory movement against the downward perturbation to the jaw is effective in maintaining labial constriction or in attaining labial contact for the production of bilabial utter- ances. We measured the stiffness of the muscle-linkage between the upper lip and jaw and proposed a model in which the muscle-linkage plays key role in coordinating articulators during speech. To further examine the compensatory mechanism using muscle-linkage, we here investigate the coordination change of the lip-jaw system under the upward perturbation condition, and examine the directional difference between the upward and downward perturbation. Based on the experimental observations, we propose an extended lip-jaw model, which could explain the directional difference.

本論文では,唇音発声時に顎摂動を与える実験を通して,機械的特性を利用した上唇・顎の協調動作について検討する. 唇摩擦音/φ/発声時に下顎を開く摂動を加えると上唇は下降することによって,下顎を閉じる方向に摂動を加えると 上唇は上昇することによって,両唇間せばめを維持するような補償応答を示した.得られた摂動時の運動変化に対して, 機械的特性を考慮した上唇・顎モデルを用いて上唇・顎間の剛性推定を行ったところ,発声タスクに応じて剛性が変化していた. また,/φ/発声時において摂動方向によって剛性が異なっており,下顎を閉じる方向に摂動を加えた場合の方が高い剛性を示した. この推定結果をもとに,下唇を含めた唇・顎モデルを用いて補償動作メカニズムの考察を行った.これらの結果は,筋スティフネスを利用した上唇・ 顎の協調的補償動作が,発声運動制御において重要であることを示唆している.

The cooperative mechanisms in articulatory movements were examined by using mechanical perturbations during bilabial phonemic tasks. The first experiment compares the differences in compensatory responses during sustained productions of the bilabial fricative /φ/ for which lip constriction is required, and /a/, for which the lips and jaw are relatively relaxed. In the second experiment, we perturbed jaw movement with different load-onsets in the sentence ‘‘kono /aφaφa/ mitai’’. In both experiments, labial distances were recovered partly or fully by the downward shifts of the upper lip. The upper lip response was frequently prior to the EMG response observed in the sustained task. Additionally, initial downward displacement of the upper lip was frequently larger when the load was supplied during /φ/ than when it was supplied during /a/ in the sustained and sentence tasks, respectively. The stiffness variation estimated by using a muscle linkage model indicates that the stiffness increases for the bilabial phonemic task in order to robustly configure a labial constriction. The results suggest that the change in passive stiffness regulated by the muscle activation level is important in generating quick cooperative articulation.

Somatosensory signals from facial skin can provide a rich source of sensory input. However, it is unknown yet how cutaneous input works on speech motor control and learning. This chapter introduces a kinesthetic role of orofacial cutaneous afferents in speech processing. We argue for specificity of the orofacial somatosensory system from anatomical and physiological perspectives. The contribution of cutaneous afferents to speech production is evident in neurophysiological and psychophysical findings. Somatosensory modulation associated with facial skin deformation induces a reflex for articulatory motion adjustment in speech production and also an adaptive motion change in speech motor learning. In addition, cutaneous mechanoreceptors are narrowly tuned at the skin lateral to the oral angle. An intriguing function of somatosensory inputs associated with facial skin deformation is to interact with the processing of speech perception. Taken together, orofacial cutaneous afferents play an important role in both speech production and perception.

Orofacial sensori-motor system has different characteristics from the other skeletal system since lack of muscle proprioceptors in most of orofacial muscles for somatosensory function and muscular hydrostat system for motor function. Experiments by observing responses to the mechanical perturbation to speech articulators showed clues to understand a fundamental role of orofacial sensori-motor function in speech motor control and perception. A mechanical perturbation to the tongue showed a quick compensatory mechanism, which may be driven by reflex mechanism. Somatosensory perturbation associated with facial skin deformation showed that facial skin receptors provide kinesthetic information in speech motor learning. One intriguing role of somatosensory inputs associated with facial skin deformation is to change the processing of speech sounds and to tune the representation of speech perception suggesting somatosensory inputs associated with facial skin deformation may works not only for the tuning of motor processing, but also for the perception of speech sounds. The results illuminate new aspect of sensori-motor function concerning speech motor control and perception. It also suggests a linkage between speech production and perception mechanisms.

The link between speech production and perception and their underlying representations lie at the core of the field of speech science. While most work in these research areas has focused on motor, auditory, and, to some extent, visual processes, the somatosensory system has been largely overlooked despite its importance in speech processing. The present talk introduces a possible role of the somatosensory system in the processing of speech production and perception.