Stimulating the lip motor cortex with transcranial magnetic stimulation

Engagement of the speech motor system in challenging speech perception: Activation likelihood estimation meta-analyses

The relationship between speech production and perception is a topic of ongoing debate. Some argue that there is little interaction between the two, while others claim they share representations and processes. One perspective suggests increased recruitment of the speech motor system in demanding listening situations to facilitate perception. However, uncertainties persist regarding the specific regions involved and the listening conditions influencing its engagement. This study used activation likelihood estimation in coordinate-based meta-analyses to investigate the neural overlap between speech production and three speech perception conditions: speech-in-noise, spectrally degraded speech and linguistically complex speech. Neural overlap was observed in the left frontal, insular and temporal regions. Key nodes included the left frontal operculum (FOC), left posterior lateral part of the inferior frontal gyrus (IFG), left planum temporale (PT), and left pre-supplementary motor area (pre-SMA). The left IFG activation was consistently observed during linguistic processing, suggesting sensitivity to the linguistic content of speech. In comparison, the left pre-SMA activation was observed when processing degraded and noisy signals, indicating sensitivity to signal quality. Activations of the left PT and FOC activation were noted in all conditions, with the posterior FOC area overlapping in all conditions. Our meta-analysis reveals context-independent (FOC, PT) and context-dependent (pre-SMA, posterior lateral IFG) regions within the speech motor system during challenging speech perception. These regions could contribute to sensorimotor integration and executive cognitive control for perception and production.

Is Phonology Embodied? Evidence from Mechanical Stimulation

Across languages, certain syllables are systematically preferred to others (e.g., plaf > ptaf). Here, we examine whether these preferences arise from motor simulation. In the simulation account, ill-formed syllables (e.g., ptaf) are disliked because their motor plans are harder to simulate. Four experiments compared sensitivity to the syllable structure of labial- vs. corona-initial speech stimuli (e.g., plaf > pnaf > ptaf vs. traf > tmaf > tpaf); meanwhile, participants (English vs. Russian speakers) lightly bit on their lips or tongues. Results suggested that the perception of these stimuli was selectively modulated by motor stimulation (e.g., stimulating the tongue differentially affected sensitivity to labial vs. coronal stimuli). Remarkably, stimulation did not affect sensitivity to syllable structure. This dissociation suggests that some (e.g., phonetic) aspects of speech perception are reliant on motor simulation, hence, embodied; others (e.g., phonology), however, are possibly abstract. These conclusions speak to the role of embodiment in the language system, and the separation between phonology and phonetics, specifically.

Speech motor facilitation is not affected by ageing but is modulated by task demands during speech perception

Motor areas for speech production activate during speech perception. Such activation may assist speech perception in challenging listening conditions. It is not known how ageing affects the recruitment of articulatory motor cortex during active speech perception. This study aimed to determine the effect of ageing on recruitment of speech motor cortex during speech perception. Single-pulse Transcranial Magnetic Stimulation (TMS) was applied to the lip area of left primary motor cortex (M1) to elicit lip Motor Evoked Potentials (MEPs). The M1 hand area was tested as a control site. TMS was applied whilst participants perceived syllables presented with noise (-10, 0, +10 dB SNRs) and without noise (clear). Participants detected and counted syllables throughout MEP recording. Twenty younger adult subjects (aged 18-25) and twenty older adult subjects (aged 65-80) participated in this study. Results indicated a significant interaction between age and noise condition in the syllable task. Specifically, older adults significantly misidentified syllables in the 0 dB SNR condition, and missed the syllables in the -10 dB SNR condition, relative to the clear condition. There were no differences between conditions for younger adults. There was a significant main effect of noise level on lip MEPs. Lip MEPs were unexpectedly inhibited in the 0 dB SNR condition relative to clear condition. There was no interaction between age group and noise condition. There was no main effect of noise or age group on control hand MEPs. These data suggest that speech-induced facilitation in articulatory motor cortex is abolished when performing a challenging secondary task, irrespective of age.

The effects of continuous oromotor activity on speech motor learning: speech biomechanics and neurophysiologic correlates

Sustained limb motor activity has been used as a therapeutic tool for improving rehabilitation outcomes and is thought to be mediated by neuroplastic changes associated with activity-induced cortical excitability. Although prior research has reported enhancing effects of continuous chewing and swallowing activity on learning, the potential beneficial effects of sustained oromotor activity on speech improvements is not well-documented. This exploratory study was designed to examine the effects of continuous oromotor activity on subsequent speech learning. Twenty neurologically healthy young adults engaged in periods of continuous chewing and speech after which they completed a novel speech motor learning task. The motor learning task was designed to elicit improvements in accuracy and efficiency of speech performance across repetitions of eight-syllable nonwords. In addition, transcranial magnetic stimulation was used to measure the cortical silent period (cSP) of the lip motor cortex before and after the periods of continuous oromotor behaviors. All repetitions of the nonword task were recorded acoustically and kinematically using a three-dimensional motion capture system. Productions were analyzed for accuracy and duration, as well as lip movement distance and speed. A control condition estimated baseline improvement rates in speech performance. Results revealed improved speech performance following 10 min of chewing. In contrast, speech performance following 10 min of continuous speech was degraded. There was no change in the cSP as a result of either oromotor activity. The clinical implications of these findings are discussed in the context of speech rehabilitation and neuromodulation.

Short-Term Immobilization Promotes a Rapid Loss of Motor Evoked Potentials and Strength That Is Not Rescued by rTMS Treatment

Short-term limb immobilization results in skeletal muscle decline, but the underlying mechanisms are incompletely understood. This study aimed to determine the neurophysiologic basis of immobilization-induced skeletal muscle decline, and whether repetitive Transcranial Magnetic Stimulation (rTMS) could prevent any decline. Twenty-four healthy young males (20 ± 0.5 years) underwent unilateral limb immobilization for 72 h. Subjects were randomized between daily rTMS (n = 12) using six 20 Hz pulse trains of 1.5 s duration with a 60 s inter-train-interval delivered at 90% resting Motor Threshold (rMT), or Sham rTMS (n = 12) throughout immobilization. Maximal grip strength, EMG activity, arm volume, and composition were determined at 0 and 72 h. Motor Evoked Potentials (MEPs) were determined daily throughout immobilization to index motor excitability. Immobilization induced a significant reduction in motor excitability across time (−30% at 72 h; p < 0.05). The rTMS intervention increased motor excitability at 0 h (+13%, p < 0.05). Despite daily rTMS treatment, there was still a significant reduction in motor excitability (−33% at 72 h, p < 0.05), loss in EMG activity (−23.5% at 72 h; p < 0.05), and a loss of maximal grip strength (−22%, p < 0.001) after immobilization. Interestingly, the increase in biceps (Sham vs. rTMS) (+0.8 vs. +0.1 mm, p < 0.01) and posterior forearm (+0.3 vs. +0.0 mm, p < 0.05) skinfold thickness with immobilization in Sham treatment was not observed following rTMS treatment. Reduced MEPs drive the loss of strength with immobilization. Repetitive Transcranial Magnetic Stimulation cannot prevent this loss of strength but further investigation and optimization of neuroplasticity protocols may have therapeutic benefit.

Asymmetry of Auditory-Motor Speech Processing is Determined by Language Experience

Speech processing relies on interactions between auditory and motor systems and is asymmetrically organized in the human brain. The left auditory system is specialized for processing of phonemes, whereas the right is specialized for processing of pitch changes in speech affecting prosody. In speakers of tonal languages, however, processing of pitch (i.e., tone) changes that alter word meaning is left-lateralized indicating that linguistic function and language experience shape speech processing asymmetries. Here, we investigated the asymmetry of motor contributions to auditory speech processing in male and female speakers of tonal and non-tonal languages. We temporarily disrupted the right or left speech motor cortex using transcranial magnetic stimulation (TMS) and measured the impact of these disruptions on auditory discrimination (mismatch negativity; MMN) responses to phoneme and tone changes in sequences of syllables using electroencephalography (EEG). We found that the effect of motor disruptions on processing of tone changes differed between language groups: disruption of the right speech motor cortex suppressed responses to tone changes in non-tonal language speakers, whereas disruption of the left speech motor cortex suppressed responses to tone changes in tonal language speakers. In non-tonal language speakers, the effects of disruption of left speech motor cortex on responses to tone changes were inconclusive. For phoneme changes, disruption of left but not right speech motor cortex suppressed responses in both language groups. We conclude that the contributions of the right and left speech motor cortex to auditory speech processing are determined by the functional roles of acoustic cues in the listener's native language.SIGNIFICANCE STATEMENT The principles underlying hemispheric asymmetries of auditory speech processing remain debated. The asymmetry of processing of speech sounds is affected by low-level acoustic cues, but also by their linguistic function. By combining transcranial magnetic stimulation (TMS) and electroencephalography (EEG), we investigated the asymmetry of motor contributions to auditory speech processing in tonal and non-tonal language speakers. We provide causal evidence that the functional role of the acoustic cues in the listener's native language affects the asymmetry of motor influences on auditory speech discrimination ability [indexed by mismatch negativity (MMN) responses]. Lateralized top-down motor influences can affect asymmetry of speech processing in the auditory system.

Transcranial Magnetic Stimulation-Induced Motor Cortex Activity Influences Visual Awareness Judgments

The influence of non-visual information on visual awareness judgments has recently gained substantial interest. Using single-pulse transcranial magnetic stimulation (TMS), we investigate the potential contribution of evidence from the motor system to judgment of visual awareness. We hypothesized that TMS-induced activity in the primary motor cortex (M1) would increase reported visual awareness as compared to the control condition. Additionally, we investigated whether TMS-induced motor-evoked potential (MEP) could measure accumulated evidence for stimulus perception. Following stimulus presentation and TMS, participants first rated their visual awareness verbally using the Perceptual Awareness Scale (PAS), after which they responded manually to a Gabor orientation identification task. Delivering TMS to M1 resulted in higher average awareness ratings as compared to the control condition, in both correct and incorrect identification task response trials, when the hand with which participants responded was contralateral to the stimulated hemisphere (TMS-response-congruent trials). This effect was accompanied by longer PAS response times (RTs), irrespective of the congruence between TMS and identification response. Moreover, longer identification RTs were observed in TMS-response-congruent trials in the M1 condition as compared to the control condition. Additionally, the amplitudes of MEPs were related to the awareness ratings when response congruence was taken into account. We argue that MEP can serve as an indirect measure of evidence accumulated for stimulus perception and that longer PAS RTs and higher amplitudes of MEPs in the M1 condition reflect integration of additional evidence with visual awareness judgment. In conclusion, we advocate that motor activity influences perceptual awareness judgments.

Particle Swarm Optimization for Positioning the Coil of Transcranial Magnetic Stimulation

The distribution of the induced electric field (E-field) during transcranial magnetic stimulation (TMS) depends on the individual anatomical structure of the brain as well as coil positioning. Inappropriate stimulation may degrade the efficacy of TMS or even induce adverse effects. Therefore, optimizing the E-field according to individual anatomy and clinical need has become a research focus. In this paper, particle swarm optimization (PSO) was applied for the first time to the positioning of TMS coils with anatomical head models. We discuss the parameters of the PSO algorithm, which were optimized to achieve a reasonable convergence time suitable for in-time treatment planning. The optimizer improved the distribution of the induced E-field strength at the dedicated cortical region, with a mean value of 48.31% compared with that from the conventional treatment position. The optimization terminated after 4–11 iterations for 13 head models. The applicability and performance of the optimizer for a large population are discussed in terms of cortical complexity. This study could benefit not only clinics but also research on brain modulation.

An interactive model of auditory-motor speech perception

Mounting evidence indicates a role in perceptual decoding of speech for the dorsal auditory stream connecting between temporal auditory and frontal-parietal articulatory areas. The activation time course in auditory, somatosensory and motor regions during speech processing is seldom taken into account in models of speech perception. We critically review the literature with a focus on temporal information, and contrast between three alternative models of auditory-motor speech processing: parallel, hierarchical, and interactive. We argue that electrophysiological and transcranial magnetic stimulation studies support the interactive model. The findings reveal that auditory and somatomotor areas are engaged almost simultaneously, before 100 ms. There is also evidence of early interactions between auditory and motor areas. We propose a new interactive model of auditory-motor speech perception in which auditory and articulatory somatomotor areas are connected from early stages of speech processing. We also discuss how attention and other factors can affect the timing and strength of auditory-motor interactions and propose directions for future research.

Facilitation of motor excitability during listening to spoken sentences is not modulated by noise or semantic coherence

Comprehending speech can be particularly challenging in a noisy environment and in the absence of semantic context. It has been proposed that the articulatory motor system would be recruited especially in difficult listening conditions. However, it remains unknown how signal-to-noise ratio (SNR) and semantic context affect the recruitment of the articulatory motor system when listening to continuous speech. The aim of the present study was to address the hypothesis that involvement of the articulatory motor cortex increases when the intelligibility and clarity of the spoken sentences decreases, because of noise and the lack of semantic context. We applied Transcranial Magnetic Stimulation (TMS) to the lip and hand representations in the primary motor cortex and measured motor evoked potentials from the lip and hand muscles, respectively, to evaluate motor excitability when young adults listened to sentences. In Experiment 1, we found that the excitability of the lip motor cortex was facilitated during listening to both semantically anomalous and coherent sentences in noise relative to non-speech baselines, but neither SNR nor semantic context modulated the facilitation. In Experiment 2, we replicated these findings and found no difference in the excitability of the lip motor cortex between sentences in noise and clear sentences without noise. Thus, our results show that the articulatory motor cortex is involved in speech processing even in optimal and ecologically valid listening conditions and that its involvement is not modulated by the intelligibility and clarity of speech.

The role of hearing ability and speech distortion in the facilitation of articulatory motor cortex

Excitability of articulatory motor cortex is facilitated when listening to speech in challenging conditions. Beyond this, however, we have little knowledge of what listener-specific and speech-specific factors engage articulatory facilitation during speech perception. For example, it is unknown whether speech motor activity is independent or dependent on the form of distortion in the speech signal. It is also unknown if speech motor facilitation is moderated by hearing ability. We investigated these questions in two experiments. We applied transcranial magnetic stimulation (TMS) to the lip area of primary motor cortex (M1) in young, normally hearing participants to test if lip M1 is sensitive to the quality (Experiment 1) or quantity (Experiment 2) of distortion in the speech signal, and if lip M1 facilitation relates to the hearing ability of the listener. Experiment 1 found that lip motor evoked potentials (MEPs) were larger during perception of motor-distorted speech that had been produced using a tongue depressor, and during perception of speech presented in background noise, relative to natural speech in quiet. Experiment 2 did not find evidence of motor system facilitation when speech was presented in noise at signal-to-noise ratios where speech intelligibility was at 50% or 75%, which were significantly less severe noise levels than used in Experiment 1. However, there was a significant interaction between noise condition and hearing ability, which indicated that when speech stimuli were correctly classified at 50%, speech motor facilitation was observed in individuals with better hearing, whereas individuals with relatively worse but still normal hearing showed more activation during perception of clear speech. These findings indicate that the motor system may be sensitive to the quantity, but not quality, of degradation in the speech signal. Data support the notion that motor cortex complements auditory cortex during speech perception, and point to a role for the motor cortex in compensating for differences in hearing ability.

Dissociating Contributions of the Motor Cortex to Speech Perception and Response Bias by Using Transcranial Magnetic Stimulation

Recent studies using repetitive transcranial magnetic stimulation (TMS) have demonstrated that disruptions of the articulatory motor cortex impair performance in demanding speech perception tasks. These findings have been interpreted as support for the idea that the motor cortex is critically involved in speech perception. However, the validity of this interpretation has been called into question, because it is unknown whether the TMS-induced disruptions in the motor cortex affect speech perception or rather response bias. In the present TMS study, we addressed this question by using signal detection theory to calculate sensitivity (i.e., d') and response bias (i.e., criterion c). We used repetitive TMS to temporarily disrupt the lip or hand representation in the left motor cortex. Participants discriminated pairs of sounds from a "ba"-"da" continuum before TMS, immediately after TMS (i.e., during the period of motor disruption), and after a 30-min break. We found that the sensitivity for between-category pairs was reduced during the disruption of the lip representation. In contrast, disruption of the hand representation temporarily reduced response bias. This double dissociation indicates that the hand motor cortex contributes to response bias during demanding discrimination tasks, whereas the articulatory motor cortex contributes to perception of speech sounds.

Multifunctional Setup for Studying Human Motor Control Using Transcranial Magnetic Stimulation, Electromyography, Motion Capture, and Virtual Reality

The study of neuromuscular control of movement in humans is accomplished with numerous technologies. Non-invasive methods for investigating neuromuscular function include transcranial magnetic stimulation, electromyography, and three-dimensional motion capture. The advent of readily available and cost-effective virtual reality solutions has expanded the capabilities of researchers in recreating "real-world" environments and movements in a laboratory setting. Naturalistic movement analysis will not only garner a greater understanding of motor control in healthy individuals, but also permit the design of experiments and rehabilitation strategies that target specific motor impairments (e.g. stroke). The combined use of these tools will lead to increasingly deeper understanding of neural mechanisms of motor control. A key requirement when combining these data acquisition systems is fine temporal correspondence between the various data streams. This protocol describes a multifunctional system's overall connectivity, intersystem signaling, and the temporal synchronization of recorded data. Synchronization of the component systems is primarily accomplished through the use of a customizable circuit, readily made with off the shelf components and minimal electronics assembly skills.

Talking hands: tongue motor excitability during observation of hand gestures associated with words

Perception of speech and gestures engage common brain areas. Neural regions involved in speech perception overlap with those involved in speech production in an articulator-specific manner. Yet, it is unclear whether motor cortex also has a role in processing communicative actions like gesture and sign language. We asked whether the mere observation of hand gestures, paired and not paired with words, may result in changes in the excitability of the hand and tongue areas of motor cortex. Using single-pulse transcranial magnetic stimulation (TMS), we measured the motor excitability in tongue and hand areas of left primary motor cortex, while participants viewed video sequences of bimanual hand movements associated or not-associated with nouns. We found higher motor excitability in the tongue area during the presentation of meaningful gestures (noun-associated) as opposed to meaningless ones, while the excitability of hand motor area was not differentially affected by gesture observation. Our results let us argue that the observation of gestures associated with a word results in activation of articulatory motor network accompanying speech production.

Discrimination of speech and non-speech sounds following theta-burst stimulation of the motor cortex

Perceiving speech engages parts of the motor system involved in speech production. The role of the motor cortex in speech perception has been demonstrated using low-frequency repetitive transcranial magnetic stimulation (rTMS) to suppress motor excitability in the lip representation and disrupt discrimination of lip-articulated speech sounds (Möttönen and Watkins, 2009). Another form of rTMS, continuous theta-burst stimulation (cTBS), can produce longer-lasting disruptive effects following a brief train of stimulation. We investigated the effects of cTBS on motor excitability and discrimination of speech and non-speech sounds. cTBS was applied for 40 s over either the hand or the lip representation of motor cortex. Motor-evoked potentials recorded from the lip and hand muscles in response to single pulses of TMS revealed no measurable change in motor excitability due to cTBS. This failure to replicate previous findings may reflect the unreliability of measurements of motor excitability related to inter-individual variability. We also measured the effects of cTBS on a listener's ability to discriminate: (1) lip-articulated speech sounds from sounds not articulated by the lips ("ba" vs. "da"); (2) two speech sounds not articulated by the lips ("ga" vs. "da"); and (3) non-speech sounds produced by the hands ("claps" vs. "clicks"). Discrimination of lip-articulated speech sounds was impaired between 20 and 35 min after cTBS over the lip motor representation. Specifically, discrimination of across-category ba-da sounds presented with an 800-ms inter-stimulus interval was reduced to chance level performance. This effect was absent for speech sounds that do not require the lips for articulation and non-speech sounds. Stimulation over the hand motor representation did not affect discrimination of speech or non-speech sounds. These findings show that stimulation of the lip motor representation disrupts discrimination of speech sounds in an articulatory feature-specific way.