Speech Motor System Research Articles

How much is a word? Predicting ease of articulation planning from apraxic speech error patterns

BackgroundAccording to intuitive concepts, ‘ease of articulation’ is influenced by factors like word length or the presence of consonant clusters in an utterance. Imaging studies of speech motor control use these factors to systematically tax the speech motor system. Evidence from apraxia of speech, a disorder supposed to result from speech motor planning impairment after lesions to speech motor centers in the left hemisphere, supports the relevance of these and other factors in disordered speech planning and the genesis of apraxic speech errors. Yet, there is no unified account of the structural properties rendering a word easy or difficult to pronounce. AimTo model the motor planning demands of word articulation by a nonlinear regression model trained to predict the likelihood of accurate word production in apraxia of speech. MethodWe used a tree-structure model in which vocal tract gestures are embedded in hierarchically nested prosodic domains to derive a recursive set of terms for the computation of the likelihood of accurate word production. The model was trained with accuracy data from a set of 136 words averaged over 66 samples from apraxic speakers. In a second step, the model coefficients were used to predict a test dataset of accuracy values for 96 new words, averaged over 120 samples produced by a different group of apraxic speakers. ResultsAccurate modeling of the first dataset was achieved in the training study (R2adj = .71). In the cross-validation, the test dataset was predicted with a high accuracy as well (R2adj = .67). The model shape, as reflected by the coefficient estimates, was consistent with current phonetic theories and with clinical evidence. In accordance with phonetic and psycholinguistic work, a strong influence of word stress on articulation errors was found. ConclusionsThe proposed model provides a unified and transparent account of the motor planning requirements of word articulation.

Recruitment of the speech motor system in adverse listening conditions

Background noise is detrimental to speech comprehension. The decline-compensation hypothesis posits that deficits in sensory processing regions caused by background noise can be counteracted by compensatory recruitment of more general cognitive areas. We are exploring the role of the speech motor system as a compensatory mechanism during impoverished sensory representations in the auditory cortices. Prior studies using functional magnetic resonance imaging (fMRI) have revealed an increase in prefrontal activity when peripheral and central auditory systems cannot effectively process speech sounds. Our research using event-related fMRI revealed a negative correlation between brain activation and perceptual accuracy in speech motor regions (i.e., left ventral premotor cortex (PMv) and Broca’s area), suggesting a compensatory recruitment of the motor system during speech perception in noise. Moreover, multi-voxel pattern analysis revealed effective phoneme categorization in the PMv and Broca’s area, even in adverse listening conditions. This is in sharp contrast with phoneme discriminability in auditory cortices and left posterior superior temporal gyrus, which showed reliable phoneme classification only when the noise was extremely weak. Better discriminative activity in the speech motor system may compensate for the loss of specificity in the auditory system by forward sensorimotor mapping during adverse listening conditions.

The effect of Parkinson disease on voice onset time: Temporal differences in voicing contrast

Parkinson disease (PD) affects the basal ganglia, which is involved with the selection, sequencing, and implementation of movement. Investigations suggest an extension of deficits to the speech motor and linguistic systems. Previous studies examining voice onset time (VOT) suggest that VOTs are neither systematically delayed nor systematically advanced in PD as compared to controls. The VOTs for voiced and voiceless stops are expected to differ for both individuals with PD and controls. However, inefficiencies in the sequencing and implementation of the voicing gesture may result in a smaller difference between VOT values of voiced and voiceless cognates. The current study examined VOT in individuals with PD and controls. Participants produced a corpus of stimuli to evaluate VOT using the carrier phrase “CVp again.” The four corner vowels and all stop consonants were used. Speech was recorded and VOT values were manually measured for every utterance. Overall VOTs were significantly shorter for voiceless stops across all speakers. However, the average difference between the voice and voiceless cognates produced by the PD group was significantly smaller than the control group. These data suggest that the PD group exhibited less temporal distinction between voiced and voiceless stops.

A causal test of the motor theory of speech perception: a case of impaired speech production and spared speech perception

The debate about the causal role of the motor system in speech perception has been reignited by demonstrations that motor processes are engaged during the processing of speech sounds. Here, we evaluate which aspects of auditory speech processing are affected, and which are not, in a stroke patient with dysfunction of the speech motor system. We found that the patient showed a normal phonemic categorical boundary when discriminating two non-words that differ by a minimal pair (e.g., ADA–AGA). However, using the same stimuli, the patient was unable to identify or label the non-word stimuli (using a button-press response). A control task showed that he could identify speech sounds by speaker gender, ruling out a general labelling impairment. These data suggest that while the motor system is not causally involved in perception of the speech signal, it may be used when other cues (e.g., meaning, context) are not available.

Control and prediction components of movement planning in stuttering versus nonstuttering adults.

Stuttering individuals show speech and nonspeech sensorimotor deficiencies. To perform accurate movements, the sensorimotor system needs to generate appropriate control signals and correctly predict their sensory consequences. Using a reaching task, we examined the integrity of these control and prediction components separately for movements unrelated to the speech motor system. Nine stuttering and 9 nonstuttering adults made fast reaching movements to visual targets while sliding an object under the index finger. To quantify control, we determined initial direction error and end point error. To quantify prediction, we calculated the correlation between vertical and horizontal forces applied to the object-an index of how well vertical force (preventing slip) anticipated direction-dependent variations in horizontal force (moving the object). Directional and end point error were significantly larger for the stuttering group. Both groups performed similarly in scaling vertical force with horizontal force. The stuttering group's reduced reaching accuracy suggests limitations in generating control signals for voluntary movements, even for nonorofacial effectors. Typical scaling of vertical force with horizontal force suggests an intact ability to predict the consequences of planned control signals. Stuttering may be associated with generalized deficiencies in planning control signals rather than predicting the consequences of those signals.

Stuttering, induced fluency, and natural fluency: A hierarchical series of activation likelihood estimation meta-analyses

Developmental stuttering is a speech disorder most likely due to a heritable form of developmental dysmyelination impairing the function of the speech-motor system. Speech-induced brain-activation patterns in persons who stutter (PWS) are anomalous in various ways; the consistency of these aberrant patterns is a matter of ongoing debate. Here, we present a hierarchical series of coordinate-based meta-analyses addressing this issue. Two tiers of meta-analyses were performed on a 17-paper dataset (202 PWS; 167 fluent controls). Four large-scale (top-tier) meta-analyses were performed, two for each subject group (PWS and controls). These analyses robustly confirmed the regional effects previously postulated as “neural signatures of stuttering” (Brown, Ingham, Ingham, Laird, & Fox, 2005) and extended this designation to additional regions. Two smaller-scale (lower-tier) meta-analyses refined the interpretation of the large-scale analyses: (1) a between-group contrast targeting differences between PWS and controls (stuttering trait); and (2) a within-group contrast (PWS only) of stuttering with induced fluency (stuttering state).

Rewarding Research

Rewarding Research

The Effect of Deep Brain Stimulation on the Speech Motor System

Chronic deep brain stimulation of the nucleus ventralis intermedius is an effective treatment for individuals with medication-resistant essential tremor. However, these individuals report that stimulation has a deleterious effect on their speech. The present study investigates one important factor leading to these effects: the coordination of oral and glottal articulation. Sixteen native-speaking German adults with essential tremor, between 26 and 86 years old, with and without chronic deep brain stimulation of the nucleus ventralis intermedius and 12 healthy, age-matched subjects were recorded performing a fast syllable repetition task (/papapa/, /tatata/, /kakaka/). Syllable duration and voicing-to-syllable ratio as well as parameters related directly to consonant production, voicing during constriction, and frication during constriction were measured. Voicing during constriction was greater in subjects with essential tremor than in controls, indicating a perseveration of voicing into the voiceless consonant. Stimulation led to fewer voiceless intervals (voicing-to-syllable ratio), indicating a reduced degree of glottal abduction during the entire syllable cycle. Stimulation also induced incomplete oral closures (frication during constriction), indicating imprecise oral articulation. The detrimental effect of stimulation on the speech motor system can be quantified using acoustic measures at the subsyllabic level.

Plasticity in the Human Speech Motor System Drives Changes in Speech Perception

Recent studies of human speech motor learning suggest that learning is accompanied by changes in auditory perception. But what drives the perceptual change? Is it a consequence of changes in the motor system? Or is it a result of sensory inflow during learning? Here, subjects participated in a speech motor-learning task involving adaptation to altered auditory feedback and they were subsequently tested for perceptual change. In two separate experiments, involving two different auditory perceptual continua, we show that changes in the speech motor system that accompany learning drive changes in auditory speech perception. Specifically, we obtained changes in speech perception when adaptation to altered auditory feedback led to speech production that fell into the phonetic range of the speech perceptual tests. However, a similar change in perception was not observed when the auditory feedback that subjects' received during learning fell into the phonetic range of the perceptual tests. This indicates that the central motor outflow associated with vocal sensorimotor adaptation drives changes to the perceptual classification of speech sounds.

Articulatory–acoustic vowel space: Application to clear speech in individuals with Parkinson's disease

Background/purposeIndividuals with Parkinson disease (PD) often exhibit decreased range of movement secondary to the disease process, which has been shown to affect articulatory movements. A number of investigations have failed to find statistically significant differences between control and disordered groups, and between speaking conditions, using traditional vowel space area measures. The purpose of the current investigation was to evaluate both between-group (PD versus control) and within-group (habitual versus clear) differences in articulatory function using a novel vowel space measure, the articulatory–acoustic vowel space (AAVS). MethodsThe novel AAVS is calculated from continuously sampled formant trajectories of connected speech. In the current study, habitual and clear speech samples from twelve individuals with PD along with habitual control speech samples from ten neurologically healthy adults were collected and acoustically analyzed. In addition, a group of listeners completed perceptual rating of speech clarity for all samples. ResultsIndividuals with PD were perceived to exhibit decreased speech clarity compared to controls. Similarly, the novel AAVS measure was significantly lower in individuals with PD. In addition, the AAVS measure significantly tracked changes between the habitual and clear conditions that were confirmed by perceptual ratings. ConclusionsIn the current study, the novel AAVS measure is shown to be sensitive to disease-related group differences and within-person changes in articulatory function of individuals with PD. Additionally, these data confirm that individuals with PD can modulate the speech motor system to increase articulatory range of motion and speech clarity when given a simple prompt. Learning outcomesThe reader will be able to (i) describe articulatory behavior observed in the speech of individuals with Parkinson disease; (ii) describe traditional measures of vowel space area and how they relate to articulation; (iii) describe a novel measure of vowel space, the articulatory–acoustic vowel space and its relationship to articulation and the perception of speech clarity.

Noise differentially impacts phoneme representations in the auditory and speech motor systems.

Although it is well accepted that the speech motor system (SMS) is activated during speech perception, the functional role of this activation remains unclear. Here we test the hypothesis that the redundant motor activation contributes to categorical speech perception under adverse listening conditions. In this functional magnetic resonance imaging study, participants identified one of four phoneme tokens (/ba/, /ma/, /da/, or /ta/) under one of six signal-to-noise ratio (SNR) levels (-12, -9, -6, -2, 8 dB, and no noise). Univariate and multivariate pattern analyses were used to determine the role of the SMS during perception of noise-impoverished phonemes. Results revealed a negative correlation between neural activity and perceptual accuracy in the left ventral premotor cortex and Broca's area. More importantly, multivoxel patterns of activity in the left ventral premotor cortex and Broca's area exhibited effective phoneme categorization when SNR ≥ -6 dB. This is in sharp contrast with phoneme discriminability in bilateral auditory cortices and sensorimotor interface areas (e.g., left posterior superior temporal gyrus), which was reliable only when the noise was extremely weak (SNR > 8 dB). Our findings provide strong neuroimaging evidence for a greater robustness of the SMS than auditory regions for categorical speech perception in noise. Under adverse listening conditions, better discriminative activity in the SMS may compensate for loss of specificity in the auditory system via sensorimotor integration.

Speaking rate effects on articulatory pattern consistency in talkers with mild ALS

This study investigated speaking rate effects on articulatory pattern consistency in talkers with mild amyotrophic lateral sclerosis (ALS) to better understand speech rate declines during the early stages of speech deterioration. Eight talkers with mild ALS and 11 controls repeated a sentence at their typical rate, an accelerated rate, and a reduced rate. Lip and jaw movements were captured using a 3-D motion capture system. Results showed that talkers with ALS produced more consistent articulatory patterns during typical speech than did controls. Further, rate reduction resulted in diminished pattern consistency in both groups. Fast speech also elicited less consistent articulatory patterns in talkers with ALS. Controls, by contrast, tended to produce more consistent patterns during fast speech. Relatively inconsistent patterns during fast speech suggest that ALS may negatively affect articulatory control when the speech motor system operates near its performance limit. Relatively consistent patterns during typical speech indicate a successful adaption to disease-related articulatory deficits. Rate reduction does not appear to benefit articulatory stability during early stages of speech decline.

Towards the computation of a variability index in orofacial muscle activity patterns during speech production using performance verification studies

In previous work (Wohlert and Smith, 2002), it was determined that variability in children's speech production is reflected in upper lip muscle activity using electromyographic (EMG) data across repetitions of a phrase. Later studies (MacPherson and Smith, 2013) showed that a lip aperture variability index, which represents the difference in upper lip displacement and lower lip displacement, can also be used as a reliable variability measure from EMG data to determine the effects of multiple repetitions of the same utterance on speech motor production. This study is an extension of previous work and examines the orofacial muscle activity patterns (i.e., EMG data) during speech production in efforts to quantify EMG variability. This information can yield significant insights into how well the speech motor system is functioning in different groups of speakers (e.g., healthy young adults vs. healthy older adults). Initially for this study, kinematic measures of articulatory variability in healthy young adults and healthy older adults were verified using previously developed customized, interactive Matlab programming. The analysis provided the platform for studying the quantification of variability in EMG data. The results of these performance verification analyses are described in this paper.

Spatiotemporal coupling between speech and manual motor actions

Much evidence has been found for pervasive links between the manual and speech motor systems, including evidence from infant development, deictic pointing, and repetitive tapping and speaking tasks. We expand on the last of these paradigms to look at intra- and cross-modal effects of emphatic stress, as well as the effects of coordination in the absence of explicit rhythm. In this study, subjects repeatedly tapped their finger and synchronously repeated a single spoken syllable. On each trial, subjects placed an emphatic stress on one finger tap or one spoken syllable. Results show that both movement duration and magnitude are affected by emphatic stress regardless of whether that stress is in the same domain (e.g., effects on the oral articulators when a spoken repetition is stressed) or across domains (e.g., effects on the oral articulators when a tap is stressed). Though the size of the effects differs between intra-and cross-domain emphases, the implementation of stress affects both motor domains, indicating a tight connection. This close coupling is seen even in the absence of stress, though it is highlighted under stress. The results of this study support the idea that implementation of prosody is not domain-specific but relies on general aspects of the motor system.

Effective cerebral connectivity during silent speech reading revealed by functional magnetic resonance imaging.

Seeing the articulatory gestures of the speaker (“speech reading”) enhances speech perception especially in noisy conditions. Recent neuroimaging studies tentatively suggest that speech reading activates speech motor system, which then influences superior-posterior temporal lobe auditory areas via an efference copy. Here, nineteen healthy volunteers were presented with silent videoclips of a person articulating Finnish vowels /a/, /i/ (non-targets), and /o/ (targets) during event-related functional magnetic resonance imaging (fMRI). Speech reading significantly activated visual cortex, posterior fusiform gyrus (pFG), posterior superior temporal gyrus and sulcus (pSTG/S), and the speech motor areas, including premotor cortex, parts of the inferior (IFG) and middle (MFG) frontal gyri extending into frontal polar (FP) structures, somatosensory areas, and supramarginal gyrus (SMG). Structural equation modelling (SEM) of these data suggested that information flows first from extrastriate visual cortex to pFS, and from there, in parallel, to pSTG/S and MFG/FP. From pSTG/S information flow continues to IFG or SMG and eventually somatosensory areas. Feedback connectivity was estimated to run from MFG/FP to IFG, and pSTG/S. The direct functional connection from pFG to MFG/FP and feedback connection from MFG/FP to pSTG/S and IFG support the hypothesis of prefrontal speech motor areas influencing auditory speech processing in pSTG/S via an efference copy.

Auditory plasticity and sensorimotor learning in speech production

Numerous studies have shown the speech motor system to be highly flexible and responsive to changes in sensory input, revealing a central role for both auditory and somatosensory feedback in the acquisition and maintenance of speech motor control. Consistent with these studies, models of speech production have highlighted the role of accurate, stable sensory representations that serve, in part, as the goals of speech movements. A separate (and considerable) body of work has demonstrated that auditory-sensory representations of speech sounds are not perfectly stable, but rather exhibit rapid adaptation to changing input conditions in both children and adults. The plasticity of auditory representations has important implications for the control of speech production, both in early speech motor development and in the sensory-based maintenance of speech accuracy that characterizes adult speech motor control. In this talk, I will describe a series of studies that explore the link between sensory and motor plasticity in the speech motor system. The studies combine the paradigm of sensorimotor adaptation (altering auditory feedback during speech production) with measures and manipulations of auditory-perceptual representations of speech sounds. The results reveal not only that auditory speech targets are flexible under conditions of altered auditory feedback, but that changes in sensory representations can have a direct impact on speech motor learning and performance.

Silent lipreading and covert speech production suppress processing of non-linguistic sounds in auditory cortex

Previous studies have suggested that speech motor system mediates suppression by silent lipreading of electromagnetic auditory cortex responses to pure tones at about 100 ms from sound onset. We used sparse sampling functional magnetic resonance imaging (fMRI) at 3 Tesla to map auditory-cortex foci of suppressant effects during silent lipreading and covert self-production. Streams of video clips were presented simultaneously with 1/3 octave noise bursts centered at 250 Hz (low frequency, LF) or 2000 Hz (mid-frequency, MF), or during no auditory stimulation. In different conditions, the subjects were a) to press a button whenever they lipread the face articulate the same consecutive Finnish vowels /a/, /i/, /o/, and /y/, b) covertly selfproducing vowels while viewing still face image, or c) to press a button whenever a circle pictured on top of the lips expanded into oval shape of the same orientation twice in a row. The regions of interest (ROIs) within the superior temporal lobes of each hemisphere were defined by contrasting MF and LF stimulation against silence. Contrasting the nonlinguistic (i.e., expanding circle) vs. linguistic (i.e., lipreading and covert self-production) conditions within these ROIs showed significant suppression of hemodynamic activity to MF sounds in the linguistic condition in left hemisphere first transverse sulcus (FTS) and right hemisphere superior temporal gyrus (STG) lateral to Heschl's sulcus (HS). These findings suggest that the speech motor system mediates suppression of auditory-cortex processing of non-linguistic sounds during silent lipreading and covert self-production in left hemisphere FST and right hemisphere STG lateral to HS.

Weak responses to auditory feedback perturbation during articulation in persons who stutter: evidence for abnormal auditory-motor transformation.

Previous empirical observations have led researchers to propose that auditory feedback (the auditory perception of self-produced sounds when speaking) functions abnormally in the speech motor systems of persons who stutter (PWS). Researchers have theorized that an important neural basis of stuttering is the aberrant integration of auditory information into incipient speech motor commands. Because of the circumstantial support for these hypotheses and the differences and contradictions between them, there is a need for carefully designed experiments that directly examine auditory-motor integration during speech production in PWS. In the current study, we used real-time manipulation of auditory feedback to directly investigate whether the speech motor system of PWS utilizes auditory feedback abnormally during articulation and to characterize potential deficits of this auditory-motor integration. Twenty-one PWS and 18 fluent control participants were recruited. Using a short-latency formant-perturbation system, we examined participants’ compensatory responses to unanticipated perturbation of auditory feedback of the first formant frequency during the production of the monophthong [ε]. The PWS showed compensatory responses that were qualitatively similar to the controls’ and had close-to-normal latencies (∼150 ms), but the magnitudes of their responses were substantially and significantly smaller than those of the control participants (by 47% on average, p<0.05). Measurements of auditory acuity indicate that the weaker-than-normal compensatory responses in PWS were not attributable to a deficit in low-level auditory processing. These findings are consistent with the hypothesis that stuttering is associated with functional defects in the inverse models responsible for the transformation from the domain of auditory targets and auditory error information into the domain of speech motor commands.

Do principles of motor learning enhance retention and transfer of speech skills? A systematic review

Background: It is well documented in limb motor research that providing the optimal practice and feedback conditions can have positive outcomes for the learning of new movements. However, it remains unclear if the training conditions used for limb movements can be directly applied to the speech motor system of healthy adults and individuals with acquired motor speech disorders. Collectively these practice and feedback conditions are known as the principles of motor learning (PML), and they have recently been applied to the rehabilitation of motor speech disorders with promising results. Aims: The purpose of this systematic review is to identify which PML have been examined in the speech motor learning literature, to determine the effectiveness of these principles, and to ascertain future lines of research. Methods & Procedures: A systematic search of the literature was completed that involved the combination of a primary search term with a secondary search term. All articles were independently reviewed and scored by the first two authors. To guide the selection process strict inclusion and exclusion criteria were implemented. Additionally, authors used a 15-category evidence-rating system to judge the overall quality of each study. After the study was scored, points were totalled into an overall quality rating of high, intermediate, or low with respect to methodological rigour and interpretability. Outcomes & Results: Seven articles met inclusion criteria, including three randomised controlled trials and four single-participant designs. Five of the articles focused on motor speech disorders, including investigations of apraxia of speech (four studies) and hypokinetic dysarthria from Parkinson's disease (one study), while two studies focused on speech motor performance in healthy adults. Five of the articles were judged to be of high quality while two were judged to be of intermediate quality. Conclusions: Although limited, the current level of evidence for the application of the PML to speech motor learning in both healthy adults and individuals with motor speech disorders is promising and continued investigation is warranted.

LSVT LOUD and LSVT BIG: Behavioral Treatment Programs for Speech and Body Movement in Parkinson Disease.

Recent advances in neuroscience have suggested that exercise-based behavioral treatments may improve function and possibly slow progression of motor symptoms in individuals with Parkinson disease (PD). The LSVT (Lee Silverman Voice Treatment) Programs for individuals with PD have been developed and researched over the past 20 years beginning with a focus on the speech motor system (LSVT LOUD) and more recently have been extended to address limb motor systems (LSVT BIG). The unique aspects of the LSVT Programs include the combination of (a) an exclusive target on increasing amplitude (loudness in the speech motor system; bigger movements in the limb motor system), (b) a focus on sensory recalibration to help patients recognize that movements with increased amplitude are within normal limits, even if they feel “too loud” or “too big,” and (c) training self-cueing and attention to action to facilitate long-term maintenance of treatment outcomes. In addition, the intensive mode of delivery is consistent with principles that drive activity-dependent neuroplasticity and motor learning. The purpose of this paper is to provide an integrative discussion of the LSVT Programs including the rationale for their fundamentals, a summary of efficacy data, and a discussion of limitations and future directions for research.