Motor Speech Impairments

C H A P T E R

78 Motor Speech Impairments Wolfram Ziegler and Anja Staiger Clinical Neuropsychology Research Group, Clinic for Neuropsychology, City Hospital, Munich, Germany

78.1 INTRODUCTION

78.2 MOTOR IMPAIRMENTS WITHIN A NEUROLOGICAL FRAMEWORK

According to conventional clinical taxonomies for neurological speech and language disorders, the term motor speech impairment comprises the different dysarthria syndromes as well as the syndrome of apraxia of speech (AOS). Although the dysarthria syndromes are considered to result from pathologies afflicting the control and execution of speech movements, apraxia of speech is usually ascribed to a dysfunction of speech motor planning or programming functions (Duffy, 2013). Historically, many neurolinguistic theories have explicitly or implicitly embraced a fundamental divide between the biological foundations of language and those of motor speech (and auditory processing). Aphasiologists have willingly adopted the langue parole distinction made by de Saussure and its continuance in structuralist and early generative phonology, in which the motor aspects of speaking (and the auditory aspects of understanding) are almost completely excluded from the arena of language biology. From this standard perspective, the dysarthrias and AOS have been neglected because they are viewed as disorders of a physical organ whose relationship to language is incidental or external rather than structurally or functionally linked. In this chapter, we regard motor speech disorders from three vantage points. First, we describe them as syndromes resulting from the recognized neuropathologies of body movement disorders, more or less following the standard view of speech as a motor function that is sealed from its overarching linguistic framework. Second, we discuss how the speech motor system is specialized to serve its linguisticcommunicative goal. Third, we expand on the sensorimotor aspects of speech motor impairments with the aim of illuminating the different neural stages during which auditory, somatosensory, and motor information is integrated.

Neurobiology of Language. DOI: http://dx.doi.org/10.1016/B978-0-12-407794-2.00078-X

Since Darley’s seminal work, the taxonomy of neurogenic motor speech impairments largely mirrors the taxonomy of (body) movement disorders, with the dysarthric syndromes corresponding to the paretic (flaccid, spastic), ataxic, akinetic, and dyskinetic motor syndromes. AOS, in this terminology, is considered to correspond with the syndrome of limb apraxia (Darley, Aronson, & Brown, 1975). In this section we describe the major CNS neuropathologies leading to speech impairment, focusing on the question of how a pathomechanism described for disorders of body movements may translate into speech motor mechanisms (for a fully comprehensive survey and detailed descriptions of clinical symptoms, see Duffy, 2013).

78.2.1 Spastic Paresis Lesions on areas representing the speech muscles in the ventral part of the Rolandic motor cortex and on the corticobulbar motor pathways cause a syndrome characterized by a paresis of the musculature involved in speaking. This dysarthria type, termed spastic dysarthria, may arise from lesions either at the motor cortex level or along the descending motor neuron fiber tracts to the pontine and medullary motor nuclei. Such lesions can be caused, for example, by infarctions or by disseminated MS plaques, by traumatic brain injuries, by congenital or very early brain damage, as in cerebral palsy (CP), or by progressive disorders affecting the upper motor neuron pathways, such as progressive supranuclear palsy (PSP) or motor neuron disease (MND). Because all bulbar motor nuclei, except the facial nucleus, receive considerable bilateral motor cortical input, lesions restricted to one hemisphere often

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lead to only mild and transient speech impairment (Muellbacher, Artner, & Mamoli, 1999), whereas lesions affecting the upper motor neuron system of both hemispheres, such as in the Foix Chavany Marie syndrome (with infarctions in the left and right motor cortices) or after bilateral brainstem stroke or traumatic brain injury, may lead to a persisting syndrome including, among others, severe dysarthria and dysphagia (Duffy, 2013). Although the presence of spasticity, as defined in the limb muscles, cannot easily be verified in the lingual, pharyngeal, or laryngeal musculature, the general understanding is that lesions to the corticobulbar motor system cause a spastic speech syndrome characterized by muscle weakness and loss of fine motor skill (as a consequence of lesions to the monosynaptic fibers of the upper motor neuron system) in combination with excessive muscle tone (as a consequence of lesions to indirect fibers targeting the motor nuclei via multiple “extrapyramidal” synapses; Duffy, 2013). The combination of upper motor neuron weakness and spasticity is considered to cause reduction of respiratory support, slowing of articulator movements, imprecise consonant articulation, or, when the velopharynx is affected, hypernasality. Increased muscle tone can be visible in the lower face muscles or, via endoscopic inspection, in the larynx. Hypertonicity may lead to a strained or strangled voice because of glottal hyperadduction or increased tension in the hypopharynx.

78.2.2 Ataxia Dysarthria may also arise when cerebellar contributions to speech motor control are compromised due to lesions of either the cerebellum itself or its efferent or afferent projections. Such lesions may result, for example, from cerebellar infarctions, multiple sclerosis, or hereditary ataxic disorders (e.g., Friedreich’s ataxia, spino-cerebellar ataxias). Conflicting theories exist regarding the lateralization and particular parts of the cerebellum implicated in ataxic dysarthria (Marie¨n’s contribution in Manto et al., 2012). Recently, two functional subsystems have been hypothesized on the basis of functional imaging data. They include a superior cerebellar circuit (encompassing connections of superior parts of the cerebellar hemispheres with the inferior frontal gyrus, anterior insular cortex, and the supplementary area), mainly involved in preparatory and motor planning aspects of speech production, and an inferior cerebellar circuit (encompassing inferiorcerebellar thalamo-cortical connections), mainly involved in the motor execution aspects of speaking (Ackermann, 2008). On the basis of clinical considerations, a similar distinction between two functional

levels of cerebellar contributions to motor speech has been proposed that distinguishes between a motor planning/programming circuit (mainly involving connections of the right cerebellar hemisphere with left inferior-frontal speech planning centers in the cerebral cortex) and a motor execution circuit involving superior parts of both cerebellar hemispheres (Spencer & Slocomb, 2007). The clinical pattern of ataxic dysarthria may vary considerably across patients. Among the ataxic pathomechanisms, impaired motor timing, sequencing, and movement coordination have been considered preeminent explanations for speech characteristics such as irregular articulatory inaccuracy, slow articulatory rates, prolonged phonemes, inappropriate pitch and loudness variation, voice tremor, or temporally disorganized or paradoxical respiratory movement patterns during speech breathing (Brendel et al., 2013; Duffy, 2013). Other symptoms may result from compensatory mechanisms serving to suppress tremor or dysmetria, such as a strained or strangled voice quality or a regularly paced, scanning speech rhythm. Overall, the pattern of cerebellar speech impairment cannot be easily divided into a group of features reflecting a planning deficit and others reflecting impaired motor execution, as would be predicted by the two-level model of cerebellar speech motor functions mentioned. Cerebellar speech signs fit into the symptom pattern traditionally described as ataxia, with dysmetria, incoordination, and deficient timing as its preeminent pathomechanisms; they differ substantially from the apraxic signs observed after lesions to the anterior perisylvian and subsylvian cortex of the left cerebral hemisphere (Ziegler’s contribution in Marie¨n et al., 2013; Section 78.2.5).

78.2.3 Akinesia The pathomechanism of akinesia in movement disorders has been ascribed to dysfunction at the level of the striato-thalamo-cortical motor circuit, with idiopathic Parkinson’s disease as the prototypic clinical model of the akinetic condition (Jankovic, 2008). Akinesia is considered to encompass a hypokinetic component, mainly characterized by a reduction in the range of rhythmical movements (e.g., gait, breathing) as a result of excessive inhibition of central pattern generators in the brainstem, and a bradykinetic component, characterized by slowness of movements as a result of reduced striato thalamic “energisation” of appropriately selected motor commands at the motor cortical level, through a defect of the striatal motor circuit that facilitates recruitment of cortical motoneurons for an intended movement. Both mechanisms, hypokinesia and bradykinesia, are

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presumably the result of a progressive loss of dopaminergic striatal innervation (Rodriguez-Oroz et al., 2009). It is not clear whether parkinsonian speech impairment originates from exactly the same mechanisms, especially because dysarthria in Parkinson’s disease is much less (if at all) responsive to dopaminergic drugs and deep brain stimulation treatment than akinetic motor impairment of the limbs and trunk (Rodriguez-Oroz et al., 2005). Moreover, symptoms such as a slowing of the serial control of sequential movements involving different body parts, which are considered characteristic of Parkinson’s disease (Rodriguez-Oroz et al., 2009), are not typically present in hypokinetic dysarthria. On the contrary, Parkinson’s patients often demonstrate a normal or even accelerated speaking rate and chains of tightly conjoined and rapidly produced syllables (“short rushes of speech”; Ackermann & Ziegler, 1992). Nonetheless, the overall pattern of speech motor signs in Parkinson’s disease, with its visible undershooting of labial and mandibular movements, its loss of sufficiently distinct consonant and vowel articulations, its hypophonic voice, and its monotonous intonation, is largely compatible with the predicted consequences of an akinetic condition. In addition, Rodriguez-Oroz et al. (2009) suggest that increased responsiveness of the subthalamic nucleus (STN) and internal segment of the globus pallidus (GPi) in Parkinsonism may contribute to a progressive attenuation of sequential movements as in handwriting (micrographia), which might also serve as an explanation of progressive hypophonia and articulatory undershoot and progressive acceleration phenomena occurring across stretches of speech. Reduced spontaneous speech and hypophonia may also be observed in patients with lesions on the medial premotor areas (supplementary motor area [SMA]) and the anterior cingulate gyrus (ACG), for instance, during recovery from akinetic mutism (Krainik et al., 2003). These data fit within a model of a limbic striatal circuit encompassing the ACG and SMA, which may serve as a starting mechanism for speech production and as a gateway for motivational and affective modulation of vocal and articulatory processes (Ackermann, Hage, & Ziegler, 2014; see Section 78.3.1).

78.2.4 Dyskinesia Although akinesia is attributed to excessive striatal inhibition of thalamo-cortical and brainstem mechanisms, opposite mechanisms may lead to a loss of inhibition or an imbalance of thalamo-cortical motor activations as a consequence of reduced STN-GPi activity (Hallett, 2011). These pathophysiological conditions result in hyperkinetic or dystonic motor impairments characterized by abnormal postures,

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uncontrollable movements or muscle spasms, and a loss of selectivity of muscular activation. They are classified into rhythmic (tremor) and arrhythmic variants, with the latter including the dystonias as a form of sustained motor abnormalities and chorea, myoclonus, and tics as rapid forms (Albanese & Jankovic, 2012). Dyskinetic conditions may result from a variety of neuropathologies, including genetic, drug-induced, toxic or metabolic, traumatic, or vascular etiologies. In many cases, especially in the dystonias, the etiology is unknown. Speech can be afflicted by all types of dyskinetic syndromes. Leaving aside the different variants of vocal tremor (e.g., in Parkinson’s disease, cerebellar ataxia, essential tremor), the dyskinetic dysarthria syndromes that have received particular consideration are the choreatic and athetotic forms occurring in Huntington’s disease (Duffy, 2013) and in a subtype of CP, and the focal dystonias of the laryngeal and the oromandibular muscles (spasmodic dysphonia, oromandibular lingual dystonia; Duffy, 2013). Choreatic hyperkinesias of the speech muscles may lead to intermittent disruptions of respiratory activity, uncontrolled vocalizations, intermittent noise productions with the tongue or lips, excessive pitch and loudness variations, overshooting and undershooting of articulatory movements, intermittent hyponasality and hypernasality, and irregular pauses or sound prolongations (Duffy, 2013). Spasmodic dysphonia may affect the adductor and/or abductor muscles of the larynx and lead to a strained strangled or breathy or aphonic voice quality, respectively (Simonyan, Berman, Herscovitch, & Hallett, 2013). Oromandibular lingual dystonia may interfere with speaking secondary to, for example, excessive jaw opening or closing and involuntary tongue protrusion (Ushe & Perlmutter, 2012).

78.2.5 Apraxia of Speech The concept of AOS dates back to Liepmann (1900), who considered the articulation impairment of a patient with Broca’s aphasia (then termed “motor aphasia”) an “apraxia of the language muscles” (“Apraxie der Sprachmuskeln”; Liepmann, 1900, p. 129). Much later, in their neurologically based classification of motor speech impairments, Darley et al. (1975) resurrected the term to describe a speech impairment occurring after infarction of the anterior branch of the left middle cerebral artery whose characteristics were not compatible with any of the neuromotor pathomechanisms of the dysarthrias. Similar to limb apraxia, the original definition of AOS consisted of largely negative designations; the speech symptoms cannot be explained by “slowness, weakness, incoordination, or

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change of tone” of the involved musculature (Darley et al., 1975, p. 251). The problem was interpreted as a consequence of impaired motor programming (Darley et al., 1975, p. 255). A specific feature of limb apraxia is that lesions of the left hemisphere may lead to motor impairment of not only the contralesional (right) but also the ipsilesional (left) limb. In speech this corresponds, in a way, with the observation that AOS is a left hemisphere syndrome and that, unlike (hemi-) paretic dysarthria, it cannot be compensated for by innervations from intact right hemisphere homologues. However, modern theories of apraxia, with their strong focus on failures of motor functions of the upper extremities, such as pantomime or tool use (Goldenberg, 2013), confuse the issue of drawing analogies between speech and limb apraxia. Without any specific reference to the latter, AOS may best be characterized as a loss of the acquired implicit knowledge of how the muscular aerodynamic apparatus of the speech organs is manipulated for the generation of syllables, words, and phrases (Ziegler, Aichert, & Staiger, 2012). Regarding the sites of the lesions responsible for AOS, there remains controversy about the roles of left inferior frontal gyrus (area 44) and left anterior insular cortex (Richardson, Fillmore, Rorden, Lapointe, & Fridriksson, 2012). Greater agreement exists about the implication of left ventral premotor and motor regions in the origin of AOS (Graff-Radford et al., 2014). In the majority of cases, AOS results from left middle cerebral artery stroke, but other etiologies have also been reported. Furthermore, speech abnormalities consistent with AOS have been observed as a primary progressive condition (Josephs et al., 2006). However, the patient groups with alleged primary progressive AOS described so far are probably rather heterogeneous. In particular, patients showing substance loss in the region of the SMA, as described, for instance, by Josephs et al. (2006), may suffer from dysfluencies due to mesiofrontal speech initiation problems (Ziegler, Kilian, & Deger, 1997) rather than from a frontolateral speech motor planning impairment. In modern psycholinguistic terminology, AOS is allocated to the phonetic planning stage of speech production (Ziegler, 2008). Speech errors in AOS differ from dysarthria in that they can be inconsistent and often are only evident at a segmental level rather than spreading over larger parts of an utterance. For instance, excess nasality may occur selectively and locally on single phonemes in AOS, whereas in dysarthric speakers hypernasality, if present, occurs more as a global feature that extends, as a consequence of velar weakness or slowness, almost uniformly across the segments of a word or phrase. This makes apraxic speech errors less predictable than dysarthric

distortions (Staiger, Finger-Berg, Aichert, & Ziegler, 2012). Nonetheless, unlike the phoneme errors observed in many aphasic patients without AOS, the distortions observed in AOS appear motoric because they often lack the quality of well-articulated phonemes through, for example, excess plosive aspirations, audible phoneme transitions, or nasal releases in stop consonants. AOS patients usually are fully aware of their speech problems and, unlike individuals with dysarthria, tend to grope for the correct articulation before they start speaking. They often self-correct their false starts and speech errors, which renders their speech dysfluent and halting.

78.3 MOTOR IMPAIRMENTS FOR SPOKEN LANGUAGE PRODUCTION Speaking differs from other motor functions in several ways. It is more skillful than many other activities within the human motor repertoire. Among the more advanced motor skills, such as playing a musical instrument, it is the only one that every healthy child acquires without specific instruction. Speaking is acquired over more than the first decade of life, is usually exercised daily, and is continuously adapted to the gradual and sometimes abrupt anatomical changes that occur from childhood to old age. Speech movements are tuned to the generation of (speech) sounds through aerodynamic mechanisms and, hence, are conducted within an acoustic rather than a spatial reference frame (Perkell, 2012). Finally, the evolution of the speech motor system is intrinsically tied to the requirements of spoken communication and is entrenched with the linguistic framework of human language and of a speaker’s native language. For these reasons, speaking should be considered a highly specific motor activity that, through mechanisms of practice-related plasticity, shapes a neural basis dedicated to its linguistic and communicative goals (Ziegler & Ackermann, 2013). In the following subsections we discuss evidence from speech disorders that illuminate the particularities of speaking across the different motor activities of the respiratory, laryngeal, and vocal tract muscles.

78.3.1 Speech and Emotional Expression The laryngeal and facial muscles are used not only in speech but also in emotional expressions such as laughter and crying. According to a dual pathway model of acoustic communication developed by Ju¨rgens and Ploog (Ju¨rgens, 2002), the motor activities of these muscles during emotional expression versus propositional speech are tied to different brain

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networks. The motor pathways engaged in emotional and intrinsic vocalizations, as understood from primate vocalization studies, have their origin in mesiofrontal cortex (ACG) and travel through the midbrain periaqueductal gray and adjacent tegmentum to the reticular formation and brainstem motor nuclei (Ackermann et al., 2014). The motor pathway involved in voluntary motor activities of the vocal tract, especially in speaking, takes a separate route encompassing corticobulbar, striatal, and cerebellar systems (Section 78.2). Several clinical observations corroborate the distinct courses of these two motor systems. A striking example in this regard relates to the Foix Chavany Marie syndrome, which results from bilateral lesions to the upper motor neuron system (Section 78.2.1). Patients with this syndrome are severely dysarthric or anarthric because of bilateral upper motor neuron paralysis of the bulbar speech muscles. At the same time, these patients are able to completely adduct their vocal folds and activate their facial musculature during emotionally driven laughter or crying (Mao, Coull, Golper, & Rau, 1989). This socalled automatic voluntary movement dissociation is taken as evidence for the separate courses of emotional and volitional motor pathways for the speech muscles. A less dramatic but equally convincing dissociation may occur after unilateral cerebral lesions that may cause contralateral lower facial paresis during speaking and volitional mouth spreading (“show your teeth”) but symmetric spreading during spontaneous smiles (“volitional facial paresis”) or, conversely, asymmetric smiling but symmetric lip spreading in speech and the “show your teeth” task (“emotional facial paresis”; Hopf, Mu¨ller-Forell, & Hopf, 1992). Although these dissociations relate to distinct actions with either a volitional/linguistic or an emotional content, natural speaking is usually linked to motivational and emotional states. The impact of a speaker’s attitudes, motivations, and emotions on his/her speech movements is reflected in the prosodic modulation of spoken utterances. This interaction implies the existence of a neural interface through which the emotional/intrinsic (“limbic”) vocalization system modulates speech motor pathway activity. In a recent extension of Ju¨rgens’ (2002) dual pathway model, Ackermann et al. (2014) proposed that the basal ganglia provide a platform for the integration of limbic mechanisms of acoustic communication with articulate speech. According to this theory, a cascade of striato-nigro-striatal circuits extending from ventromedial (limbic) to dorsolateral (motor) components of the striatum is the substrate for the limbic-motor integration process. These circuits interconnect two parallel cortico basal ganglia thalamo cortical loops, one conveying motivation-related information via ventromedial dorsolateral pathways and the other

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conveying speech motor information via the corticostriatal motor loop (Ackermann et al., 2014). The hypokinetic dysarthria associated with Parkinson’s disease may illustrate how this integration can fail. That is, the depletion of striatal dopamine leads to a diminished impact of attitudinal, motivational, and emotional states on speech motor control and thereby results in the flattened, monotonous prosody and the hypophonic voice that is characteristic of many people with Parkinson’s disease. This illustrates how evidence from speech motor impairments may contribute to a deeper understanding of the neural organization and the interaction of laryngeal and oral motor activity for speech and for emotional expression.

78.3.2 Speech Versus Volitional Nonspeech Vocal Tract Movements In the automatic voluntary motor dissociations described in the preceding section, speech was subsumed among a broader class of willed motor actions involving the vocal tract, including movements such as volitional lip spreading, tongue protrusion, and the like. If we assume, as outlined, that the speech motor system co-evolves within the structural framework of linguistic communication and that the domain-specific properties of speech motor control are represented at the neural level, then we would expect that brain lesions may selectively affect or preserve speech relative to nonspeech volitional motor activities. There is substantial evidence that speech and nonspeech motor impairments can be dissociated. Patients with brain lesions may show impairments of the oral (voluntary) phase of swallowing or may have problems imitating tongue protrusion or other labial or lingual displays, but at the same time have normal or almost normal speech. Conversely, some patients may present with marked dysarthric or speech apraxic symptoms but have a relatively preserved ability to perform nonspeech vocal tract movements. Dissociations have been found for a number of nonspeech motor tasks, including chewing and swallowing, movement imitation, strength and endurance tasks, rapid syllable production, or visuomotor tracking (Ziegler, 2003). One of the reasons for these findings is that nonspeech oromotor activities used in research and assessment differ from speech along a number of dimensions, such as timing, strength, and airflow requirements, the degree of interaction between subsystems, the rhythmical entrainment of movements, and the role of acoustic output as a reference frame. The specific requirements of producing intelligible and naturally sounding speech by manipulating the respiratory, laryngeal, and supralaryngeal muscles in a particular way, and the fact that the highly

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adaptive interplay of these muscles in speaking is acquired over time during childhood, call for the engagement of a specialized neural network in adult motor speech. This conclusion receives strong theoretical support from investigations into experience-dependent neuroplasticity (Ostry, Darainy, Mattar, Wong, & Gribble, 2010; Zatorre, Fields, & Johansen-Berg, 2012). For example, neuroimaging studies comparing speech with oral nonspeech movements converge on the observation that motor speech is more lateralized to the left hemisphere and is associated with less neural activation than nonspeech oral motor tasks (Moser et al., 2009; for a more extensive review of these arguments see Ziegler and Ackermann, 2013). In summary, the selective and domain-specific nature of motor speech impairments is consistent with highly specialized neural organization of the motor aspects of speaking. This contributes to evidence that the speech motor system constitutes an integral part of the biology of human language.

78.3.3 Language-Specific Phonological Structure Interacts with Speech Motor Impairment From early childhood, speech acquisition is shaped by the phonological structures the child encounters in his/her native language. The maturation of vocal tract, laryngeal, and respiratory functions for speaking separates rather early from nonspeech metabolic motor patterns (Moore, Caulfield, & Green, 2001) and takes a course toward mastering the specific motor requirements of the language’s phoneme repertoire and the phonotactic and prosodic patterns of the child’s ambient language (Astruc, Payne, Post, Vanrell, & Prieto, 2013). As a consequence, the speech motor system in the adult brain is shaped by the properties of the speaker’s native language. When the system breaks down after a brain lesion, observed motor speech failures reflect universal and languageparticular aspects of phonological structure (Ziegler & Ackermann, 2013). This is quite obviously the case in AOS. The sound level errors and phonetic distortions of speakers with AOS are sensitive to syllable structure, respect syllable boundaries, and are influenced by language-particular frequency-related properties of syllables and words (Aichert & Ziegler, 2004; Romani & Galluzzi, 2005; Schoor, Aichert, & Ziegler, 2012; Staiger & Ziegler, 2008). A further potential source of influence on apraxic errors is the metrical pattern of the speaker’s language. For example, in German speakers with AOS, more errors are observed on disyllabic nouns with stress on the second (iambic) as compared with the

first syllable (trochaic), which conforms to expectations because trochees are by far more frequent than iambs in German (Aichert, Bu¨chner, & Ziegler, 2011). We examined the influences of phonological structure on speech errors in German speakers with AOS using a nonlinear probabilistic model based on the hierarchical architecture of words, extending from the level of articulatory gestures to the level of metrical feet (Ziegler, 2009). The model was built to estimate the likelihood that a word with a particular syllabic and metrical structure would be produced accurately by an apraxic speaker. It was fitted to a large sample of words for which accuracy data from 120 apraxic productions were available. The findings revealed a motor planning hierarchy that was consistent with phonological models of syllable constituency and metrical structure of German with, for instance, relatively strong bonds between nucleus and coda gestures compared with onset rime gesture combinations or between syllable pairs forming a trochaic foot relative to nontrochaic combinations. Analyses of apraxic speech errors demonstrate that the architecture of speech motor programs conforms to the phonological architecture of words, implying that speech motor control and linguistic structure are mutually interconnected. These findings are at odds with theories postulating a strict dualism of linguistic versus motor functions.

78.4 SENSORY-MOTOR ASPECTS OF SPEECH SOUND PRODUCTION IMPAIRMENT The interaction between sensory and motor processing mechanisms has been a core issue in the understanding of the biology of language ever since the development of the Wernicke Lichtheim model (Lichtheim, 1885). In that model, language production relies on the activation of information from sensory centers in which the sound images (German Klangbilder) of words are stored (Lichtheim, 1885, p. 211). This theory was derived from Wernicke’s observation that lesions to auditory association centers in the left posterior superior temporal lobe not only caused auditory comprehension problems but also was associated with paraphasic language production. Later, Liepmann (1900) developed his influential theory that motor action (of the upper extremities) relies on a posteriorto-anterior stream of information located in the left hemisphere that conveys an ideatory blueprint (German: ideatorischer Entwurf) based primarily on sensory information (Liepmann, 1913, pp. 488 490). In modern accounts of speech production, these ideas are vested in computational models based on auditory goals—not

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of words but rather of phonemes or syllables—that provide an acoustic reference frame for speaking (Guenther, Hampson, & Johnson, 1998). More recently, new techniques enabling online perturbations of somatosensory information during articulation have led to an extension of sensory-motor theories of speech production by including a proprioceptive feedback processing route (Tremblay, Shiller, & Ostry, 2003). Speech production models based on experimental data relating to auditory and somatosensory feedback processing have invoked the concept of internal models that guide motor action (Hickok, 2014; Tourville & Guenther, 2011). Parallel to these developments, a new research focus based on fiber-tracking methods has refined the connectional anatomy related to Liepmann’s model of anterior-posterior information processing streams in language production (Dick & Tremblay, 2012; Rauschecker, 2012). Clinical data from patients with speech sound impairments have not yet had much influence on this research.

78.4.1 Auditory and Somatosensory Feedback and Speech Impairment The role of auditory feedback in the genesis of speech impairment has been addressed in research focused on stuttering (Cai et al., 2012) and the effects on speech in those with hearing loss and cochlear implants (Perkell, 2012). Although this research is clearly relevant to the field, it is not considered further here. Regarding the role of somatosensory afferent information in speech, only scarce clinical data exist because lesions causing a complete extinction of trigeminal somatosensory input are rare and the degree to which such information is still available cannot be assessed reliably enough by clinical methods. Duffy (2013) conjectures the existence of a “sensory dysarthria” syndrome resulting from impaired oral somatosensory processing that may lead to imprecise articulation that could reflect compensations for reduced sensory input through increased range of articulatory movement (e.g., exaggerated jaw movements). Hoole (1987) described a patient who, after a closed head trauma and whiplash injury, had experienced substantial sensory deficits in the oral-facial region as assessed by standard clinical methods. After initial severe dysarthria the patient’s speech recovered quickly, although his sensory deficits remained unresolved. Experimental investigations of isolated vowel articulation after motor speech recovery revealed reduced ability of this patient to compensate for proprioceptive perturbations with a biteblock, especially when auditory feedback was blocked by noise-masking. This result corroborates assumptions that somatosensory

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processing supports adaptive mechanisms in speaking and that auditory and somatosensory processing can partially complement each other in this role (Perkell, 2012). Yet, clinical cases of this kind will always leave the possibility that residual sensory information that evades clinical physiological detection may still provide sufficient afferent information to support normal speech, at least when the lesion is acquired in adulthood.

78.4.2 Cerebellar Sensorimotor Integration Mechanisms in Speech Impairment The cerebellum is classically considered as a site in the brain where sensorimotor integration takes place (Bhanpuri, Okamura, & Bastian, 2013). Ataxic dysarthria after cerebellar lesions might therefore be considered as a clinical model of impaired integration of proprioceptive and motor information in speech. However, most current theories conceptualize cerebellar dysarthria to result from impaired feedforward processing mechanisms (Spencer & Slocomb, 2007; Marie¨n, in Manto et al., 2012). Yet, for instance, the fact that Friedreich’s ataxia is primarily viewed as an afferent ataxic syndrome suggests that the dysarthric impairment observed in these patients may at least partly reflect an impairment of proprioceptive feedback processing in speech (Pandolfo, 2009). The disturbance of sensory feedback mechanisms in ataxic patients becomes more apparent in paraspeech or nonspeech oral motor tasks that involve strong feedback integration capacities, such as sustained vowel production, visuomotor tracking, or rapid syllable repetition. Maintenance of a stable pitch and loudness level in sustained vowels is often disproportionately impaired in ataxic patients who often demonstrate fluctuating pitch or voice tremor during sustained phonation over several seconds, but not necessarily during speaking (Ackermann & Ziegler, 1991; Brendel et al., 2013). This may reflect a failure of correction mechanisms based on reafferent laryngeal proprioceptive or auditory information. Severely impaired adaptive sensorimotor mechanisms of patients with hereditary ataxias were also observed in a visuomotor tracking task requiring control of airflow velocity during expiration to track a ramp signal (Deger, Ziegler, & Wessel, 1999), but there was no correlation between the tracking and the speech impairment. Finally, in several studies patients with cerebellar pathology have had difficulty adapting to the specific demands of a task requiring repetition of a syllable (e.g., puh, tuh, kuh) at maximum speed (Brendel et al., 2013; Ziegler & Wessel, 1996), a task considered to rely strongly on sensory mechanisms for the selection of a

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jaw angle that supports maximally rapid labial or lingual opening and closing movements. The patients had dramatically reduced acceleration ratios of syllable repetition relative to speaking rates (Ziegler, in Marie¨n et al., 2013). Taken together, these results point to vulnerability of patients with cerebellar lesions to motor demands requiring a strong reliance on sensorimotor adaptation mechanisms.

78.4.3 Striatal Mechanisms of Sensorimotor Integration in Speech Impairment The motor functions of the basal ganglia are considered to rely, at least partly, on striatal and pallidal sensory processing mechanisms involved in kinaesthesia or somatosensory discrimination (Maschke, Gomez, Tuite, & Konczak, 2003). Altered sensory processing is thought to contribute to the motor deficits of patients with Parkinson’s and Huntington’s disease (Boecker et al., 1999), and a somatosensory disinhibition mechanism has been proposed to underlie dystonic motor impairment (Frasson et al., 2001). According to a hypothesis advanced by Yin (2014), the basal ganglia control movement velocity through kinaesthetic reafferent input. It is not known if similar sensory mechanisms can also serve as an explanation for the speech motor patterns of patients with basal ganglia dysfunction. Patients with Parkinson’s disease demonstrate consistent proprioceptive deficits in the oral region, but the relationship of these abnormalities to speech impairment is unclear (Schneider, Diamond, & Markham, 1986). In a recent report of two patients with oromandibular dystonia (Møller et al., 2013), evidence was found supporting abnormal sensorimotor integration or somatosensory dysfunction for afferent input from the oral region as an explanation for the motor impairment. Other sensory processing functions of the basal ganglia relevant for speech relate to the self-perception of speech loudness. Ho, Bradshaw, and Iansek (2000) examined the hypothesis that reduced loudness (hypophonia) in Parkinson’s disease is due to impaired motor scaling mechanisms based on patients’ misperception of their own speech loudness. In an “immediate self-perception rating,” Parkinson’s patients overestimated their loudness relative to normal subjects, which the authors interpreted as an exaggeration of self-perceived effort during speaking. Likewise, the patients also overestimated their speech volume in a playback condition, even though they had spoken more quietly than controls. Ho et al. (2000) suggested that their findings provide support for the presence of sensory anomalies in Parkinson’s disease, which may cause inappropriate scaling of loudness.

78.4.4 Sensorimotor Connectivity at the Cortical Level As already discussed, recent functional neuroanatomic accounts of the dorsal stream system connecting posterior superior-temporal cortex with inferiorparietal and posterior-inferior frontal areas constitute a modern version of Liepmann’s idea of a posterior-toanterior stream of information governing motor actions through sensory-based goals (Saur et al., 2008). The cortical target areas of this system are interconnected by a massive fiber bundle, the superior longitudinal fascicle, which constitutes a circuit that includes auditory, somatosensory, and motor association areas. The dorsal pathway of the left (dominant) hemisphere is considered to be involved in higher sensorimotor integration by mapping acoustic speech sounds, and eventually also somatosensory representations, onto their corresponding articulatory actions. The prototype task targeting this system in functional imaging studies is the word repetition task (Saur et al., 2008), but more complex tasks involving phonological transformations of words have also been used (Kellmeyer et al., 2013). Although a simplification, the anterior target area of the left dorsal stream approximately coincides with the lesion site reported for AOS. This could support an inference that the pathomechanism of AOS predominantly reflects damage to feedforward processing components of speech motor control while leaving sensory feedback mechanisms intact. This is compatible with clinical experience and some experimental evidence that patients with AOS have intact monitoring of their speech errors and preserved auditory speech processing. Jacks (2008) performed a biteblock experiment with apraxic speakers to test this feedforward hypothesis and found that AOS participants compensated for the biteblock perturbation in a manner similar to normal speakers. This normal adaptation to proprioceptive perturbation was taken as evidence of intact somatosensory feedback. Historically, and in modern research, the dorsal pathway of the left hemisphere has been associated with aphasic phonological impairment. A longstanding hypothesis is that the phonemic paraphasias of patients with conduction aphasia result from a disconnection of the auditory from the motor representation areas of words (Geschwind, 1965). In conventional aphasiology, a strict boundary is drawn between aphasic phonological impairment as purely abstract-symbolic, and AOS as purely motor by nature. However, one may question whether such a clear-cut dichotomy is tenable, taking the aforementioned dorsal pathway hypothesis into consideration. As said, the dorsal pathway interfaces motor with sensory information (Hickok, 2014; Rauschecker, 2012). This interface can be assumed to

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REFERENCES

act on a representational level, which is sufficiently abstract to make any additional assumptions about symbolic representations dispensable.

78.5 CONCLUSION In this chapter we have characterized the nature of the speech motor system and its integration with sensory processes as a part of human linguistic behavior. Future theories and clinical models of motor speech disorders should consider, to a greater extent, the evidence consistent with such an account.

Acknowledgment We are grateful to Joe Duffy for his valuable comments and suggestions regarding a former version of this manuscript.

References Ackermann, H. (2008). Cerebellar contributions to speech production and speech perception: Psycholinguistic and neurobiological perspectives. Trends in Neurosciences, 31, 265 272. Ackermann, H., Hage, S. R., & Ziegler, W. (2014). Brain mechanisms of acoustic communication in humans and nonhuman primates: An evolutionary perspective. Behavioral and Brain Sciences, 37, 529 604. Ackermann, H., & Ziegler, W. (1991). Cerebellar voice tremor: An acoustic analysis. Journal of Neurology, Neurosurgery, and Psychiatry, 54, 74 76. Ackermann, H., & Ziegler, W. (1992). Articulatory deficits in parkinsonian dysarthria: An acoustic analysis. Journal of Neurology, Neurosurgery, and Psychiatry, 54, 1093 1098. Aichert, I., Bu¨chner, M., & Ziegler, W. (2011). Why is [‘ju:do] easier than [ju’ve:l]? Perceptual and acoustic analyses of word stress in patients with apraxia of speech. Stem-, Spraak- en Taalpathologie, 17, 15. Aichert, I., & Ziegler, W. (2004). Syllable frequency and syllable structure in apraxia of speech. Brain and Language, 88, 148 159. Albanese, A., & Jankovic, J. (2012). Hyperkinetic movement disorders: Differential diagnosis and treatment. Hoboken, NJ: John Wiley & Sons. Astruc, L., Payne, E., Post, B., Vanrell, M., & Prieto, P. (2013). Tonal targets in early child English, Spanish, and Catalan. Language and Speech, 56, 229 253. Bhanpuri, N. H., Okamura, A. M., & Bastian, A. J. (2013). Predictive modeling by the cerebellum improves proprioception. The Journal of Neuroscience, 33, 14301 14306. Boecker, H., Ceballos-Baumann, A., Bartenstein, P., Weindl, A., Siebner, H. R., Fassbender, T., et al. (1999). Sensory processing in Parkinson’s and Huntington’s disease investigations with 3D H215O-PET. Brain, 122, 1651 1665. Brendel, B., Ackermann, H., Berg, D., Lindig, T., Scholderle, T., Schols, L., et al. (2013). Friedreich ataxia: Dysarthria profile and clinical data. Cerebellum, 12, 475 484. Cai, S., Beal, D. S., Ghosh, S. S., Tiede, M. K., Guenther, F. H., & Perkell, J. S. (2012). Weak responses to auditory feedback perturbation during articulation in persons who stutter: Evidence for abnormal auditory-motor transformation. PLoS ONE, 7(7), e41830. Available from: http://dx.doi.org/10.1371/journal.pone.0041830.

993

Darley, F. L., Aronson, A. E., & Brown, J. R. (1975). Motor speech disorders. Philadelphia, PA: W.B. Saunders. Deger, K., Ziegler, W., & Wessel, K. (1999). Airflow tracking in patients with ataxic disorders. Clinical Linguistics and Phonetics, 13, 433 447. Dick, A. S., & Tremblay, P. (2012). Beyond the arcuate fasciculus: Consensus and controversy in the connectional anatomy of language. Brain, 135, 3529 3550. Duffy, J. R. (2013). Motor speech disorders: Substrates, differential diagnosis, and management (3rd ed.). St. Louis, MO: Elsevier Mosby. Frasson, E., Priori, A., Bertolasi, L., Mauguiere, F., Fiaschi, A., & Tinazzi, M. (2001). Somatosensory disinhibition in dystonia. Movement Disorders, 16, 674 682. Geschwind, N. (1965). Disconnexion syndromes in animal and man. Part I. Brain, 88, 237 294. Graff-Radford, J., Jones, D. T., Strand, E. A., Rabinstein, A. A., Duffy, J. R., & Josephs, K. A. (2014). The neuroanatomy of pure apraxia of speech in stroke. Brain and Language, 129, 43 46. Goldenberg, G. (2013). Apraxia. The cognitive side of motor control. Oxford: Oxford University Press, 79. Guenther, F. H., Hampson, M., & Johnson, D. (1998). A theoretical investigation of reference frames for the planning of speech movements. Psychological Review, 105, 611 633. Hallett, M. (2011). Neurophysiology of dystonia: The role of inhibition. Neurobiology of Disease, 42, 177 184. Hickok, G. (2014). Towards an integrated psycholinguistic, neurolinguistic, sensorimotor framework for speech production. Language, Cognition and Neuroscience, 29, 52 59. Ho, A. K., Bradshaw, J. L., & Iansek, R. (2000). Volume perception in parkinsonian speech. Movement Disorders, 15, 1125 1131. Hoole, P. (1987). Bite-block speech in the absence of oral sensibility. In Proceedings of the 11th International Congress of Phonetic Sciences (Vol. 4, pp. 16 19). Tallinn: Academy of Sciences of the Estonian SSR. Hopf, H. C., Mu¨ller-Forell, W., & Hopf, N. J. (1992). Localization of emotional and volitional facial paresis. Neurology, 42, 1918 1923. Jacks, A. (2008). Bite block vowel production in apraxia of speech. Journal of Speech Language and Hearing Research, 51, 898 913. Jankovic, J. (2008). Parkinson’s disease: Clinical features and diagnosis. Journal of Neurology, Neurosurgery and Psychiatry, 79, 368 376. Josephs, K. A., Duffy, J. R., Strand, E. A., Whitwell, J. L., Layton, K. F., Parisi, J. E., et al. (2006). Clinicopathological and imaging correlates of progressive aphasia and apraxia of speech. Brain, 129, 1385 1398. Ju¨rgens, U. (2002). Neural pathways underlying vocal control. Neuroscience and Biobehavioral Reviews, 26, 235 258. Kellmeyer, P., Ziegler, W., Peschke, C., Juliane, E., Schnell, S., Baumgaertner, A., et al. (2013). Fronto-parietal dorsal and ventral pathways in the context of different linguistic manipulations. Brain and Language, 127, 241 250. Krainik, A., Lehericy, S., Duffau, H., Capelle, L., Chainay, H., Cornu, P., et al. (2003). Postoperative speech disorder after medial frontal surgery role of the supplementary motor area. Neurology, 60, 587 594. Lichtheim, L. (1885). Ueber Aphasie. Aus der medicinischen Klinik in Bern. Deutsches Archiv fu¨r klinische Medicin, 36, 204 268. Liepmann, H. (1900). Das Krankheitsbild der Apraxie (“motorischen Asymbolie”) auf Grund eines Falles von einseitiger Apraxie (II). Monatsschrift fu¨r Psychiatrie und Neurologie, VIII, 102 132. Liepmann, H. (1913). Motorische Aphasie und Apraxie. Monatsschrift fu¨r Psychiatrie und Neurologie, XXXIV, 485 494. Manto, M., Bower, J. M., Conforto, A. B., Delgado-Garcı`a, J. M., Farias da Guarda, S. N., Gerwig, M., et al. (2012). Consensus

O. LANGUAGE BREAKDOWN

994

78. MOTOR SPEECH IMPAIRMENTS

paper: Roles of the cerebellum in motor control—The diversity of ideas on cerebellar involvement in movement. Cerebellum, 11, 457 487. Mao, C.-C., Coull, B. M., Golper, L. A. C., & Rau, M. T. (1989). Anterior operculum syndrome. Neurology, 39, 1169 1172. Marie¨n, P., Ackermann, H., Adamaszek, M., Barwood, C. H., Beaton, A., Desmond, J., et al. (2013). Consensus paper: Language and the cerebellum: An ongoing enigma. The Cerebellum, 1 25. Maschke, M., Gomez, C. M., Tuite, P. J., & Konczak, J. (2003). Dysfunction of the basal ganglia, but not the cerebellum, impairs kinaesthesia. Brain, 126, 2312 2322. Moore, C. A., Caulfield, T. J., & Green, J. R. (2001). Relative kinematics of the rib cage and abdomen during speech and nonspeech behaviors of 15-month-old children. Journal of Speech, Language, and Hearing Research, 44, 80 94. Moser, D., Fridriksson, J., Bonilha, L., Healy, E. W., Baylis, G., Baker, J. M., et al. (2009). Neural recruitment for the production of native and novel speech sounds. NeuroImage, 46, 549 557. Muellbacher, W., Artner, C., & Mamoli, B. (1999). The role of the intact hemisphere in recovery of midline muscles after recent monohemispheric stroke. Journal of Neurology, 246, 250 256. Møller, E., Bakke, M., Dalager, T., Werdelin, L. M., Lonsdale, M. N., Højgaard, L., et al. (2013). Somatosensory input and oromandibular dystonia. Clinical Neurology and Neurosurgery, 115, 1141 1143. Ostry, D. J., Darainy, M., Mattar, A. A., Wong, J., & Gribble, P. L. (2010). Somatosensory plasticity and motor learning. The Journal of Neuroscience, 30, 5384 5393. Pandolfo, M. (2009). Friedreich ataxia: The clinical picture. Journal of Neurology, 256(Suppl. 1), 3 8. Perkell, J. S. (2012). Movement goals and feedback and feedforward control mechanisms in speech production. Journal of Neurolinguistics, 25, 382 407. Rauschecker, J. P. (2012). Ventral and dorsal streams in the evolution of speech and language. Frontiers in Evolutionary Neuroscience, 4, 7. Richardson, J. D., Fillmore, P., Rorden, C., Lapointe, L. L., & Fridriksson, J. (2012). Re-establishing Broca’s initial findings. Brain Language, 123, 125 130. Rodriguez-Oroz, M. C., Jahanshahi, M., Krack, P., Litvan, I., Macias, R., Bezard, E., et al. (2009). Initial clinical manifestations of Parkinson’s disease: Features and pathophysiological mechanisms. Lancet Neurology, 8, 1128 1139. Rodriguez-Oroz, M. C., Obeso, J. A., Lang, A. E., Houeto, J. L., Pollak, P., Rehncrona, S., et al. (2005). Bilateral deep brain stimulation in Parkinson’s disease: A multicentre study with 4 years follow-up. Brain, 128, 2240 2249. Romani, C., & Galluzzi, C. (2005). Effects of syllabic complexity in predicting accuracy of repetition and direction of errors in patients with articulatory and phonological difficulties. Cognitive Neuropsychology, 22, 817 850. Saur, D., Kreher, B. W., Schnell, S., Kummerer, D., Kellmeyer, P., Vry, M. S., et al. (2008). Ventral and dorsal pathways for language. Proceedings of the National Academy of Sciences of the United States of America, 105, 18035 18040.

Schneider, J. S., Diamond, S. G., & Markham, C. H. (1986). Deficits in orofacial sensorimotor function in Parkinson’s disease. Annals of Neurology, 19, 275 282. Schoor, A., Aichert, I., & Ziegler, W. (2012). A motor learning perspective on phonetic syllable kinships: How training effects transfer from learned to new syllables in severe apraxia of speech. Aphasiology, 26, 880 894. Simonyan, K., Berman, B. D., Herscovitch, P., & Hallett, M. (2013). Abnormal striatal dopaminergic neurotransmission during rest and task production in spasmodic dysphonia. The Journal of Neuroscience, 33, 14705 14714. Spencer, K. A., & Slocomb, D. L. (2007). The neural basis of ataxic dysarthria. The Cerebellum, 6, 58 65. Staiger, A., Finger-Berg, W., Aichert, I., & Ziegler, W. (2012). Error variability in apraxia of speech: A matter of controversy. Journal of Speech, Language, and Hearing Research, 55, S1544 S1561. Staiger, A., & Ziegler, W. (2008). Syllable frequency and syllable structure in the spontaneous speech production of patients with apraxia of speech. Aphasiology, 22, 1201 1215. Tourville, J. A., & Guenther, F. H. (2011). The DIVA model: A neural theory of speech acquisition and production. Language and Cognitive Processes, 26, 952 981. Tremblay, S., Shiller, D. M., & Ostry, D. J. (2003). Somatosensory basis of speech production. Nature, 423, 866 869. Ushe, M., & Perlmutter, J. S. (2012). Oromandibular and lingual dystonia associated with spinocerebellar ataxia type 8. Movement Disorders, 27, 1741 1742. Yin, H. H. (2014). Action, time and the basal ganglia. Philosophical Transactions of the Royal Society B: Biological Sciences, 369, 20120473. Zatorre, R. J., Fields, R. D., & Johansen-Berg, H. (2012). Plasticity in gray and white: Neuroimaging changes in brain structure during learning. Nature Neuroscience, 15, 528 536. Ziegler, W. (2003). Speech motor control is task-specific. Evidence from dysarthria and apraxia of speech. Aphasiology, 17, 3 36. Ziegler, W. (2008). Apraxia of speech. In G. Goldenberg, & B. Miller (Eds.), Handbook of clinical neurology (pp. 269 285). London: Elsevier. Ziegler, W. (2009). Modelling the architecture of phonetic plans: Evidence from apraxia of speech. Language and Cognitive Processes, 24, 631 661. Ziegler, W., & Ackermann, H. (2013). Neuromotor speech impairment: It’s all in the talking. Folia Phoniatrica et Logopaedica, 65, 55 67. Ziegler, W., Aichert, I., & Staiger, A. (2012). Apraxia of speech: Concepts and controversies. Journal of Speech, Language, and Hearing Research, 55, S1485 S1501. Ziegler, W., Kilian, B., & Deger, K. (1997). The role of the left mesial frontal cortex in fluent speech: Evidence from a case of left supplementary motor area hemorrhage. Neuropsychologia, 35, 1197 1208. Ziegler, W., & Wessel, K. (1996). Speech timing in ataxic disorders: Sentence production and rapid repetitive articulation. Neurology, 47, 208 214.

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