Phonological encoding in apraxia of speech and aphasia

How to cut the pie is no piece of cake: Toward a process-oriented approach to assessment and diagnosis of speech sound disorders

BACKGROUND

'Speech sound disorder' is an umbrella term that encompasses dysarthria, articulation disorders, childhood apraxia of speech and phonological disorders. However, differential diagnosis between these disorders is a persistent challenge in speech pathology, as many diagnostic procedures use symptom clusters instead of identifying an origin of breakdown in the speech and language system.

AIMS

This article reviews typical and disordered speech through the lens of two well-developed models of production-one focused on phonological encoding and one focused on speech motor planning. We illustrate potential breakdown locations within these models that may relate to childhood apraxia of speech and phonological disorders.

MAIN CONTRIBUTION

This paper presents an overview of an approach to conceptualisation of speech sound disorders that is grounded in current models of speech production and emphasises consideration of underlying processes. The paper also sketches a research agenda for the development of valid, reliable and clinically feasible assessment protocols for children with speech sound disorders.

CONCLUSION

The process-oriented approach outlined here is in the early stages of development but holds promise for developing a more detailed and comprehensive understanding of, and assessment protocols for speech sound disorders that go beyond broad diagnostic labels based on error analysis. Directions for future research are discussed.

WHAT THIS PAPER ADDS

What is already known on the subject Speech sound disorders (SSD) are heterogeneous, and there is agreement that some children have a phonological impairment (phonological disorders, PD) whereas others have an impairment of speech motor planning (childhood apraxia of speech, CAS). There is also recognition that speech production involves multiple processes, and several approaches to the assessment and diagnosis of SSD have been proposed. What this paper adds to existing knowledge This paper provides a more detailed conceptualisation of potential impairments in children with SSD that is grounded in current models of speech production and encourages greater consideration of underlying processes. The paper illustrates this approach and provides guidance for further development. One consequence of this perspective is the notion that broad diagnostic category labels (PD, CAS) may each comprise different subtypes or profiles depending on the processes that are affected. What are the potential or actual clinical implications of this work? Although the approach is in the early stages of development and no comprehensive validated set of tasks and measures is available to assess all processes, clinicians may find the conceptualisation of different underlying processes and the notion of potential subtypes within PD and CAS informative when evaluating SSD. In addition, this perspective discourages either/or thinking (PD or CAS) and instead encourages consideration of the possibility that children may have different combinations of impairments at different processing stages.

Psycholinguistic effects, types of impairments and processing levels in word production: Can we reduce confusions?

This commentary highlights three common difficulties faced by the literature that aims to specify models of speech production based on the performance of aphasic speakers, taking as a springboard a recent study by Mailend et al. These include: (1) difficulties with theoretical assumptions which linki psycholinguistic effects unequivocally to one processing level; (2) difficulties using clinical classifications to localize experimental effects; (3) difficulties making theoretical inferences given the controversial nature of the representations that characterize different processing levels. We argue that these three types of difficulties could be ameliorated by studies in which: (1) the level of psycholinguistic effects is demonstrated with converging analyses; (2) clinical classification is not taken as a starting point in studies investigating the nature of an impairment, but, instead, associations between clusters of symptoms are carefully analysed; (3) The nature of processing levels associated with deficits is made clear and results are not over-interpreted as supporting models whose characteristics go beyond an explanation of the results.

Apraxia of speech and the study of speech production impairments: Can we avoid further confusion? Reply to Romani (2021)

We agree with Cristina Romani (CR) about reducing confusion and agree that the issues raised in her commentary are central to the study of apraxia of speech (AOS). However, CR critiques our approach from the perspective of basic cognitive neuropsychology. This is confusing and misleading because, contrary to CR's claim, we did not attempt to inform models of typical speech production. Instead, we relied on such models to study the impairment in the clinical category of AOS (translational cognitive neuropsychology). Thus, the approach along with the underlying assumptions is different. This response aims to clarify these assumptions, broaden the discussion regarding the methodological approach, and address CR's concerns. We argue that our approach is well-suited to meet the goals of our recent studies and is commensurate with the current state of the science of AOS. Ultimately, a plurality of approaches is needed to understand a phenomenon as complex as AOS.

Examining speech motor planning difficulties in apraxia of speech and aphasia via the sequential production of phonetically similar words

This study investigated the underlying nature of apraxia of speech (AOS) by testing two competing hypotheses. The Reduced Buffer Capacity Hypothesis argues that people with AOS can plan speech only one syllable at a time Rogers and Storkel [1999. Planning speech one syllable at a time: The reduced buffer capacity hypothesis in apraxia of speech. Aphasiology, 13(9-11), 793-805. https://doi.org/10.1080/026870399401885]. The Program Retrieval Deficit Hypothesis states that selecting a motor programme is difficult in face of competition from other simultaneously activated programmes Mailend and Maas [2013. Speech motor programming in apraxia of speech: Evidence from a delayed picture-word interference task. American Journal of Speech-Language Pathology, 22(2), S380-S396. https://doi.org/10.1044/1058-0360(2013/12-0101)]. Speakers with AOS and aphasia, aphasia without AOS, and unimpaired controls were asked to prepare and hold a two-word utterance until a go-signal prompted a spoken response. Phonetic similarity between target words was manipulated. Speakers with AOS had longer reaction times in conditions with two similar words compared to two identical words. The Control and the Aphasia group did not show this effect. These results suggest that speakers with AOS need additional processing time to retrieve target words when multiple motor programmes are simultaneously activated.

To Lump or to Split? Possible Subtypes of Apraxia of Speech

BACKGROUND

The speculation that apraxia of speech (AOS) is not a unitary diagnosis, but consists of different subtypes instead, has been around for decades. However, attempts to empirically substantiate such a notion remain few and far between.

AIMS

The primary objective of this article is to consider the different bases for identifying subtypes of AOS, review existing evidence regarding subtypes under each classification basis, and provide discussion and implications for future research.

MAIN CONTRIBUTION

AOS subtypes have been proposed on the basis of clinical symptomatology, theoretical constructs, and an analogy to limb apraxia. Different possible subtypes of AOS are reviewed, along with their empirical support and limitations. Empirical evidence, particularly in the context of a progressive disease, supports the idea that AOS diagnosis may capture different underlying impairments of speech motor planning. Future research to advance our understanding of AOS should carefully consider the basis for subtype classification, and include large sample sizes to differentiate individual variability from possible subtypes.

CONCLUSIONS

Several proposed AOS subtypes have found some support in the literature. Further research is needed to determine the validity, coherence and utility of possible AOS subtypes for theoretical and clinical purposes.

Do Patients With Neurogenic Speech Sound Impairments Benefit From Auditory Priming With a Regular Metrical Pattern?

Purpose Earlier investigations based on word and sentence repetition tasks had revealed that the most prevalent metrical pattern in German (the trochee)-unlike the iambic pattern-facilitates articulation in patients with apraxia of speech (AOS; e.g., Aichert, Späth, & Ziegler, 2016), confirming that segmental and prosodic aspects of speech production interact. In this study, we investigated if articulation in apraxic speakers also benefits from auditory priming by speech with a regular rhythm. Furthermore, we asked if the advantage of regular speech rhythm, if present, is confined to impairments at the motor planning stage of speech production (i.e., AOS) or if it also applies to phonological encoding impairments. Method Twelve patients with AOS, 12 aphasic patients with postlexical phonological impairment (PI), and 36 neurologically healthy speakers were examined. A sequential synchronization paradigm based on a sentence completion task was conducted in conditions where we independently varied the metrical regularity of the prime sentence (regular vs. irregular prime sentence) and the metrical regularity of the target word (trochaic vs. iambic). Results Our data confirmed the facilitating effect of regular (trochaic) word stress on speech accuracy in patients with AOS (target effect). This effect could, for the first time, also be demonstrated in individuals with PI. Moreover, the study also revealed an influence of the metrical regularity of speech input in both patient groups (prime effect). Conclusions Patients with AOS and patients with PI exploited rhythmic cues in the speech of a model speaker for the initiation and the segmental realization of words. There seems to be a robust metrical influence on speech at both the phonological and the phonetic planning stages of speech production.

Speech motor planning in the context of phonetically similar words: Evidence from apraxia of speech and aphasia

The purpose of this study was to test two competing hypotheses about the nature of the impairment in apraxia of speech (AOS). The Reduced Buffer Capacity Hypothesis argues that people with AOS can hold only one syllable at a time in the speech motor planning buffer. The Program Retrieval Deficit Hypothesis, states that people with AOS have difficulty accessing the intended motor program in the context where several motor programs are activated simultaneously. The participants included eight speakers with AOS, most of whom also had aphasia, nine speakers with aphasia without AOS, and 25 age-matched control speakers. The experimental paradigm prompted single word production following three types of primes. In most trials, prime and target were the same (e.g., bill-bill). On some trials, the initial consonant differed in one phonetic feature (e.g., bill-dill; Similar) or in all phonetic features (fill-bill; Different). The dependent measures were accuracy and reaction time. The results revealed a switch cost - longer reaction times in trials where the prime and target differed compared to trials where they were the same words - in all groups; however, the switch cost was significantly larger in the AOS group compared to the other two groups. These findings are in line with the prediction of the Program Retrieval Deficit Hypothesis and suggest that speakers with AOS have difficulty with selecting one program over another when several programs compete for selection.

Vowel Formant Dispersion Reflects Severity of Apraxia of Speech

BACKGROUND

Apraxia of Speech (AOS) has been associated with deviations in consonantal voice-onset-time (VOT), but studies of vowel acoustics have yielded conflicting results. However, a speech motor planning disorder that is not bound by phonological categories is expected to affect vowel as well as consonant articulations.

AIMS

We measured consonant VOTs and vowel formants produced by a large sample of stroke survivors, and assessed to what extent these variables and their dispersion are predictive of AOS presence and severity, based on a scale that uses clinical observations to rate gradient presence of AOS, aphasia, and dysarthria.

METHODS & PROCEDURES

Picture-description samples were collected from 53 stroke survivors, including unimpaired speakers (12) and speakers with primarily aphasia (19), aphasia with AOS (12), primarily AOS (2), aphasia with dysarthria (2), and aphasia with AOS and dysarthria (6). The first three formants were extracted from vowel tokens bearing main stress in open-class words, as well as VOTs for voiced and voiceless stops. Vowel space was estimated as reflected in the formant centralization ratio. Stepwise Linear Discriminant Analyses were used to predict group membership, and ordinal regression to predict AOS severity, based on the absolute values of these variables, as well as the standard deviations of formants and VOTs within speakers.

OUTCOMES AND RESULTS

Presence and severity of AOS were most consistently predicted by the dispersion of F1, F2, and voiced-stop VOT. These phonetic-acoustic measures do not correlate with aphasia severity.

CONCLUSIONS

These results confirm that the AOS affects articulation across-the-board and does not selectively spare vowel production.

Speech and nonspeech: What are we talking about?

Understanding of the behavioural, cognitive and neural underpinnings of speech production is of interest theoretically, and is important for understanding disorders of speech production and how to assess and treat such disorders in the clinic. This paper addresses two claims about the neuromotor control of speech production: (1) speech is subserved by a distinct, specialised motor control system and (2) speech is holistic and cannot be decomposed into smaller primitives. Both claims have gained traction in recent literature, and are central to a task-dependent model of speech motor control. The purpose of this paper is to stimulate thinking about speech production, its disorders and the clinical implications of these claims. The paper poses several conceptual and empirical challenges for these claims - including the critical importance of defining speech. The emerging conclusion is that a task-dependent model is called into question as its two central claims are founded on ill-defined and inconsistently applied concepts. The paper concludes with discussion of methodological and clinical implications, including the potential utility of diadochokinetic (DDK) tasks in assessment of motor speech disorders and the contraindication of nonspeech oral motor exercises to improve speech function.

A Novel Investigation of Generalized Motor Program Theory: Syllable Stress as a Motor-Class Variable

PURPOSE

This experiment evaluated syllable-stress position as a motor class directed by a syllable-sized generalized motor program. Reaction times were predicted to be slower for stimuli with untrained stress patterns outside the trained motor class. Furthermore, reaction times were predicted to be stable for untrained stimuli within the same motor class regardless of phonetic similarity.

METHOD

Twenty-three young adults with typical speech and hearing participated in a motor-learning study comprised of motor-class training and a judgment task. Reaction times derived from the judgment task were evaluated using a within-subject repeated-measures design to evaluate the effects of syllable-stress position on stimulus type.

RESULTS

Reaction times were not significantly different across proposed syllable-stress motor-class boundaries. However, reaction times for the stimuli within the same motor class were significantly different. To be specific, slower reaction times were associated with untrained stimuli that were phonetically similar to the trained stimuli.

CONCLUSIONS

The proposed hypotheses for syllable stress as a motor class were not met. Results indicate that multiple stimulus features, including syllable stress and phoneme similarity, may be encoded into memory during motor learning. Future research should evaluate how phonetic similarity of stimuli may influence motor-learning outcomes.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.5116837.

Identification and Remediation of Phonological and Motor Errors in Acquired Sound Production Impairment

Purpose

This study aimed to test whether an approach to distinguishing errors arising in phonological processing from those arising in motor planning also predicts the extent to which repetition-based training can lead to improved production of difficult sound sequences.

Method

Four individuals with acquired speech production impairment who produced consonant cluster errors involving deletion were examined using a repetition task. We compared the acoustic details of productions with deletion errors in target consonant clusters to singleton consonants. Changes in accuracy over the course of the study were also compared.

Results

Two individuals produced deletion errors consistent with a phonological locus of the errors, and 2 individuals produced errors consistent with a motoric locus of the errors. The 2 individuals who made phonologically driven errors showed no change in performance on a repetition training task, whereas the 2 individuals with motoric errors improved in their production of both trained and untrained items.

Conclusions

The results extend previous findings about a metric for identifying the source of sound production errors in individuals with both apraxia of speech and aphasia. In particular, this work may provide a tool for identifying predominant error types in individuals with complex deficits.