Neural bases of childhood speech disorders: lateralization and plasticity for speech functions during development
Current models of speech production in adults emphasize the crucial role played by the left perisylvian cortex, primary and pre-motor cortices, the basal ganglia, and the cerebellum for normal speech production. Whether similar brain-behaviour relationships and leftward cortical dominance are found in childhood remains unclear. Here we reviewed recent evidence linking motor speech disorders (apraxia of speech and dysarthria) and brain abnormalities in children and adolescents with developmental, progressive, or childhood-acquired conditions. We found no evidence that unilateral damage can result in apraxia of speech, or that left hemisphere lesions are more likely to result in dysarthria than lesion to the right. The few studies reporting on childhood apraxia of speech converged towards morphological, structural, metabolic or epileptic anomalies affecting the basal ganglia, perisylvian and rolandic cortices bilaterally. Persistent dysarthria, similarly, was commonly reported in individuals with syndromes and conditions affecting these same structures bilaterally. In conclusion, for the first time we provide evidence that longterm and severe childhood speech disorders result predominantly from bilateral disruption of the neural networks involved in speech production.
Previous research has suggested that the left anterior insula, specifically the superior precentral gyrus of the insula (SPGI), is a critical brain region for the coordination of complex articulatory movements. However, previous studies have not determined which articulatory factors are specifically dependent on this brain region. In the current study, 33 left hemisphere stroke patients with varying degrees of speech impairment were asked to perform multiple repetitions of single words that varied along three separate dimensions: number of syllables, degree of articulatory travel (i.e., change between places of articulation for consonants), and presence/absence of an initial consonant cluster. The role of the SPGI in performance across the three conditions was determined using voxel-based lesion symptom mapping (VLSM), a statistical approach to lesion analysis that does not require separating patients based on lesion site or symptom profile. Rather, continuous performance data are entered, along with lesions reconstructed in normalized space. Based on preliminary analyses, there was adequate power to detect differences in the SPGI, which was the focus of our predictions. We found that the SPGI was critical for performance on the articulation task across all three conditions, namely, when words were multi-syllabic, required a high degree of travel, or involved an initial consonant cluster. As a control, we also generated a VLSM map for articulation of words with minimal articulatory complexity (i.e., single-syllable words with no initial cluster and a minimal change in place of articulation). In this case, the SPGI was not implicated. The current results suggest that the left SPGI is a critical area for intra- and inter-syllabic coordination of complex articulatory movements, prior to end-stage execution of speech commands.
The term phrase-level reduction refers to transformations of the phonetic forms of words in connected speech. They are a characteristic property of fluent speech in normal speakers. Phrase-level reductions contribute to a reduction of articulatory-motor effort and constitute an important aspect of speech naturalness. So far, these phenomena have not been subject to clinical investigations in patients with apraxia of speech. Since apraxia of speech is considered a phonetic encoding deficit, this condition might interfere with the mechanisms involved in phrase-level reduction. In this study we investigated the types and frequencies of reduction phenomena in an individual with pure apraxia of speech and compared it with normative data from two control speakers. Analyses were based on large samples of spontaneous speech. The results are discussed against the background of theoretical accounts of phrase-level reduction.
Augmented kinematic feedback provided by an EMA system improved production for some, but not all, treated targets. Generalisation to untreated probes was also evident. Predictions concerning the effects of feedback frequency on the acquisition, maintenance, and transfer of trained behaviours were not supported.
Variability in apraxia of speech: Perceptual analysis of monosyllabic word productions across repeated sampling times
Repeated sampling or method of speech elicitation did not influence errors, with a similar overall mean percentage of errors for the group in both conditions of stimulus presentation across the three sampling times. Distortions were found to be the predominant error type for the majority of target sounds. A comparison of the number of error types produced by the group in each condition across the three sampling times found no obvious pattern of responding by the group in either condition for individual phonemes. That is, condition of elicitation did not appear to influence the variability of error type for any given sound.
Foreign Accent Syndrome (FAS) is a rare acquired syndrome following stroke, manifesting as a perceived change in the speaker’s accent. We present acoustic-phonetic analyses of the speech of a patient, RD, with FAS presenting as an apparent accent shift from Southern Ontarian to Atlantic Canadian who was first described in Naidoo, Warriner, Oczkowski, Sévigny, and Humphreys (2008). As well as more fully documenting this case, this paper also seeks to examine whether the accompanying articulatory deficits constitute a mild form of some other motor speech disorder such as apraxia of speech (AOS) or dysarthria. Acoustic-phonetic analyses showed increased vowel formant variability, overlap of voice onset times for phonemically contrastive stops, inconsistent consonant distortions, and global prosodic attenuation. These, combined with the patient’s fluent speech, can account for the perception of a foreign accent as opposed to disordered speech. The observed inconsistency of consonant distortions coupled with other articulatory deficits suggest that at least in this case, FAS can be considered to be a mild form of AOS.
from the Journal of Neurolinguistics
The results provide some preliminary evidence of reduced anticipatory lingual movement in AOS, and have demonstrated that this can have a significant impact on absolute speech timing. However, measures of relative timing were suggestive of either unimpaired or more extensive coarticulation. Additional research is required to resolve this issue.
Conclusions: Results for this speaker with severe AOS and verbal perseverations were similar to those previously reported for SPT. The decrease in performance from 10 weeks to 15 weeks indicated that changes in behaviour had not been sufficiently instantiated. Furthermore, these findings suggested that maintenance probing may need to be conducted over a considerably longer period of time than has previously been reported in the literature.
Conclusion: Comparison with normative acoustic measures suggested that vowel production at the word level is unimpaired in the current speakers with AOS, supporting previous studies that have shown vowel production is relatively intact in AOS.
Progressive supranuclear palsy syndrome presenting as progressive nonfluent aphasia: A neuropsychological and neuroimaging analysis
There is currently considerable interest in the clinical spectrum of progressive nonfluent aphasia (PNFA) and progressive supranuclear palsy (PSP) and the intersection of these two entities. Here, we undertook a detailed prospective clinical, neuropsychological, and neuroimaging analysis of 14 consecutive patients presenting with PNFA to identify cases meeting clinical criteria for PSP. These patients had further detailed assessment of extrapyramidal and oculomotor functions. All patients had high-resolution MR brain volumetry and a cortical thickness analysis was undertaken on the brain images. Four patients presenting with PNFA subsequently developed features of a PSP syndrome, including a typical oculomotor palsy. The neuropsychological profile in these cases was similar to other patients with PNFA, however, with more marked reduction in propositional speech, fewer speech errors, less marked impairment of literacy skills but more severe associated deficits of episodic memory and praxis. These PSP-PNFA cases had less prominent midbrain atrophy but more marked prefrontal atrophy than a comparison group of five patients with pathologically confirmed PSP without PNFA and more prominent midbrain atrophy but less marked perisylvian atrophy than other PNFA cases. In summary, although the PSP-PNFA syndrome overlaps with PNFA without PSP, certain neuropsychological and neuroanatomical differences may help predict the development of a PSP syndrome. © 2010 Movement Disorder Society
from Movement Disorders
This paper briefly explores the relevance of patterns of related symptoms of nonfluent aphasia arising from left inferior frontal brain damage for the evolution of speech, language and gesture. I discuss aphasic lexical speech automatisms (LSAs) and their resolution with recovery into agrammatism with apraxia of speech and draw parallels between this recovery and the early evolution of language to protospeech and protosyntax. I focus attention on the most common forms of LSAs, expletives and the pronoun + modal/aux subtype, and propose that further research into these phenomena can contribute to the debate on the evolution of speech and language.
from the Journal of Neurolinguistics
This paper for the first time documents two patients who presented with FAS on a developmental basis. The finding that FAS does not only occur in the context of acquired brain damage or psychogenic illness but also exists as developmental motor speech impairment requires a re-definition of FAS as a clinical syndrome.
In theories of spoken language production, the gestural code prescribing the movements of the speech organs is usually viewed as a linear string of holistic, encapsulated, hard-wired, phonetic plans, e.g., of the size of phonemes or syllables. Interactions between phonetic units on the surface of overt speech are commonly attributed to either the phonological encoding stage or the peripheral mechanisms of the speech apparatus. Apraxia of speech is a neurogenic disorder which is considered to interfere with the mechanisms of phonetic encoding. Analyses of apraxic speech errors have suggested that phonetic representations have a non-linear, hierarchically nested structure. This article presents a non-linear probabilistic model of the phonetic code, which embraces units from a sub-segmental level up to the level of metrical feet. The model is verified on the basis of accuracy data from a large sample of apraxic speakers.
Temporal parameters of speech production in bilingual speakers with apraxic or phonemic paraphasic errors
Conclusions: The results of this study suggest that L2 speech production is more challenging than speech production in L1 for participants with either AOS or PP. The current study could not differentiate between all speakers with a disorder in phonological planning and speakers with a motor planning and programming disorder. Individual patterns appear to exist in speakers with PP. Increased processing demands seem to impact on the phonological planning and the speech motor planning and programming levels of speech production.
The influence of visual and auditory information on the perception of speech and non-speech oral movements in patients with left hemisphere lesions
Patients with lesions of the left hemisphere often suffer from oral-facial apraxia, apraxia of speech, and aphasia. In these patients, visual features often play a critical role in speech and language therapy, when pictured lip shapes or the therapist’s visible mouth movements are used to facilitate speech production and articulation. This demands audiovisual processing both in speech and language treatment and in the diagnosis of oral-facial apraxia. The purpose of this study was to investigate differences in audiovisual perception of speech as compared to non-speech oral gestures. Bimodal and unimodal speech and non-speech items were used and additionally discordant stimuli constructed, which were presented for imitation. This study examined a group of healthy volunteers and a group of patients with lesions of the left hemisphere. Patients made substantially more errors than controls, but the factors influencing imitation accuracy were more or less the same in both groups. Error analyses in both groups suggested different types of representations for speech as compared to the non-speech domain, with speech having a stronger weight on the auditory modality and non-speech processing on the visual modality. Additionally, this study was able to show that the McGurk effect is not limited to speech.