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Purpose: This study was conducted to evaluate the clinical utility of error variability for differentiating between apraxia of speech (AOS) and aphasia with phonemic paraphasia.
Method: Participants were 32 individuals with aphasia after left cerebral injury. Diagnostic groups were formed on the basis of operationalized measures of recognized articulatory and prosodie characteristics of AOS and phonemic paraphasia. Sequential repetitions of multisyllabic words were elicited as part of a motor speech evaluation and transcribed phonetically. Four metrics of variability at the syllable and word levels were derived from these transcripts.
Results: The measures yielded different magnitudes of variability. There were no group differences between participants who displayed speech profiles consistent with AOS and participants who displayed speech profiles indicative of aphasia with phonemic paraphasia. Rather, correlation coefficients and analyses of covariance showed that the variability metrics were significantly mediated by overall error rate. Additionally, variability scores for individuals with salient diagnoses of AOS and conduction aphasia were inconsistent with current diagnostic guidelines.
Conclusions: The results do not support diagnostic validity of error variability for differentiating between AOS and aphasia with phonemic paraphasia. Future research using error variability metrics should account for overall error rate in the analysis and matching of participant groups.
Key Words: aphasia, apraxia of speech, articulation, assessment, diagnostics, speech production, prosody
The differential diagnosis of apraxia of speech (AOS) relative to other acquired speech production difficulties caused by left-hemisphere lesions is based on the subjective evaluation and synthesis of multiple speech properties. In the context of stroke or cerebral trauma, the distinction between AOS and dysarthria is relatively uncontroversial. In contrast, the differentiation between AOS and aphasia with phonemic paraphasia is considerably more challenging, largely because these conditions share overlapping lesion location and because both are most prominently characterized by the perception of sound substitutions in the speech output.
Because AOS is defined as a phonetic-motor disorder specific to speech production (McNeil, Robin, & Schmidt, 2009), it is conceptually distinct from aphasia. However, when seen following left-hemisphere stroke or trauma, the disorder almost always coexists with aphasia. AOS is characterized by several features other than sound substitutions, including sound distortions and impaired fluency and prosody (Duffy, 2005; McNeil et al., 2009). In contrast, phonemic paraphasia refers specifically to the behavioral presentation of...