| Abstract: |
BACKGROUND: The impairment underlying apraxia of speech (AOS) is usually attributed to the phonetic encoding stage of the speech production process, where speech motor programs are accessed (e.g., Code, 1998). At this processing stage, Levelt, Roelofs, and Meyer (1999) postulate a store of motor patterns for frequently used syllables ("syllabary"). These syllable gestures are assumed to be holistically represented. However, the fact that syllable structure influences the error mechanism of AOS is in conflict with the assumption of holistic syllable gestures. AIMS: This study examined the assumption of holistic syllable-sized motor programs in apraxia of speech by a learning paradigm. We investigated if training of phonologically simple syllables, which were derived from more complex target syllables, showed a generalisation effect on these target syllables. If the assumption of holistic syllable programs is appropriate, no generalisation effects are expected. METHODS and PROCEDURES: A learning experiment was conducted with four patients with AOS. For each of 24 complex target syllables a set of 15 training syllables was derived by deleting one or two of the onset and/or coda consonants or by assimilating consonantal features. The learning trials comprised repetitions of the training syllables, block-wise for each target syllable. To assess generalisation effects, segmental errors and disfluencies were counted and syllable durations were measured before and immediately after training, for the target syllables as well as for matched control syllables. OUTCOMES and RESULTS: In the patients as a group, the training resulted in significant and specific improvements on the complex target syllables. The strongest effect was found in RK, a patient with pure AOS. This patient additionally exhibited a significant reduction of target but not of control syllable durations. CONCLUSIONS: In this learning study, patients with apraxia of speech showed specific generalisation effects from phonologically simple syllables to more complex syllables. These effects cannot be explained by the assumption of holistically stored syllable programs (Levelt et al., 1999). In contrast, the results suggest that syllabic motor programs comprise an internal phonological structure. |
