A nonlinear model of word length effects in apraxia of speech

Abstract

Previous studies of length effects in normal and disordered spoken language production were based on a linear view of words as strings of phonemes or syllables. This approach was limited by an inherent interaction of numbers of phonemes and syllables with syllable complexity. The present investigation was therefore based on a metrical tree model of word form representations, which was implemented by a nonlinear regression model predicting accurate word production in apraxia of speech. One hundred data samples from 20 apraxic speakers were used to estimate the probabilties of accurate production for a data corpus of 72 carefully controlled words and nonwords of varying lengths. The nonlinear regression model explained almost 80% of the variance in these data. The shape of the model indicated a preserved cohesion of phonetic representations at the rime and the foot level in apraxia of speech.

Acknowledgement

This study was supported by grant Zi 469 / 4-1 from the German Research Council (DFG). I am indebted to Bettina Brendel and Michaela Liepold for their contribution to the development of the materials utilised here and for their assistance in auditory evaluation. David Howard, Lyndsey Nickels, and Ingrid Aichert are acknowledged for their valuable comments on an earlier version of the manuscript. David Howard's contribution led to a substantial methodological improvement of the approach taken here. Preliminary versions of this study were presented at the EURESCO-conference The Science of Aphasia, Giens, France, 2001, and the Satellite Conference to the 15th International Congress of Phonetic Sciences, Ontogeny and Phylogeny of Syllable Organization, Barcelona, Spain, 2003.

Notes

Fluency ratings considered increased response delays, within-word pauses, phoneme or syllable iterations, false starts, and self-corrections