A diagnostic marker for childhood apraxia of speech: the lexical stress ratio

This report includes an extended review of the contemporary inclusionary criteria used to identify children with suspected apraxia of speech (sAOS) and describes findings supporting a lexical stress marker for sAOS. The thesis is that although a deficit in speech praxis is the core disorder in sAOS, only a few diagnostic markers for sAOS assess this speech motor control construct. The proposed marker is a composite lexical stress ratio (LSR) that quantifies the acoustic correlates of stress (frequency, intensity, duration) in bisyllabic word forms. Responses to a lexical stress task were obtained from 35 participants referred for a study of apraxia of speech. Eleven of the children were classified as sAOS, because they met one or both of two investigator groups' provisional criteria for sAOS. The 24 remaining children who did not meet either group's criteria were classified as having speech delay (SD). The first question posed was whether the LSR scores of children with sAOS differed from those of children with SD. Findings were affirmative. Of the six LSRs at the upper and lower extremes of the obtained distributions of LSR scores (approximately 8% of scores at each end), five (83%) were from speakers with sAOS (p < 0.003). The second question was whether findings for the sAOS speakers were more consistent with deficits in speech motor control or with deficits in underlying phonological representational aspects of lexical stress. A parsimonious interpretation of the present findings, together with findings from other studies, suggests that they reflect the prosodic consequences of a praxis deficit in speech motor control.