Acoustic phenotypes for speech-genetics studies: an acoustic marker for residual backslash 3 backslash distortions

A companion paper addresses the need for phenotype markers for speech-genet ics studies and provides reference data for US English rhotics that can be used for phenotype research. The present paper uses these reference data to derive and test an acoustic marker to discriminate the residual /3(iota)/ distortions of adolescents with two speech disorder histories. One speech d isorder history includes significant speech delay; the other history is a s peech disorder limited to only speech sound distortions of /r/, /3(iota) / and/or /partial derivative (iota)/. The first subtype of speech delay is po sited to be genetically transmitted, whereas the origins of the second subt ype are posited to be associated with shared and non-shared environmental v ariance. Speech samples from 84 9 to 17-year-old speakers were divided into four groups based on speech history and speech errors at assessment. Group 1 children had prior speech delay and residual rhotic distortions, Group 2 children had only prior and residual rhotic distortions, and children in t he two control groups had normal or normalized speech. Statistically signif icant logistic regression models indicated that an acoustic marker successf ully discriminated residual derhotacized /3(iota)/ tokens produced by speak ers in Group 1 from residual derhotacized / / tokens produced by speakers i n Group 2. The marker was a z score less than 6.0 for Formant 2 subtracted from Formant 3 (i.e. zF3-F2< 6.0) as measured at the constriction interval for / / targets. Sensitivity (percentage of correctly identified derhotaciz ed /3/ tokens from Group 1 speakers) for the acoustic marker was 85%. Speci ficity ( percentage of correctly rejected derhotacized /3/ tokens from Grou p 2 speakers) was 79%. Discussion considers methodological, phonological, a nd genetic perspectives that might account for the articulatory differences in the residual /3(<iota>)/ distortions of adolescents with the two differ ent speech histories.