Intrinsic Fundamental Frequency of Vowels in Children with Childhood Apraxia of Speech

Intrinsic pitch or fundamental frequency (IF0) is an inherent property of vowels where close vowels are produced with a higher fundamental frequency than open vowels. Although well studied in adults, it remains underexplored in children. IF0 reflects combined biomechanical effects as well as a deliberate effort from speakers to produce distinct vowels and enhance vowel contrasts. Vowel errors and inconsistency in vowel production are well-known characteristics in Childhood Apraxia of Speech (CAS). We aimed to investigate if children with CAS exhibit IF0 and if present, how it compares with typically developing (TD) children.

A total of 17 CAS children and 8 TD children were asked to repeat simple bisyllabic nonword utterances of the type [dəCV] six times. The stimuli contained a consonant, C (/b, d/), and a vowel V, which comprised the corner vowels of the Dutch vowel space (/a, i, u/). The target stimulus was produced in a carrier sentence (/he dəCV wɪːr/; "hey the CV again"). Mean pitch (F0) and formant (F1 to F3) values were extracted from the recorded speech samples around vowel midpoint and bark transformed prior to further analyses. Statistical analyses were carried out using linear mixed models for each outcome measure separately.

The main finding of our study is that IF0 is present in children with CAS, showing a pattern that is generally similar to TD children. Additionally, we observed differences in vowel characteristics in children with CAS that are ambiguous; rather, we observed vowel-specific differences. Children with CAS produced /i/ less closed and /a/ with an exaggerated openness, whereas they produced /u/ more fronted compared to TD children. Also, children with CAS produced their vowels generally with a higher pitch and a longer duration compared to TD children. Pitch and duration were only correlated (negatively) in the vowel /a/ in both groups.

Where IF0 appears to be preserved in children with CAS, they do show differences in articulatory dimensions of vowel production compared to TD that are vowel specific. Clinicians should take these vowel-specific differences into account when choosing therapeutic targets.