Intelligibility: Do sentence length and word structure matter?

Speech intelligibility is a complex construct and many aspects are known to influence intelligibility in motor speech disorders. This includes the way people pronounce words, but also how well listeners can tune into people’s speech. Context also helps, which is why connected speech is usually more intelligible than single words. In a recent study, Kristen Allison and Katie Hustad explored another important aspect influencing speech intelligibility: the linguistic properties of speech. Two aspects were of particular interest to them: sentence length and phonetic complexity, i.e. the motoric difficulty of a word’s sound structure (e.g. the word ”no” involves less complex motoric movements than the word “complex”). Both, longer sentences and higher phonetic complexity are known to increase the motoric demands of producing an utterance, which can affect intelligibility.

In order to quantify their contribution to intelligibility problems in children with CP the authors asked 119 listeners to orthographically transcribe speech samples of 24 5-year old children. 16 children had CP, of which 8 were diagnosed with dysarthria. The speech of 8 typically developing children was transcribed as well. The speech samples were sentences of 2 to 7 words in length. Each listener was asked to transcribe about 60 sentences.

Results showed that both factors had an effect on intelligibility, but the effect was found to be greater for children with dysarthria. They were best understood in short sentences of 2 to 3 words, whereas children without speech problems were easily understood up to a sentence length of 6 words. A similar result was observed for phonetic complexity: whilst the effect for children without speech problems was small, intelligibility of children with dysarthria was significantly reduced in sentences with words that required more complex motoric movements. The findings indicate that reducing length as well as phonetic complexity may enhance intelligibility. However, considerable individual variation suggests that for some children sentence length was more of an issue and for others complexity. This highlights the importance of a child’s individual motor profile when considering treatment options.

Allison, K. & Hustad, K. (2014). Impact of sentence length and phonetic complexity on intelligibility of 5-year-old children with cerebral palsy. International Journal of Speech-Language Pathology.