Conversations with SLPs (1)

I have been enjoying conversations with SLPs about Developmental Phonological Disorders, discussing questions either about the book or about their patients in relation to the clinical advice offered in the book. I am going to share the gist of some of these exchanges on this blog from time to time, with the permission of the parties in involved.

Gabrielle has been reading the book, valiantly page by page, with a view to the implications for her primary clientele which is students with dyslexia as well as speech and language disorders. She has two interesting questions regarding Chapter 7. The questions relate to the discussion of the perceptual abilities of children with DPD generally and the results of Rvachew and Jamieson (1989) in particular.

In Rvachew and Jamieson (1989), the participants who were preschoolers with phonological disorders were required to identify the words seat and sheet via a picture pointing task when presented with the words live-voice, in order to be enrolled in the study. Even though the children were able to complete the live-voice task, about half of them were unable to identify synthetic versions of these words. I relate these results to Nittrouer’s finding that some children have immature cue weighting strategies such that they identify these fricatives based on the formant transitions between the fricative and the vowel rather than attending the steady-state spectrum of the fricative noise. Gabrielle asks: “What is the difference between a spectral cue and a formant transition, and if the kids can hear the difference during “live” speech does it matter that they can’t use spectral information during synthetic listening tasks?” The way in which these fricative sounds are perceived in words by adults and children is described in pages 43 to 45 (see also Nittrouer, 2002). Like all speech sounds, the perception of fricatives requires the integration of many acoustic cues that are spread across the word.  When the child attends primarily to one cue (the dynamic formant frequency changes between consonant and vowel) and ignores other cues (such as the peak frequency of the fricative noise) there are consequences for both perception and production of these phonemes. The child can achieve accurate perception with live-voice stimuli under some conditions with an immature cue-weighting strategy but this strategy will be unreliable, leading to perceptual errors under difficult listening conditions (unfamiliar listeners, fast speech, background noise). The consequences for speech production are greater however; if the child believes that the primary cues to the /s/-/ʃ/ contrast lie in the dynamic transition between the fricative and the vowel, the child is not attending to a critical piece of information that will help him or her learn to manipulate tongue placement and groove width in order to produce /s/ with higher frequency centroid frequencies and /ʃ/ with lower frequency centroid frequencies. Testing the child with synthetic speech that isolates these cues highlights the fact that even though the child can perceive the contrast in some circumstances, the child’s perceptual knowledge is incomplete. Developmentally, perceptual and productive knowledge of phonological contrasts is acquired gradually, with perceptual knowledge leading somewhat but iterative gains occurring over time in both domains.

The second question was “about auditory processing difficulties (on page 551 and 552), as different from speech perception difficulties it was my understanding that often “auditory processing” evaluations include speech perception tasks. Was the original concept of auditory processing based on non-speech tasks?” Indeed, in this section I am making a distinction between the hypothesis that auditory processing deficits cause speech or language disorders (such as Tallal’s proposal that temporal processing difficulties are a causal factor) versus the hypothesis that speech perception deficits play a role (as in Ramus’ proposal about phonological representations). Traditionally, auditory processing skills would be attributed to lower levels of the auditory system, midbrain and primary auditory cortex for example, whereas the formation of phonological representations necessary for speech perception takes place at higher cortical levels (see discussion of dual-stream model on pages 127-130). Of course the development of the system is influenced by all kinds of auditory input and perception of speech requires the entire system working together. However, certain nonspeech tasks such as gap detection or the binaural masking level difference are supposed to measure functioning at lower levels of the auditory system. It is true that batteries of tests that are supposed to measure central auditory processing involve speech input and, at least to me, often appear to be measures of language processing. It is not for me to explain the choice of tests in those batteries (although you may enjoy this RALLI video in which Dorothy Bishop discusses the controversy about central auditory processing disorder as a diagnosis). My point was that it is important to not mix up the literature on the auditory processing skills of children with speech and language deficits with the literature on the speech perception and phonological processing skills of children with speech and language deficits. The unreliable findings on auditory processing have no bearing on the more consistent finding that children with speech production difficulties have problems with speech perception.

I welcome interesting questions such as these and invite more from pre-practice and practicing speech-language pathologists.

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