AOS, CAS and STM

I greatly enjoyed this new Frontiers in Neuroscience paper by Hickok and colleagues called “Partially overlapping sensorimotor networks underlie speech praxis and verbal short-term memory: Evidence from apraxia of speech following acute stroke”. These researchers evaluated 76 patients during the acute phase of their stroke using behavioral and MRI measures. They found a strong relationship between apraxia (AOS) and verbal short- term memory (vSTM) difficulties as well as weak relationships between aphasia and AOS and vSTM upon behavioral testing. For patients with AOS, the MRIs revealed tissue damage along a sensorimotor network of motor-related areas and sensory-related areas. The motor related areas that were implicated were as follows: primary motor cortex (proposed site of motor programs for opening and closing vocal tract gestures that correspond roughly to consonant and vowel phonemes); pars opercularis (a part of Broca’s area involved in phonological processing and suppression of response tendencies);  premotor cortex (planning and sequencing of speech units and sensory guidance of movement; motor programs for syllables); and insula (specialized for motor planning of speech). The sensory-related areas  associated with AOS were primary somatosensory cortex (site of  somatosensory targets for speech); secondary somatosensory cortex (sensorimotor integration); parietal operculum (sensory motor interface for speech); and auditory cortex (processing of auditory information; auditory targets for speech). The areas associated with vSTM deficits overlapped those associated with AOS but only in the motor-related areas, specifically pars opercularis and par triangularis (i.e., Broca’s area), premotor cortex and primary motor cortex.

With regard to the network associated with AOS, the authors concluded that the findings demonstrate “that the targets for speech are sensory in nature” and that “motor control generally and speech motor control specifically is dependent on sensorimotor integration”. I found these conclusions to be interesting in view of our interventions studies with children who have childhood apraxia of speech. As I reported in a previous blog, we are having success with an approach in which we encourage strengthening of both articulatory-phonetic and acoustic-phonetic representations for target words and the connections between them.

With regard to vSTM, the authors indicate that “the involvement of motor areas is predicted as vSTM involves an articulatory rehearsal component”. They seem  surprised however that “posterior, sensory related regions” were not implicated in this study as correlates of the hypothesized “storage” component in short-term memory. This finding reminded me of a paper I wrote in 2008 in which I pointed out that children’s nonword repetition performance, supposedly a measure of vSTM, factors with speech production accuracy rather than language ability in large scale studies involving children with either typical or atypical language development. I interpreted these findings in relation to a connectionist model of working memory proposed by MacDonald and Christensen (2002). According to this model there is no short term memory store per se because  working memory is not differentiated from linguistic knowledge and processing. Individual differences in working memory task performance reflect differences in precision of phonological representations and processing efficiency due to experiential and biological factors. The processes and representations involved in working memory are the same as those used in speech planning.  Many of the children that we are working with have difficulty planning an utterance – I have described these children with phonological planning difficulties in a previous blog. The children have difficulty with consistent repetition of nonwords and complex real words. The successful intervention for these children involves providing multimodal external cues to support the child’s efforts to construct and execute a plan to produce new words, as described in a previous blog. It is important that the SLP avoid providing an auditory  model for imitation by the child however although the SLP may imitate the child’s production to reinforce successful attempts or correct failed attempts.

Hickok et al interpret their findings in light of their hierarchical model although I remain uncertain about this notion of a hierarchical organization of these components just because I can never quite sort out what ‘higher” versus “lower” means when placing these kinds of components in a hierarchical relationship.  The importance of acquiring knowledge of different forms of linguistic representation – acoustic, articulatory, phonological and semantic – and linking across multiple representations to achieve functional goals has implications for typical and atypical language development however.

Tanya and I will be discussing these issues further (with video demonstrations) at ASHA2014:

Topic Area: Speech Sound Disorders in Children Session Number: 1037 Title: Differential Diagnosis of Severe Phonological Disorder & Childhood Apraxia of Speech Session Format: Seminar 2-hours Day: Thursday, November 20, 2014 Time: 10:30 AM ─ 12:30 PM Author(s): Susan Rvachew and Tanya Matthews

Nonword Repetition and Speech Therapy

I am grateful to Sofia Strömbergsson (Ph.D. student at Kungliga Tekniska högskolan in Stockholm) for sending me the link to a new paper on the nonword repetition task: Melby-Lervåg, M., Lervåg, A., Lyster, S.-A. H., Klem, M., Hagtvet, B., & Hulme, C. (2012). Nonword-Repetition Ability Does Not Appear to Be a Causal Influence on Children’s Vocabulary Development. Psychological Science, Online First, accessed October 6, 2012. The results of a 3-year longitudinal study of 219 children are reported in which nonword repetition and vocabulary performance was measured at regular intervals. Simplex models and latent growth curve models were used to show that each variable was very stable over time with earlier performance predicting later performance in each domain. However, neither variable had significant longitudinal influence on the other over time (in fact the strongest cross-loading that was observed was from vocabulary to nonword repetition at the earliest time point although the link was not statistically significant). The result is at variance with the hypothesis that the phonological loop is a fundamental cognitive mechanism that underlies language learning in general and vocabulary development in particular. The outcome is of interest to me because when Meghan Grawburg and I published our model of the correlates of phonological awareness in children with speech delay in 2006 (inserted Figure is simplified version of the published LSE model), Hartman (2008) suggested that our model was deficient due to the exclusion of phonological memory as a variable; it was further suggested that phonological memory would explain both vocabulary and phonological awareness skills in our model whereas we had shown that phonological awareness emerged from the interaction of children’s speech perception and vocabulary skills. We argued in reply that phonological memory, as measured by a nonword repetition task, reflects the quality of children’s phonological representations and would most likely factor with our measure of speech production accuracy or our measure of speech perception. Therefore it rather pleases me to see the two variables developing along independent paths despite their initial correlation in this new paper by Melby-Lervåg et al.  These authors point out (as we did in Rvachew and Grawburg, 2008) that training studies are required to most definitively test hypotheses about possible causal relationships between phonological memory and vocabulary skills. Two new nonword repetition tests are available to support these kinds of studies. First, Shula Chiat has developed the Preschool Repetition Test which can be used with children as young as two years (see also, Chiat and Roy, 2008). I really like this one because it takes prosody into account. Second, Shriberg and colleagues  have developed a Syllable Repetition Test that is made up solely of early developing phonemes so it is very appropriate for children with speech problems. Following from our model and the intervention work that we are doing in our lab right now I would predict that vocabulary interventions in young children with speech and language delays would lead to improvements in phonological processing skills (including nonword repetition and phonological awareness skills). I’d be surprised if training phonological memory transfers directly to vocabulary skills but it would certainly be an interesting study! I stress however that these would be interesting studies from an academic point of view and not good clinical practice.

In terms of clinical relevance, I think this new study supports my general rule and that is to always treat the primary functional goals of interest – if the child’s speech is unintelligible target speech accuracy. If there are concomitant problems with phonological processing by all means work on those skills at the same time given that phonological processing skills are important precursors to both speech accuracy and reading. In this case however, focus on those skills that have the greatest functional relevance – word identification/error detection tasks are directly relevant to speech perception and self-monitoring for the improvement of speech accuracy; phonological awareness is essential for the acquisition of decoding. If the child’s vocabulary size is small it will be necessary to target lexical knowledge as well. Under no circumstances would I recommend targeting phonological memory or any so-called executive functions as a means of promoting language or cognitive skills (e.g., morphosyntax, phonology, reading and so on) as is often recommended by the purveyors of so many popular ‘brain training’ regimens.  The other point of clinical relevance has to do with screening – nonword repetition has proven to be an important marker in genetics studies but it is not by itself a clinically useful screening or diagnostic tool. Most of the time, as SLPs, we want to know whether a given child has a clinically significant delay in speech or language functioning right now and to answer that question standardized tests of articulation accuracy and language ability are still the best tools in our kit. It remains to be seen whether nonword repetition tests (in combination with other measures) will prove to be a reliable means of sorting speech and language caseloads  into psycholinguistic subtypes that require distinct approaches to intervention – I have been hopeful that this might be possible but I am not convinced that the science is there yet.