Characterizing the Link Between Behavior and Neurobiology of Verbal Working Memory in Young Adults With and Without Developmental Language Disorder

Deficits in verbal working memory commonly occur in developmental language disorder (DLD) and are hypothesized to contribute to the language impairment due to its importance in early childhood language acquisition (Ellis Weismer et al., 1999; Evans et al., 2018; Gathercole, 1998). Individuals with DLD perform poorly on verbal working memory tasks (e.g., nonword repetition, n-back, sentence- and word-span tasks), and this performance correlates to linguistic proficiency in lexical and sentence-level comprehension. While deficits in verbal working memory and language are well documented in DLD, due to the current diagnostic methods, it has been debated whether individuals with DLD represent a distinct clinical population, as opposed to the bottom percentage of language users in the normal population (Dollaghan, 2004, 2011). Recently, research has shown that a mismatch between neural activation patterns and behavior may be a potential neurobiological marker of DLD (Brown et al., 2014; Haebig et al., 2017). To examine whether DLD represents a true clinical population or is rather representative of low language ability typical individuals, this manuscript-based dissertation contains three studies examining the pattern of vii cortical activation across the prefrontal cortex (PFC) for verbal working memory in neurotypical adults and individuals with developmental language disorder (DLD) using functional near infrared spectroscopy (fNIRS). The first study (Chapter 3) identified the neural response across the PFC during an auditory 2-back task in adults with no history of language or learning disorders. Results from this study showed differences in the change in hemoglobin level patterns for normal language adults in response to high- and low-frequency words, as well as faster reaction times for low-, and greater accuracy for high-frequency words. The second study (Chapter 4) expanded upon the findings of the first study and showed that while two adults with DLD had behavioral performance within normal limits of a group of age- and gender-matched controls, individual z-score analysis revealed the adults with DLD exhibited qualitative differences in HbO2 and Hb in response to high- and low-frequency words in the PFC as compared to normal controls. In addition, the second study compared the adults with DLD to a low-performing normal language control, showing that while both DLD subjects had increased levels of hemoglobin across the PFC, the low-performing normal language control displayed significant reduction in hemoglobin levels across the PFC in response to word frequency in the verbal working memory task. The findings of this study suggest that not only do the individuals with DLD have atypical neural response to word frequency during a verbal working memory task that distinguishes them from normal language peers, but also that their atypical neural response does not resemble that of the low-performing normal language population. The third study (Chapter 5) examined the ability of fNIRS to monitor changes in hemoglobin levels across the PFC during the auditory 2-back task both before and after a stimulation-based training that combined high-definition transcranial direct current stimulation (HD tDCS) with a phonological working memory task. Using a single-subject design this study confirmed the feasibility of fNIRS to track individual differences in changes in brain activity immediately before and after working memory training and showed that neural response for both individuals with DLD moved to within the expected range similar to that of the normal controls after training. Collectively these three studies enhance our understanding of the relationship between atypical neural activation patterns paired with normal behavior in adults with DLD to address the theoretical question of whether DLD represents a true clinical population, or merely low-normal language users. The findings support a clinical definition of DLD and offer a potential neural biomarker to detect these individuals that may otherwise go undetected due to their high functional, but compensatory, performance.