Word Learning Ability Varies Across Contexts and Time: A Longitudinal Study of Primary School Children with Developmental Language Disorder.

Background and Aims: Children with developmental language disorder (DLD) have difficulty learning new words, but we know little about whether and to what extent their word-learning ability improves over time. Our primary goal was to compare the rate of development of word learning abilities, specifically the encoding of new lexical-semantic information, in children with DLD and their age-mates with typical language development (TLD). Secondary goals were to examine variation in outcomes according to the aspect of word knowledge under consideration, the word-learning context, and the cognitive abilities of the learners. Methods: Children with DLD (n= 38) and TLD (n = 45) participated in two experiments each year from Grades 1 to 4. In each, they were taught a new set of 12 novel words and their referents. The experiments involved a cross-situational word learning context (Study 1) and ostensive naming and mutual exclusivity contexts (Study 2). Post-training probes measured knowledge of the phonological form of the word and the link between the word and its referent. In Year 1, we measured various aspects of cognition. Linear mixed models with fixed effects for diagnostic group, year, and context yielded main outcomes. Results: Children with DLD were less able than peers with TLD to learn form- and link information in all three contexts and in all four years, but their relative rate of growth across the years was similar. Neither form- nor link-learning was consistently harder than the other for the DLD group; however, the size of the TLD-DLD performance gap was especially large for form learning in the ostensive naming (most direct) training context. The lower cognitive abilities of children with DLD, especially phonological short-term memory and receptive vocabulary knowledge, accounted for variance in form and link learning. With cognitive scores in the statistical models, the TLD-DLD gap in link learning was no longer significant. Conclusions: As a group, primary school children with DLD present with weaknesses in word learning abilities but age-appropriate rates of improvement in those abilities over time. The problem reflects their lower verbal cognition. The extent of the problem varies with the context in which the word is learned and the aspect of word knowledge that is measured as an outcome. Implications: The implication for scientists is that a diversity of contexts and outcome measures must be included in future research. The implication for clinicians is two-fold. First, brief opportunities to learn word forms in ostensive naming contexts may be leveraged for dynamic assessments, as it is this learning goal and this learning context that most effectively distinguished DLD from TLD. Second, when treating a child with DLD, the common practice of direct teaching is unlikely to be effective unless active engagement, sufficiently high dosage, optimally sized target sets, and rich vocabulary instruction are included.