Hierarchical Organisation of the Mental Lexicon in Children with Specific Language Impairment

This study examines the lexical organisation and differentiation of nouns and verbs within spontaneous speech of children with SLI and their age-matched controls. Subjects were 10 4-year-old children and 10 9-year-old children (5 SLI children and 5 controls in each age group). Spontaneous samples of each participant were transcribed with particular attention being paid to linguistic variables (e.g., number of different words, total number of words, number of utterances, MLU). For an in-depth semantic analysis a classification system has been developed, and all nouns and verbs from the transcripts were classified and coded according to this scheme. The results show that preschool aged children with SLI clearly suffer from an insufficient semantic organisation. The noun lexicon contains predominantly basic-level words but marginal super- and subordinate referents. Within the verb lexicon only a small amount of specific verbs could be found. In contrast, the 4-year-old typically developing children already showed a well-organised lexicon comparable to the semantic organisation of the older controls. At age 9, SLI children produce significantly shorter utterances than their age-matched peers, but there were no group differences with respect to semantic organisation. Consequently, the development of an organised and hierarchically structured mental lexicon is delayed in SLI.