Word Retrieval in Acquired and Developmental Language Disorders: A Bit more on Processing

This thesis offers a new scientific approach to word finding difficulties in language-disordered individuals. Concepts from information theory are used to calculate complexity of word forms and the effect of this complexity on processing is investigated for Dutch adults with aphasia and children with SLI. Unlike previous research, this project is the first to provide quantitative measures of “complexity” of linguistic material, using statistical measures from information theory to calculate complexity of specific word forms and inflectional paradigms encompassing both frequency and linguistic factors. An application of information theory to visual processing is also explored. Research questions: 1.To what extent can limitation of information processing capacity as measured within an information theoretic paradigm explain apparent agrammatic behaviour in Dutch people with aphasia? 2.Is the above limitation language - specific or it is observable in other cognitive domains, specifically visual cognition? 3.To what extent can limitation of information processing capacity as measured within an information theoretic paradigm explain the article retrieval difficulties of Dutch children with Specific Language Impairment? The thesis is divided into two parts: Part A: the experimental part describes an auditory lexical decision and a visual search experiment conducted with three groups: healthy young, elderly and aphasic participants. The effects of several information theoretic measures (two of which are new) on lexical retrieval in these three groups are discussed. In chapter 4 the results of a visual search task are discussed and related to the auditory lexical decision results. Part B: spontaneous speech analyses looks at data from both aphasic speakers and children with SLI. The first chapter discusses a new application of Zipf’s law and speaker entropy and the use of these tools in aphasic speech samples. The second chapter explores a third way to use information theoretic measures to investigate reduced processing capacity: the Kullback- Leibler divergence between input and output distribution. Article production is investigated for adults with aphasia and children with SLI. Overall the results suggest that linguistic structure is not impaired in adults with aphasia, but rather that they have difficulty processing linguistic (and non-linguistic) complex material. Furthermore, the final chapter shows that, although children with SLI and adults with aphasia have similar apparent difficulties with article production, the underlying cause is different. This thesis provides a first attempt to investigate reduced processing in language-impaired individuals within an information theoretic framework. The results on an auditory lexical decision task, a visual search task and two types of spontaneous speech analyses for language disordered individuals and healthy controls showed that a) information theory provides us with the tools to quantify linguistic material and that b) we can relate the effect of this complexity to the (reduced) processing capacity in adults with aphasia and children with SLI.