The experiments presented in this work are aimed at gaining a greater understanding of the semantic impairment in semantic dementia (SD) in terms of modality-specificity and category-specificity, and to consider the implications for the organisation of the semantic system. To this end, the semantic abilities of a group of twelve SD patients with varying impairment severity were examined using a variety of methods - tests from a traditional semantic battery and novel tests, which examined the verbal and nonverbal knowledge of concepts and the semantic attributes of those concepts. The methods of assessment were directly contrasted and relationships between them explored. Throughout this work, comparisons with normal performance were drawn using data collected from age-matched healthy subjects. Longitudinal analyses of the performance of a subset of the patients were also presented to investigate any decline in patient performance over time. The findings suggest a remarkable degree of consistency in semantic performance in the patient group, regardless of stimulus modality or feature type, with few exceptions. However, this consistency was not reflected in the influence of semantic domain. Some patients showed category-specificity while others did not. These differences could not be explained by reference to psycholinguistic variables or evolutionarily determined categories. Differential processing of feature types was more satisfactory as an explanation but required the implication of more fine-grained distinctions than the binary sensory/functional knowledge classification. Recent models of which consider multiple principles of organisation within the semantic system are more likely to be able to account for all the data showing both consistency and inconsistency within the present cohort of SD patients, and the myriad findings in the semantic memory literature.

This thesis applies Conversation Analysis (CA) to the phenomenon of agrammatism, a particular type of aphasia (a language difficulty acquired most commonly after stroke) which is characterised by grammatical impairment. Although mainstream research has done much to characterise the nature of the underlying disorder, most studies have analysed elicited, task-based data by applying the theoretical concepts of a standard grammar the well-formed sentence, clause and phrase. As a result, little is known about the grammar that people with agrammatism use in real, everyday talk-in-interaction with habitual conversational partners. This study investigates the utility of CA as a tool for the exploration of conversational grammar in agrammatic aphasia. The data comprise video-recordings of the conversation of two adults with agrammatic aphasia, recorded in the home talking to a family member or friend. Conversation is contrasted with single word-, sentence- and narrative-level language samples elicited via commonly used clinical assessments. The data-driven procedures of CA reveal recurring turn construction formats in the talk of the individuals with aphasia. Cognitive neuropsychological, linguistic and psycholinguistic methodologies are drawn on to analyse the elicited language samples, in order to produce the type of clinical profile of agrammatism on which speech and language therapy is based. A comparison of the two samples finds that turn construction for conversation differs from sentence construction for testing. The thesis concludes that the conversation of both aphasic speakers exhibits structure and systematicity, a 'grammatical' organisation, but that the constructions documented do not resemble the sentences, clauses and phrases of a standard grammar. Rather, their form is shaped by the interactional demands of taking a turn at talk. The study questions the widely-held assumption that elicited language tests provide a view of grammatical impairment that is synonymous with the reality of the condition for the person who lives, and most crucially talks with it.

This thesis is in two parts: the first focuses on theories of SLI and the development of argument structure while the second focuses on intervention. Chapter 1 reviews experimental findings and theories of SLI and finds that while some areas of language are well-researched, others (including argument structure) have received relatively little attention. Chapter 2 reviews the literature regarding the development of argument structure and concludes that studies of typical development have not investigated use of alternations and omissions of obligatory arguments, whereas studies of SLI have little focus on alternations or overgeneralisations. Chapters 4 and 5 therefore consider the performance of typically developing children and children with SLI on all these areas. I find typically developing children differ from adults in their use of the causative alternation and overgeneralisation of the locative alternation. The children with SLI have difficulties with argument structure, avoiding the ditransitive form of the dative alternation and making more errors with change of state verbs and omission of arguments. A secondary focus (Chapter 6) is on the influence of phonological complexity and length (measured by a non-word repetition test) on the language abilities of children with SLI. The results show a bimodal split where half the children with SLI show normal abilities and half have significant difficulties. Chapter 7 discusses the implications of the experimental findings for theories of SLI. Part 2 reviews intervention studies for SLI (Chapter 8) and presents an intervention study focusing on argument structure (Chapter 9). 27 secondary-aged children with SLI are randomly assigned to three groups, one control and two target therapies focusing on semantics vs constructions. Both target groups show significant progress. Thus, this thesis shows that detailed investigations of the nature of the deficit in SLI can lead to successful interventions even for children with severe, persistent difficulties.

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Semantic dementia (SD) is a focal degenerative disorder characterised by multimodal loss of semantic knowledge. A number of studies have reported preservation of numerical and arithmetic knowledge in SD, contrasting with patients' profound impairment of object and word knowledge, a finding interpreted as evidence for the independent cerebral representation of number knowledge.