Abstract/Summary

Jargon aphasia describes the profile of individuals producing fluent but largely unintelligible output, often comprising high proportions of non-word errors. This thesis presents a series of studies exploring possible mechanisms underlying the output difficulties in these individuals, drawing on data from 19 individuals from the Moss Aphasia Psycholinguistic Database and six individuals who participated in experimental testing. The studies addressed four research questions: a) to what extent target phonological information is preserved in non-word errors; b) how manipulating semantic and phonological activation affects output; c) whether a phonological-level breakdown is responsible for output difficulties in all individuals or does impairment at prior/subsequent levels play a role; and d) the origin of non-target phonemes in non-word errors. Analysis of phonological overlap between non-word errors and their targets suggested that participants were consistently accessing target phonology, with non-word errors falling along a continuum of how much target phonology was available. Experiments involving manipulation of phonological and semantic activation through characteristics of target words (Chapters 2 & 3: Principal Components Analysis comprising variables such as length, frequency and imageability; Chapter 5: Phonological Neighbourhood Density experiment) or semantic/phonological blocking paradigms (Chapter 4) suggested that increasing the activation reaching target phonemes improves output. A comparison of performance on different tasks (picture-naming, word/non-word repetition) where the majority of individuals showed a difficulty activating phonemes across tasks, and consistent effects of phonological-level variables, supported a phonological-level breakdown as the primary difficulty. Finally, analysis of the non-target phonemes produced by participants suggested that highly-activated phonemes can be selected in place of weakly activated target phonemes. Non-target phonemes may become more activated than target phonemes because they have been previously produced (i.e. perseveration), are high frequency, or a semantic or phonological relationship exists between a competing word and the target. It is argued that non-word errors in jargon aphasia arise, at least in part, from insufficient activation of target phonology. This can result from insufficient activation at any level of the lexical system, but the deficit is primarily localised to the phonological system.