Parkinson�s disease is a neurodegenerative disorder, which is typically characterised in terms of its debilitating effects on motor function. However, ubiquitous neuropsychological deficits are also an integral feature of the progression of this disease. This thesis investigated these cognitive deficits as they manifest in language, with the overarching goal being to elucidate the word-finding problems that are associated with Parkinson�s disease. Making semantic judgements and identifying semantic relations are two processes that are particularly germane to word-finding. Therefore, the present thesis examined: 1) the ability of people with Parkinson�s disease to make judgements about semantic categories, and 2) the integrity of associative semantic networks in Parkinson�s disease. In the first series of studies, Cups and Bowls, a novel semantic categorisation task was used to investigate the ability of people with Parkinson�s disease to consistently categorise common kitchen items across a number of trials. The Parkinson�s group was impaired relative to an age-matched control group on this task. This inconsistent categorisation was particularly apparent for the less typical category exemplars at the category boundaries, suggesting that the Parkinson�s group had less salient or less elaborated semantic categories, which particularly compromised categorisation of the less typical category exemplars. This finding is discussed in terms of selective attention deficits and inappropriate weightings of semantic features.
In the second series of studies, Verbal Memory, the structure of the semantic network and access to the semantic system were further investigated using a verbal memory task, which required participants to recall word lists. These word lists consisted of semantically associated words and were designed to elicit false recall of another, non-presented, close semantic associate (the critical lure). The results of this second series of studies, particularly the fact that the Parkinson�s group recalled more of the false critical lures than the control group, suggested an intact semantic network in Parkinson�s disease and normal saliency of semantic categories. The potentiated false recall effect in the Parkinson�s group is discussed in terms of poor modulation of attention in Parkinson�s disease, both as the result of an executive deficit leading to poor controlled processing and in terms of a dopamine-modulated decrease in the signal-to-noise ratio leading to impaired automatic processing. Taken together, the results reported in the present thesis suggest that basal ganglia pathology and striatofrontal deafferentation in Parkinson�s disease do not diminish the integrity of semantic memory, but do compromise operation of semantic memory due to impaired modulation of activation/inhibition mechanisms. This finding of a selective attention deficit has implications for word-finding, suggesting that the word-finding difficulties associated with Parkinson�s disease are the result of impaired lexical access. In particular, retrieval of specific lexical items from semantic memory is impeded because of failure to modulate activation/inhibition mechanisms effectively for the target word to be distinguished from close semantic associates. An intact semantic checking mechanism in anterior language cortex prevents the production of semantic paraphasias, and results in the tip-of-the-tongue word-finding problems displayed by some people with Parkinson�s disease.
| Identifer | oai:union.ndltd.org:ADTP/217411 |
| Date | January 2006 |
| Creators | Saunders, Vickie Ellen, n/a |
| Publisher | University of Otago. Department of Psychology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Vickie Ellen Saunders |
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