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| Speaker: | Malti Patel |
| Lecturer, Department of Computing, Macquarie University | |
| Date: | Tuesday 27th October 1998 |
| Time: | 11:30 - 12:30 |
| Place: | Seminar room 357, Building E6A, Macquarie University |
Abstract:
Using the connectionist model of the Dual Route Cascaded Reading Model (DRC) and its semantic component, we have simulated some findings on semantic errors made by patients with Parkinson's Disease.
Recent clinical and experimental studies have indicated that lexical-semantic processing suffers the greatest impairment from the degradation of dopaminergic pathways in the mesolimbic forebrain and neocortex after the onset of Parkinson's Disease (PD). PD patients make more errors and have slower responses in semantic judgement tasks compared to age-matched controls, even though non-semantic performance on word-search tasks is unimpaired. In this study, these findings are used to predict the performance of the semantic system of the dual-route model of reading (DRC), where the connection strengths between word and word sense levels in the system are systematically manipulated to produce PD-like reductions in dopamine transmission (i.e. a reduction of gain). Low excitation between the word and word sense levels is predicted to produce PD errors in semantic judgements of polysemous words which have a low sense-frequency delta (s.f.d.), that is, a small difference in sense frequency between at least two word senses. For example, it is more likely that an error will be made if the two senses of the word BANK (i.e. money and river) have frequency values which are very close together, as opposed to if their frequency values are far apart. The DRC model was tested with 450 words; frequency values for all senses of these words were known. Results showed that judgements made on high s.f.d words resulted in 100% correct responses over all excitation values between the word and word sense levels, with a stable solution reached at 39-48 epochs. However, for low s.f.d words, incorrect responses were made for low to medium values of excitation (30-50% errors), only providing the correct solution for medium to high values of excitation. An interaction was also observed for low s.f.d words between high lateral inhibition in the word sense level, and low-medium excitation between the word and word sense levels, to produce the results most characteristic of PD semantic errors. The results of this study support the findings that competitive and inhibitory mechanisms related to dopaminergic gain are responsible for impaired PD performance on semantic processing tasks.Enquiries: sals@mri.mq.edu.au
| Last modified: October, 1998 |