Semantic processing deficits in patients with Parkinson's disease: degraded representation or defective retrieval?

Objective: To determine whether degraded representations (characterized by small differences between word sense frequencies), or defective competitive processes thigh levels of word sense lateral inhibition), individually or jointly, can give rise to parkinsonian semantic deficits. Design: Computer model of smeantic processing. Outcome measures: Correct sense selection, defined by the activation of the word sense unit that first reaches the 0.5 activation threshold. If Parkinson disease (PD)-like errors are observed only at high levels of lateral inhibition, independently of low or high sense frequency deltas (SFDs), this would indicate that a defective competitive process alone could account for the errors. Alternatively, if PD-like errors were observed at any level of lateral inhibition, exclusively with low SFD words, this would indicate that degraded representations alone could account for the errors. Results: Neither degraded representations nor defective competitive processes alone can account for parkinsonian semantic errors. An interaction between the 2, however, correctly reproduces both increased errors and longer latency responses. Conclusions: Competing explanations for semantic deficits in patients with Parkinson's disease need to be integrated in order to develop effective interventions (e.g., estimating the amount of context required to improve semantic processing performance).