Dysfluency in Huntington's disease, Parkinson's disease and schizophrenia

Verbal fluency was compared in Huntington's disease (n=12), Parkinson's disease (n=25) and schizophrenia (n=18) with a series of fluency tasks evaluating the effects on productivity of cuing with subordinate categories and alternation between fluency probes. Findings indicated reduced output in the patient groups across all tasks consistent with a difficulty in intrinsic generation. A significant group interaction was observed with cue provision, but not with alternation. The facilitation of performance with cuing in all groups suggests a common mechanism of disruption in these disorders, most likely reflecting interruption of the prefrontal modulation of retrieval processes. To explain the observed pattern, we propose that the semantic retrieval anomaly in these disorders reflects a difficulty in intrinsic extra-dimensional shift. Verbal productivity was significantly associated with negative dimensions of schizophrenia, such as poverty of speech and flattened affect, a finding that is discussed in terms of the neurocognitive heterogeneity of schizophrenia and theories of cognitive dysfunction in subtypes of this disorder.