Parallel distributed processing and the emergence of schizophrenic symptoms.

Computer simulations of parallel distributed processing (PDP) neural networks have increased our understanding of brain functioning. This article reviews how PDP concepts can contribute to our understanding of schizophrenic symptoms. Psychotic states induced by phencyclidine and the adult form of metachromatic leukodystrophy, as well as neurometabolic studies, suggest that schizophrenia reflects a breakdown in communication between cortical areas. A computer simulation of this type of brain pathology has suggested two neurocognitive consequences: some cortical circuits will become functionally autonomous, and a subset of these circuits will yield "parasitic foci" that slavishly reproduce the same cognitive output. Delusions of control, paranoid delusions of the idee fixe type, thought broadcasting, "voices," and certain deficit symptoms are postulated outcomes of parasitic foci located at different levels of language processing. A neurodevelopmental model of impaired corticocortical communication is described, and this model's implications for further study are outlined.