A NEURAL-NETWORK SIMULATION OF HALLUCINATED VOICES AND ASSOCIATED SPEECH-PERCEPTION IMPAIRMENTS IN SCHIZOPHRENIC-PATIENTS

The mechanism of hallucinated speech, a symptom com monly reported by schizophrenic patients, is unknown. The hypothesis that these hallucin ations arise from pathologically altered working memory underlying spe ech perception was explored. A neural network computer simulation of c ontextually guided sequential word detection based on Elman (1990a,b) was studied. Pruning anatomic connections or reducing neuronal activat ion in working memory caused word ''percepts'' to emerge spontaneously (i.e.,in the absence of external ''speech inputs''), thereby providin g a model of hallucinated speech. These simulations also demonstrated distinct patterns of word detection impairments when inputs were accom panied by varying levels of noise. In a parallel human study, the abil ity to shadow noise-contaminated, connected speech was assessed. Schiz ophrenic patients reporting hallucinated speech demonstrated a pattern of speech perception impairments similar to a simulated neural networ k with reduced anatomic connectivity and enhanced neuronal activation. Schizophrenic patients not reporting this symptom did not demonstrate these speech perception impairments. Neural network simulations and h uman empirical data, when considered together, suggested that the prim ary cause of hallucinated ''voices'' in schizophrenia is reduced neuro anatomic connectivity in verbal working memory.