New methods for studying hallucinated 'voices' in schizophrenia.

The mechanism of hallucinated speech or 'voices', a symptom commonly reported by schizophrenic patients, is poorly understood. We have undertaken two types of study to explore the hypothesis that this symptom arises from pathologically altered speech perception networks. The first consists of neural network computer simulations of narrative speech perception. We have shown that if these networks are partially disconnected or undergo a 'monamine' neuromodulatory disturbance, 'hallucinated speech' (speech percepts occurring in the absence of phonetic input) are simulated as well as specific speech perception impairments. The latter finding prompted us to conduct parallel studies of actual schizophrenic patients using a 'masked speech tracking' (MST) task. MST requires subjects to track narrative speech the phonetic clarity of which is reduced with superimposed multispeaker 'babble.' Hallucinators demonstrated speech perception impairments and experimentally induced perceptual 'illusions' which clearly differentiated them from non-hallucinators and normal controls. Our results support the hypothesis that 'voices' are spurious products of altered speech perception networks.