Psychoacoustics and hallucinating schizophrenics. A psychobiological approach to schizophrenia

Psychoacoustic experiments have been conducted for schizophrenic patients with auditory hallucinations in their case history and for reference subjects. The aim of the thesis has been to assess differences between the two groups of subjects. The experiments consisted of tests on perceptual grouping mechanisms measuring how sound with high and low pitches is separated into perceptual streams (Study I ), how sound, interrupted by noise, is restored into a meaningful percept (Study II), and how localisation of components of sound is influenced by frequency spectrum and amplitude (contralateral induction ,Study III). In one study (Study IV) eight click-sounds were used in an illusion test, normally perceived as a train of events with equidistant steps from left to right. The last study (Study V) concerned the influence of pre-test learning on streaming appearing by pitch and amplitude. The schizophrenic patients reported fewer and aberrant experiences of streaming and restoration as compared to reference subjects. They were more directed by frequency content than the reference subjects in the contralateral induction experiment. In the directional illusion test, they did not, as the reference subjects, perceive the stimulus as passing in equidistant steps, but as being directed towards the right position. Learning was found to normalize the perception of streaming for the chronic, but not for the more acutely ill schizophrenics. The results are discussed in relation to the psychoacoustic mechanisms involved and with reference to symptoms and neuropathophysiology in schizophrenia. It is concluded that schizophrenics form aberrant streams and utilize restoration to a lesser extent. Their localization perception is also aberrant. Pre-test learning exerts an ameliorating effect on perception of streaming for schizophrenics. Intraindividual variation is great among them, with patchy patterns of deficient function of various psychoacoustic systems. Cortex plays a role in the creation of illusory percepts, but disturbances of brain-stem nuclei cannot be ruled out as a contributing pathogenetic factor