Audiology and Speech Pathology - Theses
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ItemCentral auditory processing in schizophrenia patients who experience auditory hallucinations( 2013)Central Auditory Processing can be conceptualised as how the brain interprets auditory stimuli. Disordered auditory processing may lead to difficulties interpreting complex auditory signals such as speech, even where normal sound detection thresholds are recorded. Auditory verbal hallucinations are the perception of hearing voices “in the head” in the absence of auditory stimuli, and occur in over 70% of patients with schizophrenia. Impaired central auditory processing in schizophrenia patients with a history of auditory hallucinations has been reported, and evidence suggests possible dysfunction of interhemispheric transfer in auditory circuits in this group. This thesis investigated the hypothesis that connectivity between left and right auditory cortical regions may be reduced in schizophrenia patients who experience auditory hallucinations, compared with both non-hallucinating patients and healthy controls. Experiments 1 and 2 used electrophysiological techniques to investigate the hypothesis. EEG was recorded continuously while subjects listened passively to pure tones and single-syllable words, presented monaurally to each ear. Experiment 1 used temporal information obtained from latency measures of the auditory N1 evoked potential to examine interhemispheric connectivity between auditory cortical regions. The Interhemispheric Transfer Time (IHTT) was calculated by comparing latencies of the auditory N1 potentials recorded contralaterally and ipsilaterally to the ear of stimulation, for each stimulus type. No between-group differences of IHTT were found for pure tone stimuli. For word stimuli however, significant between-group differences were found: the IHTT was highest in the hallucinating patient group, close to zero in the control participants, and was negative (shorter latencies ipsilaterally) in the non-hallucinating patients. Experiment 2 used the technique of EEG spectral coherence to examine interhemispheric functional connectivity between cortical areas known to be important in the processing of auditory information. Upper alpha and upper beta band interhemispheric coherence was calculated from six pairs of electrodes located over homologous auditory areas in the left and right cerebral hemispheres. In the upper alpha band, significant between-group differences were found on four electrode pairs (C3-C4, C5-C6, Ft7-Ft8 and Cp5-Cp6). Of the three groups, coherence was lowest in the hallucinating patient group, consistent with the hypothesis that interhemispheric connectivity may be reduced in these patients. Experiment 3 used two behavioural auditory tasks which are thought to be sensitive to interhemispheric transfer difficulties – the Staggered Spondaic Words (SSW) and the Frequency Pattern Test (FPT) – to assess interhemispheric transfer function and dysfunction in schizophrenia patients. Although a wide range of abilities were found in both patient groups, overall group results from the behavioural data suggest reduced interhemispheric connectivity in auditory hallucinating patients. These behavioural results were correlated with coherence data from Experiment 2, and significant correlations were found. In conclusion, this thesis provides evidence which supports the hypothesis that, in schizophrenia patients who experience auditory hallucinations, the transfer of auditory information between the left and right auditory cortices via the corpus callosum is less efficient than in healthy people.