Who is that talking? Function and dysfunction of efference copy and corollary discharge in healthy people and schizophrenic patients
The concept of efference copy and corollary discharge is an attempt to explain the phenomenon that most of us recognize the outcome of self-performed actions as such and not as the result of externally generated stimuli. The planning of every motor act generates an efference copy, which is used to calculate the sensory experience this motor act will evoke and prepare the parts of the brain involved in the processing of sensory information for the upcoming stimulation. Thus, self-generated stimuli are "expected" and so the neural response to these stimuli is attenuated compared to externally generated sensory stimulation. This mechanism enables the brain to distinguish "self" from "other" A dysfunction in this internal model has been hypothesized by Frith and colleagues (e.g. Frith & Done, 1988; Frith, 1992) to underlie schizophrenic symptoms such as auditory verbal hallucinations and delusions of control. In this thesis the function and dysfunction of the efference copy mechanism during the motor act of speaking was investigated in 5 experiments A-E: Experiment A was designed to investigate how the efference copy/corollary discharge mechanism influences the neural responses to self produced speech sounds when they either match their internal predictions or when they are externally (experimentally) altered and do not match. The results show that the N100 event-related response measured using Electroencephalography was most suppressed when the subjects received auditory feedback of their own unaltered voice. It can be concluded that there is a specific forward model operating in the auditory modality, specifically suppressing neural responses only to auditory stimulation that matches closely its internal prediction. Experiment B used Magnetoencephalography to achieve a precise localization of the varying, feedback-dependent activation. Neuromagnetic Source Imaging clearly revealed auditory cortex as the place where stimuli are processed differently depending on their degree of match to their prediction or corollary discharge. In Experiment C, Frith's (1992) theory was investigated that some schizophrenic symptoms may be caused by a dysfunction in the corollary discharge mechanism. Using EEG and the same design as in experiments A and B, normal controls, patients with, and patients without hallucinations were investigated. Results show that while normal controls showed a suppressed N100 to the feedback of the "predicted" unaltered speech sound, the N100 in the group of Hallucinators did not differ between the 4 feedback conditions. The amount of suppression shown by the patients correlated with the extent of hallucinations reported in the clinical interview. These results strongly support the theory of the involvement of a corollary discharge deficit in the generation of auditory hallucinations. In Experiment D, the question investigated was whether the corollary discharge can be influenced by foreknowledge of an external event that will manipulate the outcome of a motor act and whether the prediction of the actual sensory feedback of that motor act is adjusted accordingly. Subjects produced speech sounds at a fast rate, which were infrequently altered in pitch. If the pitch-change occurred without foreknowledge, subjects reacted with large a Mismatch Negativity (MMN). If subjects were notified of the upcoming pitch-shift, however, the MMN was considerably reduced. If subjects were passively listening, predictability did not influence the size of the MMN. Schizophrenic patients did not show differences in MMN amplitude during Speaking. This was interpreted as further support for a deficient corollary discharge generation in schizophrenia. Finally, Experiment E was designed to clarify whether the high rate of misattributions made by patients in Experiment C, may simply be due to an inability of schizophrenic patients to discern differences between their own unaltered speech sounds, their speech sounds altered or someone else's voice. Results did not show any differences in the behavioral performance or the MMN pattern between patients and controls, suggesting that patients do not suffer from a general voice discrimination problem. In summary, this dissertation accomplished to show the extent of accuracy to which healthy auditory cortices are "prepared" for self-generated speech sounds: An attenuated auditory cortical response only occurred when the speech produced was not altered or substituted by an alien voice. This supports the idea of a precise efference copy mechanism (rather than a general gating mechanism) operating in the auditory modality. This thesis further demonstrated how a dysfunction in this mechanism is related to schizophrenia and especially auditory verbal hallucinations - a strong support of Frith & Done's (1988) model of a "neuropsychological approach toward schizophrenia".
Magnetencephalogramm, Ereignis-korrelierte Potentiale, corollary discharge, efference copy, auditory cortex, speech production, schizophrenia
Networked Digital Library of Theses and Dissertations