Cognitive and neural mechanisms underlying auditory verbal hallucinations in a non-clinical sample

Abstract

Auditory verbal hallucinations (AVHs) are the experience of hearing a voice in the absence of any speaker. Cognitive models of AVHs have suggested that they may occur when an internal mental event, such as inner speech, is misattributed to an external source. This has variously been explained by reference to biases in self-monitoring, source monitoring, or reality discrimination processes. Evidence suggests that, mechanistically, this may be related to atypical functioning of a forward model system which usually predicts the outcome of self-generated actions, attenuating activity in sensory cortices to the resulting perceptual input. At a higher level, excessive vividness and low cognitive effort associated with internal mental events may be associated with external misattributions of inner speech. Chapter 1 reviews inner speech models of AVH, as well as recent attempts to reduce the frequency of AVHs using neurostimulation. Chapter 2 then provides a methodological overview of techniques used in this thesis.
The first two empirical studies presented in this thesis, in Chapters 3 and 4, explore the cognitive mechanisms underlying AVHs by investigating the associations between self-reported hallucination-proneness and phenomenology of inner speech, and performance on source monitoring and self-monitoring tasks, in a non-clinical, student sample. The results indicated that hallucination-prone participants were more likely to misattribute self-generated auditory verbal imagery, both when instructed to generate imagery and when they retrospectively reported using imagery. Regression analysis also indicated that a tendency to use dialogic inner speech, biased performance on reality discrimination and self-monitoring tasks, and a tendency to perceive meaning in jumbled speech independently predicted hallucination-proneness.
The studies presented in Chapters 5 and 6 investigated the neural basis of performance on auditory signal detection and source monitoring tasks using transcranial direct current stimulation (tDCS). Results indicated that modulating activity in the superior temporal gyrus/temporoparietal junction (STG/TPJ) affected the number of false perceptions on the signal detection task. However, stimulation to the left STG or medial prefrontal cortex did not affect performance on a source monitoring task. These results indicate that different cortical regions may be involved in the two tasks, and hence that they may reflect different aspects of how self-generated actions are experienced as such. Together, the four experimental chapters 1) provide evidence for inner speech accounts of AVH, 2) indicate the need for a more complex account of self-monitoring and reality discrimination in which both are seen as independent predictors of AVHs, and 3) suggest that the left STG plays a key role in reality discrimination, but less so in source monitoring tasks (at least in the encoding stage). The thesis concludes with a general discussion of these issues, and recommendations for future research.