People with schizophrenia who experience auditory-verbal hallucinations experience difficulty in determining the source (self vs. other) of verbal information, and those with visual hallucinations experience a conceptually similar problem with visual information. In this study, we examined whether such source monitoring deficits extend to olfaction for olfactory hallucinators and whether they are selective to the modality in which the hallucination is experienced. To test these claims, three groups were formed: normal controls (NC), people with schizophrenia who experience olfactory hallucinations (OH), and people with schizophrenia who experience auditory-verbal hallucinations (AVH). These three groups were then tested on both an olfactory and an auditory-verbal source-monitoring task. We found evidence of a modality-specific impairment. The OH group was less accurate in determining whether an odor had been imagined or smelled relative to NC and AVH groups. In contrast, the AVH group was least accurate in determining the source of a word, relative to the OH and NC groups. These findings provide the first evidence of a source-monitoring impairment in schizophrenic participants with OHs and suggest that this impairment is modality specific.
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