Psychopathology 2015[]; 48 (6): 376-85 PMID26361209show ga
Background: Ketamine, the NMDA glutamate receptor antagonist drug, is increasingly employed as an experimental model of psychosis in healthy volunteers. At sub-anesthetic doses, it safely and reversibly causes delusion-like ideas, amotivation, and perceptual disruptions reminiscent of the aberrant salience experiences that characterize first-episode psychosis. However, auditory verbal hallucinations (AVHs), a hallmark symptom of schizophrenia, have not been reported consistently in healthy volunteers even at high doses of ketamine. Methods: Here we present data from a set of healthy participants who received moderately dosed, placebo controlled ketamine infusions in the reduced stimulation environment of the magnetic resonance imaging scanner. We highlight the phenomenological experiences of three participants who experienced particularly vivid hallucinations. Results: Participants in this series reported auditory verbal and musical hallucinations at a ketamine dose that does not induce auditory hallucination outside of the scanner. Discussion: We interpret the observation of ketamine-induced AVHs in the context of the reduced perceptual environment of the magnetic resonance scanner, and offer an explanation grounded in predictive coding models of perception and psychosis: the brain fills in expected perceptual inputs and it does so more in situations of reduced perceptual input. The reduced perceptual input of the MRI scanner creates a mismatch between top-down perceptual expectations and the heightened bottom-up signals induced by ketamine; such circumstances induce aberrant percepts including musical and auditory verbal hallucinations. We suggest that these circumstances might represent a useful experimental model of AVHs and highlight the impact of ambient sensory stimuli on psychopathology.
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Psychopathology 2015 ; 48 (6): 376-85
