• K B Böcker, R Hijman, R S Kahn, and E H De Haan.
• Department of Psychonomics and Psychology, Health Research School, Utrech University, The Netherlands. K.B.E.Bocker@pharm.uu.nl
• Br J Clin Psychol. 2000 Nov 1; 39 ( Pt 4): 397-406.

ObjectivesIn this study the hypothesis was tested that hallucinations result from confusing external and internal stimulus sources, i.e., perception and imagery, respectively.Design And MethodsThirteen hallucinating and 19 non-hallucinating schizophrenic patients, as well as 14 control participants performed multiple tests of perception, vividness of mental imagery and the ability to discriminate between them (reality discrimination). These functions were tested in both the auditory and the visual modalities.ResultsThere were no group differences on perceptual acuity. The results on one imagery task indicated that for the hallucinating patients, the relative, but not the absolute, level of vividness of mental images might be higher in the auditory modality, which was the modality in which 12 of the patients also experienced hallucinations, than in the visual modality. Finally, there was a positive relationship between severity of (auditory) hallucinations and reality discrimination problems.ConclusionsHallucinations may result from increased vividness of mental imagery, and their severity increases with larger impairments in reality discrimination. It is recommended that research into, and cognitive behavioural therapy for, hallucinations should also focus on their sensory qualities.

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