Auditory hallucinations inhibit exogenous activation of auditory association cortex.

Percepts unaccompanied by a veridical stimulus, such as hallucinations, provide an opportunity for mapping the neural correlates of conscious perception. Functional magnetic resonance imaging (fMRI) can reveal localized changes in blood oxygenation in response to actual as well as imagined sensory stimulation. The safe repeatability of fMRI enabled us to study a patient with schizophrenia while he was experiencing auditory hallucinations and when hallucination-free (with supporting data from a second case). Cortical activation was measured in response to periodic exogenous auditory and visual stimulations using time series regression analysis. Functional brain images were obtained in each hallucination condition both while the patient was on and off antipsychotic drugs. The response of the temporal cortex to exogenous auditory stimulation (speech) was markedly reduced when the patient was experiencing hallucinating voices addressing him, regardless of medication. Visual cortical activation (to flashing lights) remained normal over four scans. From the results of this study and previous work on visual hallucinations we conclude that hallucinations coincide with maximal activation of the sensory and association cortex, specific to the modality of the experience.