Functional neuroimaging of hallucinations in schizophrenia: toward an integration of bottom-up and top-down approaches

Hallucinations in schizophrenia represent an important clinical problem, an interesting neuropsychological enigma, and a significant challenge for neuroscientific research. Functional neuroimaging techniques allow the in vivo, systems-level study of brain dysfunction underlying this debilitating symptom. Clinical and scientific vantage points that can inform the design and interpretation of functional neuroimaging studies of schizophrenic hallucinations are outlined. These include considerations of the phenomenology of hallucinations, the relationship of hallucinations to other symptoms of schizophrenia, and the neuropsychological functions that are thought to be disrupted in hallucinations. They also include the anatomical and chemical brain systems in which abnormalities are implicated in schizophrenia, the neurologic conditions in which hallucinations may occur, the neurochemical contexts that are associated with hallucinations, and the methodologic details of the functional neuroimaging techniques employed. Bottom-up and top-down functional neuroimaging strategies for the investigation of schizophrenic hallucinations with positron emission tomography (PET), single photon emission computed tomography (SPECT) and functional magnetic resonance imaging (fMRI) are reviewed. Bottom-up approaches start with or measure the biology associated with hallucinations. Top-down approaches start from the specific neuropsychological dysfunctions thought to be associated with hallucinations. The distributed brain regions, systems and functions implicated in schizophrenic hallucination formation are then discussed in the context of an integration of bottom-up and top-down approaches. Focus is placed upon abnormalities in the functions of, and interactions among, auditory-linguistic association cortices, caudal and rostral limbic/paralimbic systems, prefrontal cortices, ventral striatum and (non-specific projection and associative) thalamic nuclei, as well as upon the glutamatergic, GABAergic and ventral tegmental dopaminergic modulation of these systems.