One prominent core characteristic of schizophrenia is the occurrence of hallucinations, which are perceptions in the absence of external stimuli. Almost everyone has on occasion experienced things that are not there, but for many patients with schizophrenia the perceptions are frequent and seem to be very real. Although the most common hallucinations are auditory, the experience varies from individual to individual. The voices may be familiar ones such as a parent or spouse, or unrecognized voices that may be interpreted as “foreign agents,” such as a radio transmitter implanted in the individual’s head, or as the voices of angels. In many cases words and/or their meanings are not intelligible, but the message is clearly understood by the individual. Unfortunately, although the message may be harmless, such as a suggestion to mow the lawn, often the voices are harsh, critical, or demanding. The voices may accuse the individual of inappropriate actions, such as having raped a woman when in fact he just passed her on the street. In other instances, the voices direct the individual to perform certain behaviors that can lead to violent and destructive ends. Finally, although in many cases the hallucinations persist over long periods, in other cases the voices occur primarily when the individual is not engaged in a task or has limited sensory input.
Among the more exciting research into the etiology of hallucinations is the use of PET or SPECT imaging to scan the brain during an ongoing hallucination. Several laboratories have produced intriguing results, although the functional neuroanatomy of the hallucinations is not identical in each individual. When the scans of several individuals were compared (Figure 1), some consistent results became apparent. First, in the vast majority of cases, the areas activated were located in the left cerebral hemisphere, which reflects the characteristic auditory–linguistic nature of the hallucinations and the fact that the left hemisphere specializes in language function in the majority of people. The most active brain areas are within the auditory–linguistic association cortex rather than the primary cortex, which is consistent with the internally generated nature of the experience. Psychological theories have suggested that auditory hallucinations represent “inner speech,” that is, the individual is listening to his or her own thoughts and cannot tell the difference from listening to an outside source. However, Broca’s area in the frontal lobe, which is responsible for generating fluent speech, is not consistently activated, so inner speech may be involved but may not be the central pathological process. Left temporal lobe sites (including Wernicke’s area and the middle temporal gyrus) are usually active during the hallucination, and electrical stimulation of these areas can produce auditory hallucinations in a healthy individual. Further, temporal lobe activity during epileptic seizures is often associated with auditory hallucinations.
Other neocortical areas that are active reflect the content of the individual hallucination. Activation in the visual association cortex (specialized for higher-order visual perception) accompanies visual components of a hallucination. We might add that the classic hypofrontality (reduced activity in prefrontal regions) associated with schizophrenia may also contribute to the phenomenon. Hypofrontality and the resulting lack of inhibition and internal monitoring may lead the individual to falsely attribute internally generated perceptions to the external environment.
Somewhat surprising is that it is not only cortical areas that are active. There is also a common pattern of deep brain activity: subcortical regions including the thalamus and limbic (e.g., hippocampus, cingulate cortex) and parahippocampal areas show increased activity and are probably responsible for attention and the emotional component of the experience. These areas are highly interconnected with one another and with the activated association cortices. Cellular abnormalities in these brain regions are commonly reported in schizophrenia, and abnormal dopamine and glutamate activity are believed to be involved in disrupting these cortical–subcortical circuits, producing symptoms (see textbook Chapter 19). Further details on the functional neuroanatomy of hallucinations can be found in a variety of sources (McGuire et al., 1993; Silbersweig et al., 1995).
McGuire, P. K. Shah, G. M. S., and Murray, R. M. (1993). Increased blood flow in Broca’s area during auditory hallucinations in schizophrenia. Lancet, 342, 703–706.
Silbersweig, D. A., Stern, E., Frith, C., Cahill, C., Holmes, A., Grootoonk, S., Seaward, J., McKenna, P., Chua, S. E., Schnorr, L., Jones. T. and Frackowiak, R. S. J. (1995). A functional neuroanatomy of hallucinations in schizophrenia. Nature, 378, 176–179.