The achievements of neuroimaging in psychiatry have been modest, although perhaps no less than those of any other technique for investigating the cause or mechanism of mental disorders. The cardinal reason for this limited success is the immense complexity of the human mind and brain. The problem is not a lack of studies reporting intriguing findings, but rather the lack of consistency between them. Two of the earliest imaging findings, ventricular enlargement and hypofrontality in schizophrenia, both of which were reported more than a quarter of a century ago, have eventually been confirmed in numerous studies, but fundamental aspects of these abnormalities, such as the time course of ventricular enlargement and the circumstances under which hypofrontality can be elicited, remain uncertain on account of inconsistent findings.

In contrast to the modest advances in understanding of mental disorders, the rate of evolution of imaging technology and of study design is breathtaking. This book provides a taste of a variety of contemporary strategies. Among the recent technological advances reported are diffusion tensor imaging, which provides information regarding the orientation of myelinated fibres, and the use of transcranial magnetic stimulation (TMS) with functional imaging techniques. TMS provides reversible disruption of brain function at a localised cerebral site, and hence makes it possible to examine the effect of transient loss of local function on the pattern of cerebral activity elsewhere in the brain. Thus, we now have the technology to assess both the structural connections (mediated by myelinated fibres) between brain regions, and the remote functional consequences of local malfunction. Perhaps these techniques will illuminate the way in which the coordination of activity in distributed neural networks is disrupted in disorders such as schizophrenia.

The past few years have also seen the development of many new approaches to experimental design in the imaging of regional brain function and also in the imaging of neuroreceptors. The use of positron emission tomography (PET) or single photon emission computed tomography (SPECT) with displaceable ligands to provide an indirect measure of the release of endogenous neurotransmitters has opened the door to many exciting possibilities. It has also provided a partial resolution of the perplexing conflict between previously reported findings regarding D₂ receptor levels in schizophrenia. Previous studies using the ligand raclopride indicated no elevation of D₂ receptor density in the basal ganglia in schizophrenia, whereas studies using the ligand N-methyl spiperone did indicate an elevation. Although the mechanism is imperfectly understood, it is now clear from the work of Laruelle and colleagues that substituted benzamide ligands, such as raclopride, are displaceable by endogenous dopamine, whereas ligands such as N-methyl spiperone are not. Use of radio-actively labelled raclopride to measure D₂ receptors before and after treatment with a dopamine-depleting drug reveals that the level of endogenous dopamine is abnormally high in schizophrenia, suggesting that when raclopride is used to measure D₂ receptor density, an elevated level of endogenous dopamine obscures an abnormally high concentration of D₂ receptors. Thus, the current evidence indicates not only that D₂ receptor density is increased in schizophrenia, but also that there is an excessive release of endogenous dopamine.

The technique for measuring endogenous dopamine offers the prospect of measuring the effect of various pharmacological or psychological challenges that might be expected to modify dopaminergic activity. However, enthusiasm for this technique should be tempered by the fact not only that the physiological basis of the effect is imperfectly understood, but also that the effects of interest are small and difficult to measure.

Although many psychiatric imaging studies in the past three decades have focused on schizophrenia, this book draws attention to the rapid rise in the use of elegant experimental designs to study other disorders. Whalen, Curran & Rauch provide a stimulating account of strategies for evaluating implicit information processing that appears to play a key role in anxiety disorders. Several chapters report advances in the study of mood disorders. In particular, the chapter by Drevets addresses the likely reasons for the conflicts between previously reported studies of regional cerebral activity in depression, and argues cogently for integrating structural and functional imaging modalities. Pine et al provide a clear account of the way in which functional imaging has contributed to the understanding of attention-deficit hyperactivity disorder.

Overall, this volume provides an introduction to most of the major developments in psychiatric neuroimaging technology that occurred in the final decade of the 20th century. As would be expected from a multi-author volume, the quality of the technical detail varies, but overall, the editors have done a good job in ensuring a comprehensive coverage without much overlap between chapters. The book ends with a thought-provoking chapter by Kosslyn & Plomin on neurocognitive genetics. They argue convincingly that imaging studies designed to delineate individual differences in brain structure and function might bridge the gap between genes and behaviour. Their chapter was apparently written before the publication of the study by Egan et al (Reference Egan, Goldberg and Kolachana2001) demonstrating the influence of variation in the gene specifying the enzyme catechol-O-methyltransferase on dopaminergic modulation of frontal lobe function. That study ushered in a potentially exciting era in which the complementary use of neuroimaging and genetic strategies offers the prospect of major advances in understanding of the causes and mechanisms of mental disorders.