Chance et al (Reference Chance, Esiri and Crow2002) described volumetric measurement of the amygdala and found few differences between normal and schizophrenia post-mortem samples. This fails to confirm published magnetic resonance imaging (MRI) data on hundreds of individuals which have been systematically reviewed and analysed (Reference Wright, Rabe-Hesketh and WoodruffWright et al, 2000). Chance et al (Reference Chance, Esiri and Crow2002) report mean absolute volumes (643 mm³ for nine men and 612 mm³ for nine women) that are much smaller than those reported in MRI studies. They go on to speculate on the reasons for this discrepancy and point to ‘limitations’ in both MRI and meta-analysis. The authors are right to highlight the problem of anatomical definition of the amygdala in vivo and how other imaging parameters may obscure (or reveal) laterality effects and differences between subject groups. However, they are wrong to blame meta-analysis. Systematic review and meta-analysis of MRI data is a powerful means of quantifying the precise effects that are the subject of speculation by Chance and colleagues.

We have recently carried out just such a review of the normal human amygdala (Reference Brierley, Shaw and DavidBrierley et al, 2002). Some 39 studies and 51 data-sets met our inclusion criteria, allowing comparison of 1491 amygdala pairs. The weighted mean volumes (95% CI) for the left and right amygdala were 1726.7 mm³ (35.1) and 1691.7 mm³ (37.2), respectively. The range was from 1050 to 3880 mm³. This variance is clearly worrying. We were able to examine systematically some of the causes of this and found that most imaging parameters, such as slice thickness and plane of orientation, were not particularly influential. Study size had a weak but significant effect, with larger studies tending to give smaller volumes. Anatomical definition was the most important variable. Studies which employed the Watson criteria (Reference Watson, Andermann and GloorWatson et al, 1992) gave significantly larger volumes than the remainder. Gender differences persisted (male greater than female) even in studies which attempted to control for intracranial volume. Laterality effects were not significant.

The ease of obtaining high-resolution anatomical brain images afforded by modern MRI on large samples of individuals across the life span means that MRI should be taken as the gold standard on regional volumetrics rather than post-mortem samples with all their attendant deficiencies. However, in order to exploit the advantages of MRI, researchers must pay particular attention to reliability of anatomical definitions. We have proposed that Watson's criteria be adopted generally and have recommended some minor improvements (Reference Brierley, Shaw and DavidBrierley et al, 2002).