I am writing to point out an error in our paper on cerebral perfusion correlates of depressed mood (Reference Ebmeier, Cavanagh and MoffootEbmeier et al, 1997). Although the description of methods in the paper is accurate, the interpretation of the interaction between condition (time of day) and depression scores (Befindlichkeitsscale (BFS)) is not accurate. This interaction does not represent the “within-subjects correlation with BFS change”, but the change in correlation between cerebral perfusion and symptom scores from morning to evening.

Consequently, “correlations with BFS” in the text ought to read “ diurnal changes in correlations”. The reported “positive correlation between severity of depression and anterior limbic perfusion” is, in fact, a significantly stronger correlation between symptom scores and cerebral perfusion in the morning v. evening. In other words, there is a relatively greater (positive) slope of the regression line between perfusion and symptoms in the morning, whereas the slope in the evening is significantly less positive. The correlations reported with factor scores were interpreted accurately and, for example, support the claim for a positive correlation between severity of depression/weakness/fatigue and anterior limbic perfusion.

Although this may seem an arcane point, the additional change in the ‘tightness’ of the mood-brain perfusion association from morning to evening implied by these results is actually rather exciting. It could reflect the difference between a direct and a compensatory relationship between brain activity and behaviour at different times of the day and should provide a motive for follow-up experiments.