Ambiguous results in functional neuroimaging data analysis due to covariate correlation

In this note we draw attention to a source of potential ambiguity in functi onal neuroimaging results when data analysis is based on the resolution of a linear model. This ambiguity arises whenever there exists correlation bet ween the model covariates. A single-subject PET activation experiment helps to illustrate to what extent correlation can affect statistical results in terpretation, possibly leading to misinterpretation of part of the activati on pattern. This note is intended to clarify this point and to suggest the use of a simple and well-known procedure to deal with these situations. In the Appendix, we suggest a convenient mathematical formulation for statisti cal tests particularly useful in such cases. (C) 1999 Academic Press.