Orthogonalization of regressors in FMRI models.
The occurrence of collinearity in fMRI-based GLMs (general linear models) may reduce power or produce unreliable parameter estimates. It is commonly believed that orthogonalizing collinear regressors in the model will solve this problem, and some software packages apply automatic orthogonalization. However, the effects of orthogonalization on the interpretation of the resulting parameter estimates is widely unappreciated or misunderstood. Here we discuss the nature and causes of collinearity in fMRI models, with a focus on the appropriate uses of orthogonalization. Special attention is given to how the two popular fMRI data analysis software packages, SPM and FSL, handle orthogonalization, and pitfalls that may be encountered in their usage. Strategies are discussed for reducing collinearity in fMRI designs and addressing their effects when they occur.
Center for Investigating Healthy Minds at the Waisman Center, University of Wisconsin-Madison, Madison, WI, USA.
Jeanette A
JA
Mumford
Helen Wills Neuroscience Institute, Brain Imaging Center, University of California, Berkeley, CA, USA.
Jean-Baptiste
JB
Poline
Department of Psychology, Stanford University, Palo Alto, CA, USA.
Russell A
RA
Poldrack
D000465:Algorithms; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008962:Models, Theoretical; D011930:Reaction Time; D012044:Regression Analysis; D012984:Software