Learning-related effects and functional neuroimaging

A fundamental problem in the study of learning is that learning-related cha nges may be confounded by nonspecific time effects. There are several strat egies for handling this problem. This problem may be of greater significanc e in functional magnetic resonance imaging (fMRI) compared to positron emis sion tomography (PET). Using the general linear model, we describe, compare , and discuss two approaches for separating learning-related from nonspecif ic time effects. The first approach makes assumptions on the general behavi or of nonspecific effects and explicitly models these effects, i.e., nonspe cific time effects are incorporated as a linear or nonlinear confounding co variate in the statistical model. The second strategy makes no a priori ass umption concerning the form of nonspecific time effects, but implicitly con trols for nonspecific effects using an interaction approach, i.e., learning effects are assessed with an interaction contrast. The two approaches depe nd on specific assumptions and have specific limitations. With certain expe rimental designs, both approaches may be used and the results compared, len ding particular support to effects that are independent of the method used. A third and perhaps better approach that sometimes may be practically unfe asible is to use a completely temporally balanced experimental design. The choice of approach may be of particular importance when learning-related ef fects are studied with fMRI. Hum. Brain Mapping 7:234-243, 1999. (C) 1999 W iley-Liss, Inc.