I am doing multiple regression with PET data with 7 different conditions.  I have modeled a linear and an exponential function separately and plotted the functions.  When they are the sole covariates the plots are linear and exponential functions.  However when I include both types of functions are included as covariates the plots are no longer linear or exponential.  This happens when I examine the fitted responses for the corresponding contrast's adjusted response plotted against scan or time.  They look very different from when I included modeled them as the sole covariates.  When I examine the design -> explore -> covariates the graphs look like the linear and exponential functions, not the graphs I get when plotting.  I think this may have something to do with orthogonalization, but am not entirely certain.  I have looked in the list archive, but I have not found anything that helped me track down exactly how to work around this problem.  I would appreciate any suggestions.  Thank you.

Paul Corbitt