I'm interested in using AFNI to perform a non-linear regression. For example, in studying development, often the relationships that we're interested in are curvilinear...such that age may relate to performance/brain response until a certain age and then the relationship may flatten or change. I would like to be able to determine whether a given variable (such as age) is best fit to the task response with a linear or a non-linear model. How might I go about doing this? It has been suggested that I enter both the independent variable and the squared independent variable into 3dRegAna (in 2 separate columns). Is this the appropriate way to examine this question, and if so, how might I determine whether the linear or non-linear model best fits the data?

Thanks for your help!