Andrew,
There are a few approaches to assessing the multicollinearity among explanatory variables in a model. One of them is the condition number of the design matrix that is used in 3dDeconvolve to alert the user about its severity. Some people do view that a correlation higher than 0.4 between two regressors indicate a potential multicollinearity issue, but that's not necessarily always the case. See more discussion about this and a couple of other multicollinearity assessment methods here:
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en.wikipedia.org]
Gang
