Which effect is your focus: behavior effect or the correlation with the seed? Or both? The centering issue is only applicable for the correlation with seed (intercept), not the behavior effect (slope).

> Aged animals can generally be broken down into 2 groups - high performers that perform on par
> with young and low performers who have cognitive deficits - so this is a little tricky.

If you're referring to group difference about the correlation with seed, it depends on what kind of questions you want to answer: do you want to compare the aged animals with high performance to the young animals? Or do you want to compare the aged animals with low performance who have cognitive deficits with the young animals? Or, you just want to the whole aged group and with the young at some common behavior value?

> If what these results are saying is that the slopes are different with respect to performance,
> how do I know about directionality of each of the groups?

Do you see the behavior effect (sub-brick) for each group in the output?

Gang