A few years ago I tried to understand the sign of the interaction term you get when performing a physio-physiological interaction analysis. It's a very simple model that was reported in the supplementary material to the paper "Griffiths, D.J., Tadic, S.D., Schaefer, W. & Resnick, N.M. Cerebral control of the lower urinary tract: how age-related changes might predispose to urge incontinence. Neuroimage 47, 981-6 (2009)". I'm not sure I agree with all of it now, and it refers to a different situation than the usual psycho-physiological interaction, but the model did seem to shed some light on the meaning of the sign of the interaction.
I'd be interested in your comments, if you can manage to dig out the .doc and .tiff files from the supplementary material.