Michael, this work moved far beyond Higgin's work. There was the work of Bartleson and Breneman, the work of DeMarsh and the work of Kriss. And, much of this was never published outside of EK. So,yes the slit exposures show what we now call "edge effects". These are the "ears" on the exposure. Those who worked with knife edge exposures missed a lot of information such as fill in and bloom.
Kriss' method of defining "image content" codifies all of these into an appropriate equation which I would have to scan in, but the explanation is nearly 30 pages long in his article. It can be found in the bok "Color Thory and Imaging Systems" published by the SPSE.