Interestingly, Rick’s database does not have a separate entry or link for PSI, although search of his website contains number of documents that do mention PSI. It has been around for a while, since the 70‘s.
Here is the info I was able to find regarding PSI:
One reference regarding PSI and generally a great article regarding LGAT:
What I found are mostly tidbits about PSI in the context of LGAT.