I don't remember how was it called, unfortunately. I should have an article on it somewhere, but I can't find it right now. The similiarities mostly consider the lexicon (reflexes of *pravъ, *čadja, *potъ etc. vs *desьnъ, *sadja, *znojь in the central zone). I have to hit the pillow now, but I will try to dig up the article tomorrow, as well as some videos.