Hey, welcome to Dreamviews!

I haven't read the article, but I did a quick search and found it on google and read the paragraph about the subjects. I'm just guessing, but it sounds like they mean the subjects had experience with LDing on their own. Maybe it would be clearer after reading the full article, but I'm up too ridiculously late to start. I think it's safe to assume they were natural LDers.

Good luck with your paper