After reading the examples, I'm still a little iffy. Yes, this is a college paper. Context is about Down syndrome. The question is whether both of these mean the same thing. We know that some memory deficits still function, while others don't. If you attempted to work on one, which would you choose? I think "that" is the right word because it refers to any memory deficit that works. But how does the meaning change when I use which? I'm working on a memory deficit, which still works. I think memory deficits that still function are most open to amelioration. I think memory deficits, which still function are most open to amelioration.