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.
‘I think memory deficits that still function are most open to amelioration’. That is correct, based on what you have said about some memory deficits functioning and some not. That restricts the meaning of the sentence and is essential for it to be correct. Which is non-restrictive. You should be able to remove it and the underlying meaning remain correct. ‘I think memory deficits, which still function are most open to amelioration’. Becomes: ‘I think memory deficits are most open to amelioration’. (They all function and so are all open to amelioration) Dave