I'm only speaking from experience here.
A high score in cognition wouldn't necessarily have any baring on communication but it could do -it depends on the intensity/complexity etc (as ever). Pete had a severe in cognition and a high in Communication; Pete had aphasia (receptive and expressive) -obviously due to brain damage, so there was some crossover in the two domains. He wasn't given a severe in communication because at the time of the checklist he could show communication in other ways. Like drinking from other residents cups (shows he needs a drink) taking their food at the lunch tables (hungry) rubbing tummy (constipation) etc. All of that showed that he was able to communicate in some way.
As I have said before the CHC Team were very (unusually) clued up on the crossovers between the domains.I would say though that aphasia is classed a separate symptom of brain damage from Alzheimer's-or so I was told. I never quite understood that as AD was what was causing the organic damage.
I suppose that many Dementia sufferers do not actually lose their power of speech-or understanding of speech -even if they forget what has been said /they have said within a few minutes/seconds.
Perhaps someone will be able to explain this much clearer than I can.
Also confused-do you have something specific that you think your Mum's lack of cognition should be cross referenced to the communication domain?
Love
Lyn T