It is a common symptom during the progression of the degenerative properties of CBGD for the patient to lose their ability to speak. I can say with certainty that my speaking abilities are greatly diminished both in enunciation and my ability to properly choose words. I believe that had I not been a trained public speaker and a bit on the talkative side, I might be mute now. I believe that my "baseline" of verbal skills was better than most. In other words, I had further to fall than most.
Writing is a different skill set. Certainly my "skills" there have also diminished. You need only follow my blog as evidence of that. Writing allows me to construct an idea and then choose and re-choose the words I may use to convey the idea. The process is slow and increasingly laborious.
However the "idea" remains vivid and organized. It just seems that finding the way to express it may leave me. Even my facial expressions and body language have withered.
The point I am clumsily trying to make is: If I consider my baseline verbal and cognitive skills four years ago and extrapolate their degeneration moving forward, it seems very evident that I will be able to understand and, indeed, create abstract ideas and thought long after I am no longer able to demonstrate or acknowledge that understanding.
This does not necessarily mean this is the case with your loved one or even me at some point in the future, but it seems plausible, even valid, that I may someday be fully capable mentally with no way to express it
The irony is, by the time I know the definitive answer, I will no longer be able to tell you. The Caregiver's Lament.
I took this from my office window a few days ago.