@ossobuco,
I think what tenderfoot is saying is that once the disease progresses, the person afflicted loses the capacity for understanding the emotional impact on the caregiver. Eventually, they lose the capacity to recognize the caregiver which is most disheartening to the one doing the caring.
Eva, hugs and well wishes to you and your brother. It's a long road and it will be best if you travel it together. Progressive dementia, whether from Alzheimer's or secondary to some other condition, robs everyone involved. As difficult as it is, there comes a time when it's best to put on a clinical hat (as Walter has done) and see the situation as a condition the strips away the life (soul, if you will) long before the disease is done with the body.
Tenderfoot, I watched my mother argue with my father over things that were really not important -- so what if he was wrong -- but her frustration at being bombarded with falsehoods wore her down. It's difficult to disengage emotionally or tune it out. Best wishes to you and your wife.