Each person is different, and moments change. I don't know that this can be quantified by a study. When I was speaking to a gerontology class that uses my book as a text, a young woman told me that she had never known her grandma without Alzheimer's. Her grandma didn't know anyone, anymore. But once, as the girl was hugging her grandmas goodbye, and the girl said, "I love you, Grandma," the grandma said, "I love you, too, Anna." It was out of no where.
My dad had surgically induced dementia. He often seemed to know nothing about his condition, yet once he looked me straight in the eye and I knew it was coming from the "real" him - he said," My universe has gotten so small." Then he went right back "in."
Personally, I believe that there is awareness of their condition, at least at times, and frustration and anger as well, but it depends a lot on the stages of the disease. I feel, from what I've read and observed, that toward the end, they may be seeing things we don't even know about. It is a process. Death is a process, for people suffering from disease. I think they can be "in and out" for a long time. And the times we can't reach them, they are reaching beyond what we see.
At all stages, touching and talking and doing what you are doing - it's all so important. Whether the person responds or not, they are aware on some level that someone cares and is there for them. Please keep doing what you can. You sound wonderful.