When it happens
Why it happens
As brain changes progress, even the early memories that often dominated, or at least were more vivid, during moderate dementia can now become elusive. What's more, many people develop language difficulties, making it harder to articulate and share what memory remains. The immediate present tends to dominate.
What you can do
Learn to "be in the moment" as best you can.
Focus on creating emotional comfort by treating the person with dignity, affection, and security. These are the realities that will most influence mood and behavior.
Do continue to talk about the past (yesterday, last year, the person's childhood). There's no harm or pain in it, and it may bring him or her some degree of pleasure.
Just because the person seems to have little awareness of the past doesn't mean others should avoid it.