and also dementia (whether Alzheimer's dementia or some other dementia) does not typically cause sudden rapid decline. any sudden decline in a person with dementia is actually a sign something else medical is going on. it really signals, "Get me to the doc!" that's so further in-depth exploration of what's going on can be done.
things which cause such sudden declines can often be fixable (UTI's, medication changes and so on).
in general i'd say new situations don't stimulate better responses in a person with dementia, rather the opposite. but i have seen where a person with dementia rises to the demands of a particular occasion and functions better for a while. also, laughter improves brain function in those with dementia for anything from 30 minutes to 2 hours -- those lovely endorphins!
after-effects of stroke can often involve dementia which would then correctly be called vascular dementia, not Alzheimer's. i sense that the word Alzheimer's is grotesquely over-used in our society right now.
do get your Mom checked over right now and also consider whether a medication change coincides with decreased functioning.