Do personality and behavioral changes become noticeable around the same time that biomarkers, such as the plaques and the tau tangles, become detectable?

In Alzheimer’s disease, the thought used to be that if you had amyloid in the brain, that you are not going to exhibit symptoms until you start to get close to the dementia period. And the mythology is that you don’t have behavior problems until you’re years into the illness.
Personality measures—neuroticism and introversion—are early features of the disease.
Behavior precedes the clinical diagnosis of dementia almost always with frontotemporal dementia. These are people who lose their money. They make bad financial decisions. They alienate people around them. They get in trouble at work, not because they can’t remember, but because they’re disagreeable and they don’t follow orders.
With Parkinson dementias, like Lewy Body, there’s often a very strong signal of mood and anxiety. So these are people who, years before they actually develop a motor problem, develop for the first time in their life a major depressive episode, or a huge anxiety disorder that requires medications.