In Dementia, the memory is "lost" in somewhat of a "reverse" chronological order with short term memory being the first to go followed by longer and longer term memories. As a result as dementia progresses, the person starts living increasingly in the past - something that is tough for both the caregiver and care-recipient to handle. In such cases, things that invoke memories from the past - especially the past in which the person with dementia is currently in - are helpful in engaging and relating the person. This can help combat isolation and depression that comes along with the situation created by dementia.
We met a daughter who was taking care of her father who was originally from Lithuania. She realized the importance of memories in getting her father, who suffered from Alzheimer's, engaged and excited. She and her brother used music from his youth as well as cooked food from his youth which got him talking and excited and engaged. He had trained and worked as a tailor, so they took him to meet a tailor they knew and he was fully engaged.
Another person we met was taking care of his dad in advanced stages of dementia. His dad used to wake up thinking he was back in Sicily as a kid and didn't recognize his current surroundings in suburban Chicago area. He used to get agitated. His son used to show him photos and even images from Google earth of locations in Sicily which relaxed his father and then used to put him in the car for a ride to "take him back home".