I think both can apply. In diseases like Alzheimer’s memories are lost because cells die in a region of the brain called the Hippocampus. This part of the brain is where we store our long term memories. A lot of cell loss also happens in the cortex of the brain, this causes changes in behaviour and cognition which are also associated with the disease. Head injuries can also lead to memory loss, again by causing damage to sensitive areas of the brain.
It has been said that you never forget anything, even answers to exam questions you took when you were young, or the names of people you haven’t seen for 50 years. But you do forget how to recall this information. it gets stored deep within the brain and until something happens to remind you of that information it is almost like you have lost it. It is helpful to have cues to help you remember stuff, this helps you recall the information from your brain that you associate with that image 🙂
so in answer to your question, both are true, but in terms of diseases and memory loss with age, it is definitely that the cells have died. 🙂
Specific areas of the brain are associated with the ability to perform certain tasks – say how to speak – and people who have brain injuries in these locations might be unable to perform those tasks as well as they used to. But these kinds of injuries are relatively rare.
As we get older, we lose some of our ability to recall specific facts and events. The memory loss associated with Alzhiemer’s disease and other forms of dementia are at the extreme end of a distribution, and can be associated with cell loss in specific regions. People’s general ability to recall varies, and is likely to be influenced by both genetic and environmental factors.
I don’t think that memories of things or events or people can be located to a specific part of the brain. It’s more like a bunch of neurons that are all activated together when we think of a specific person. These neurons can be distributed all over the brain, rather than being in a set location. We gradually lose brain cells over time, so there might be fewer cells in any particular circuit. So it might take us longer to access a specific memory.