Memory loss in Alzheimer's disease (AD) is a central symptom of the disease, so understanding memory deficits that AD patients experience is essential to caregivers. It gives us insights into how they see the world around them, why they behave in certain ways and, very importantly, helps us cope better and provide better supportive care.
The brain is often compared to a computer, but actually the brain is far more complex. For one thing a brain deals with emotions which can not only shape our perceptions but also our memories. This is what makes Alzheimer's disease an individual experience and why it's progression and symptoms do not happen in an exact but a similar way for each person.
The process of laying down memories is complex and involves different processes using different parts of the brain. Encoding information, for example, is different to storage and retrieval.
There is no single place or location in the brain where memories are stored although the hippocampus seems to have an important function in organising new memories and linking them with sensory information, such as sounds and smells. It then sends the components of the memory to relevant parts of the brain ready for later retrieval. Damage to the hippocampus has an immediate effect on our ability to form new memories, although established memories may be unaffected. This suggests that the hippocampus acts as an important gateway for encoding and storing new memories.
Let's look at this using an everyday example, say meeting a friend for coffee. There's the content of the conversation, the time it took place, the emotions that were evoked, the taste of the coffee, the light levels, the smells and so on. Each one of these stimuli will be processed by the brain and stored in different places.
Memory is perhaps better thought of as a kind of web or mesh of connections (called the engram). A memory can be triggered by stimulating one of the processes associated with it. A smell, an emotion, an object, or perhaps a scenario that has similar or familiar objects will do the trick. The more we recall a memory, the more it strengthens the engram and the less likely we are to forget.
People with early onset dementia often find they are unable to recall events or other things in the short term whilst memories that are older and deeper are more intact. However, as the disease progresses, the more brain damage occurs. Links between areas of the brain become more and more difficult to access leaving the person with AD unable to knit together information. Difficulty, and later failure to access memories, confusion, misinterpretation of events, people, time, the eventual loss of self. That is why Alzheimer's disease is often called the long goodbye.
View the short video on Brain changes and Alzheimer's disease for more information about how AD affects the brain.
National Institutes on Aging 2010 alzheimer’s disease progress report: A deeper understanding. (2011, December). Retrieved from http://www.nia.nih.gov/alzheimers/publication/2010-alzheimers-disease-progress-report-deeper-understanding/brief-primer