How Does Dementia Cause Memory Loss?
One of the confusing things about dementia is that memory loss rarely behaves the way you expect it to. A person can describe in detail the house they grew up in, but they can’t remember what they had for breakfast. They may ask the same question several times because they have no memory of receiving the answer. To many people this seems contradictory; if memory is failing, why do some memories disappear while others remain untouched?
Part of the answer lies in the way memory works. Most people think of memory as though it were a storage system, like a filing cabinet holds documents, everything in one location, retrievable on demand. The reality is a little more complicated because memory is a process, and different stages of that process happen in different regions of the brain.
When you experience something it is first received by the hippocampus, which acts as a consolidation centre. This small structure sitting deep inside the brain (one on each side) functions as the brain’s processing hub for new experiences. It takes the raw impression and, over time, writes it into the cortex for longer-term storage (the cortex is the brain’s outer layer, the large wrinkled surface that you see in pictures of the brain). The cortex handles thinking, language, perception, and stored knowledge but the hand-over from the hippocampus to the cortex is not an instant process. A memory can take days or weeks to become fully embedded and distributed across that outer layer. Once it is, the hippocampus is no longer needed to retrieve it; the memory has been shipped out from the deep interior and stored at the surface. The hippocampus has done its job.
Long-term memories, particularly those from decades past, are no longer dependent on the hippocampus, and the hippocampus is the region where the disease tends to attack first.
Alzheimer’s disease, the most common cause of dementia, involves the accumulation of two abnormal proteins in the brain: amyloid, which clumps together to form plaques between brain cells, and tau, which tangles and accumulates inside them. One of the earliest and most heavily affected regions is the hippocampus (the factory that takes in the new experiences). As damage accumulates there, the person gradually loses the ability to turn these new experiences into lasting memories.
The important take away for us is that, at this stage of the disease, the person is not forgetting things, but that the memory was never properly sent to storage in the first place; the consolidation process has been disrupted at the source. So when someone with Alzheimer’s can’t remember a conversation from this morning or what they had for breakfast, it is not that the memory has been lost; it is that the machinery needed to create it has been damaged and so the input was never processed into memory.
Meanwhile, everything that was consolidated and stored in the cortex years or decades ago remains largely untouched, because the disease has not yet reached those regions. This is why the distant past can feel more real and more accessible than yesterday.
As Alzheimer’s disease progresses, however, the damage does not remain confined to the hippocampus and over time it spreads into other parts of the brain, including the cortex which is holding the long term memories. This is one reason memory loss often appears to move backwards through a person’s life. Recent years may become difficult to recall first, followed by more distant decades, and eventually even memories that once seemed permanent can become difficult to access.
This progression will be a time of sadness, where the person you are caring for may know your face but not your name, or know you are someone important but be unable to place exactly who. This is not a choice or a preference; it reflects which memories were laid down earliest and most deeply.
The pattern described here is most closely associated with Alzheimer’s disease because of the way it affects the hippocampus during the early stages. Other forms of dementia often follow different paths. Frontotemporal dementia, for example, frequently affects behaviour, personality, judgement, or language before significant memory problems appear. Lewy body dementia may affect attention, alertness, and visual processing, while vascular dementia depends largely on where damage has occurred within the brain. Understanding these differences is one reason doctors place so much importance on identifying the specific type of dementia involved.
When you understand that the hippocampus is failing before the cortex, a great deal of behaviour that might otherwise seem strange or frustrating begins to make sense and how having to repeat the same question minutes after it was answered has its reasons. The consolidation process that would have stored your answer simply did not complete, so asking someone to remember a recent instruction and then becoming upset when they can’t is, in effect, asking them to remember something that you never said. There is an instinct within us to correct the person, to bring them back to the present and to remind them of the actual date or the actual situation. But with Alzheimer’s, correction does not work, the present moment is precisely what the damaged hippocampus cannot hold, so meeting the person in their world, the world that still makes sense to them, even if that world is forty years ago, can be how you keep the connection.
Further Support
If you are caring for somebody living with dementia and would like more practical guidance on day-to-day care, communication, behaviour changes, routines, safety, and the later stages of the illness, you will find The Dementia Carer’s Guide helpful.
About the author: Michael Willers writes practical plain-English resources for dementia carers and families through Bright Mind Books.