There are two main categories of memory:
Information is stored in the brain for a brief period of time (max 30 seconds) before it is either stored into long-term memory or dismissed.
Even without dementia, studies have shown that the brain's ability to store information decreases as a person ages.
We often think of long-term memories as memories that happened weeks, months, or years ago, but actually if more than 30 seconds have passed and you are able to recall something that is because it was stored into long-term memory. Now, the lifespan of long-term memories vary. They can last just a few minutes or for many many many years. We also know that memories that are attached to strong emotions tend to "live" longer. (likely because it activates the amygdala which then helps the hippocampus store the memory more effectively — fancy stuff)
There are two different types of long-term memory:
Explicit memories are memories that can be consciously recalled. For example, memories of specific personal events or facts (episodic memory) and general facts about the world, other people, words, how things work, or how a person feels/thinks about something (semantic memory). Explicit memories are typically first type of long-term memory to become impaired in people living with dementia.
Implicit memories are memories that do not require conscious thought to be remembered. For example, remembering how to walk, ride a bike, drive, feed yourself, brush your teeth, or tie your shoe. It's knowledge about how to perform a skill (procedural memory). You don't think about these things too much do you? You just do them, right? These types of memories are typically preserved in people living with dementia for quite some time before they need assistance.
Now, it's important to know that generally speaking, long-term memories are not created equally, meaning some are stronger than others and overtime memories can be revised or merged together with other memories. This is why even for people who do not have dementia, memories can often be unreliable. Think of how unreliable eyewitness accounts can be or how people can confuse reality with their dreams and create a hybrid memory — it happens!
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