Did you know that amnesia doesn’t just mean forgetting your life history? Time to find out about the different kinds of amnesia and how to write about them.
Amnesiacs are found everywhere between the pages of books. In mysteries, in thrillers, in romance and fantasy and historical fiction. Our fascination with memory—and not being able to remember—makes reading and writing about amnesic characters particularly exciting.
Most people only know about one type of amnesia, however—complete loss of memory before a certain point. This is the kind of amnesia usually portrayed in fiction, but to think this is the only type would be a grave error. Actually, there are several kinds of memory loss, each with different causes and consequences, and not all of them happen because of a bump on the head.
Broadly speaking, amnesia involves the loss of memory, which can be due to several factors: physical injury, psychological trauma, substance use and disease, to name the main ones. However, amnesia might not affect all of your memories.
Types of Memory
Short Term vs. Long Term
The name says it all: short term memories are short, temporary, fleeting. They’re limited in number (how many things can you hold in your mind at once without forgetting them?) and readily available, but they decay quickly unless they’re rehearsed.
When short term memories are rehearsed (e.g., repeating them over and over in your head), it strengthens their associations in the long term memory. In effect, short term memories transfer over to the long term memory. Because it’s unlimited in size, you can have a whole lifetime’s worth of recollections stored in the long term memory.
This next bit’s important. Evidence suggests that short term and long term memories are structurally and functionally different, which means that, if a certain part of the brain involved in memory is damaged or impaired in some way, it might not affect the other memory systems. So, should someone find that their short term memory is impaired after a head injury, their long term memories could still be intact, and vice versa.
Explicit vs. Implicit Memory
Just like there are differences between short term and long term memories, the type of memories you have differ too. First up, there are explicit (also known as declarative, intentional or direct) memories, which are used to consciously remember things you’ve done and know about, like general knowledge facts, the time of the meeting you have to go to, or the first day you ever rode a bike.
In contrast, implicit (or non-declarative, unintentional or indirect) memories involve the unconscious recollection of things you’ve learnt from past experiences. For example, once you’ve learnt how, you can ride bikes, drive cars and tie your shoelaces without consciously thinking about everything you’re doing.
In people suffering with amnesia, the implicit, explicit, short term and long term memories may not be affected equally. Explicit memories can be impaired in some cases of amnesia, in which case you may not be able to recall facts or memories, but your implicit memories, like knowing how to ride a bike, are just fine.
Also, depending on what part of the brain is damaged and what’s causing the amnesia, you could lose your ability to turn short term memories into long term ones or lose some or all of your long term memories entirely, which brings us nicely to...
Types of Amnesia
In short, this amnesia is the inability to form new memories. That example above of not being able to turn those short term memories into long term ones? Anterograde amnesia. If you’re suffering from this, you can’t lay down new memories after the event that caused the amnesia, which means you forget things a short time after they happen. Someone could talk to you one day and return the next to find you had completely forgotten the incident.
Despite this significant effect on memories, the long term memories of someone with anterograde amnesia—the things that happened before the amnesia was caused—can be completely or mostly intact. That means that, though you can’t remember the conversation you had with a friend the day before, you can still remember events from years ago.
The medial temporal lobe of the brain is usually damaged in people with anterograde amnesia, with possible causes being head injury, disease or alcohol and drug use. How long the anterograde amnesia lasts can also differ. It can be a short term bout of amnesia, as might be the case with alcohol and drug use, or a long term condition, brought on by brain trauma or disease.
This features the loss of memories before the amnesia occurred and is the one people usually think of when they hear ‘amnesia’. The ability to form new memories is intact, but the old ones, particularly those close to the event that caused the amnesia, are damaged. It’s very rare that people forget who they are entirely, but when it does happen, it’s called ‘global amnesia’.
Usually, damage to the hippocampal area and temporal lobes of the brain are found in retrograde amnesiacs, with causes ranging from brain injury and traumatic events to disease and surgical lesions. Simply telling someone suffering from retrograde amnesia about their life isn’t enough to make them remember. Often it’s a spontaneous recovery, if they ever recover at all.
Because brain damage isn’t necessarily limited to one place, you could end up with both retrograde and anterograde amnesia. How severe each type of amnesia is differs from person to person, but oftentimes, if a person has one type of amnesia, they’ll experience some of the other.
But it doesn’t end there—don’t forget the different types of memory. While the explicit memories of retrograde and anterograde amnesiacs may be damaged, their implicit memories can remain, which means they might still know how to ride a bike or tie their shoes, despite forgetting the times they’ve done so before.