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Nonfiction

Forget Everything You’ve Ever Known About Amnesia

When we think of “amnesia,” we think of the classic soap opera example. A character endures some sort of emotionally traumatizing situation or blow to the head. Critical details about the character’s entire life are forgotten, but total recall usually occurs at a crucial moment for maximal dramatic tension. The character retains all the necessary skills to survive: they don’t forget how to talk, eat, get dressed, do math, use an ATM, drive a car, etc. However, the amnesiac can’t recall any details whatsoever of their childhood. They don’t recognize friends and family. They don’t remember what they did for a living. Occasionally a skill or memory may come to them in a disjointed flash.

In most storylines, the person will end up with fully restored memories, only to find that they are a completely different person than they were before the triggering event, and that they like their new selves better. Irrespective of the damage done by the head injury, you can count on the person never, ever losing their ability to organize their lives and plan for the future.

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Amnesia Comes in More Than One Flavor

In the real world, even the types of amnesia are not as simple as the Hollywood portrayal suggests. First off, we can categorize the types of amnesia by their temporal effects. Let’s say that you, dear reader, unfortunately had a nasty fall down the stairs and hit your head. If you forget things that happened to you before the injury (for example, you forget about your first day in kindergarten), this is called retrograde amnesia, retrograde meaning the memory came before the injury. If, on the other hand, you find it difficult to form new memories (you keep forgetting where you left your car whereas it was never a problem before the injury), this is called anterograde amnesia.

The type of memory that has been lost can also be differentiated. There are two main divisions; declarative memory and nondeclarative memory. Declarative memories are those you show you learned by telling people. They’re a narrative. Say you lose all memories for specific events, or episodes, in your life; you can’t remember going to school, learning to play basketball, your first kiss, your first job, all that autobiographical knowledge. You’ve lost your episodic memory. In this specific case, since those things happened before the injury, you have retrograde episodic memory loss. You can have anterograde episodic memory impairments too, if you can’t make new autobiographical memories. There’s also semantic memory, which means general knowledge about the world. Think of it as more like trivia. What’s the capital of Rhode Island?  What’s two plus two?  Those sorts of things are considered semantic. Both semantic and episodic memories are considered to be declarative.

Nondeclarative memories are things you show you learned by actually doing: Procedural memories. Procedures are skills that you perform. I show you I know how to ride a bike by actually riding a bike. No amount of semantic knowledge is going to convince you otherwise. I can tell you how bikes are ridden until I’m blue in the face, but you won’t necessarily believe me until I get on one and start pedaling.

Note that we’ve uncoupled the skill (riding) from the semantic (describing how bikes are ridden) and the episodic (when you learned to ride). Your fall down the stairs may take away the memory for an event (e.g. when your dad taught you to ride a bike), but leave intact the skill of actually riding a bike. You may have absolutely no idea that you know how to do it! As you can guess, this is difficult for an amnesiac to cope with, but it gives rise to those tantalizing moments in movies when the character suddenly remembers they can beat up several prison guards or sing an opera.

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How Physical Damage Makes Mental Trauma

Depending on the type of injury and how widespread it is, you may find that you’re unable to remember things from the past, or to make new memories after the incident, depending on where the brain is damaged. The type of memory that is lost is also dependent upon what brain areas are affected.

How do we know what brain areas account for which types of memories? Consider three famous cases of amnesia patients and their histories; Henry Molaison, Clive Wearing, and another patient known as KC. All three of these individuals share common features of memory loss, but their deficits came about very differently; Henry Molaison suffered from intractable epileptic seizures and required surgery to stop them, Clive Wearing suffered from a viral infection known as Herpes simplex encephalitis that affected his brain, and KC was involved in a traumatic motorcycle accident that caused massive subdural hemorrhaging that required extensive surgery to treat.

Three very different patients, but some common features emerge. All three of them show damage to the temporal lobes, including structures that help form new memories. Henry Molaison had his large chunks of hippocampus, amygdala, and medial temporal cortex removed to prevent his seizures. The patient KC also had extensive damage to his medial temporal cortex and hippocampus, but showed much more widespread damage to other cortical areas around the occipital and temporal lobes. These areas are thought to be involved with producing visual imagery. Clive Wearing’s brain inflammation from Herpes simplex encephalitis wiped out his hippocampus and put pressure on much of his cortex throughout his brain. Right away we can see some similarities. All three of these famous cases have damage to the hippocampus and the surrounding medial temporal cortex, the central part of the brain, and all three of them simply cannot form new episodic or semantic memories. They are essentially trapped in a moment; they have normal short term memory (seconds to minutes), but none of that can actually be stored anywhere; it gets dumped, lost forever. All three of them can have a short conversation with you, but will have forgotten how the conversation began. Leave the room and come back in 10 seconds later, they’ll have forgotten ever having met you.

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A Post-Amnesia Life

There are some stark differences between the cases. Only Henry Molaison had his damage restricted to the temporal lobe. In him alone, we find relatively minor retrograde memory impairments. He could remember going to the doctor and being told he would have a surgery to treat his seizures, although he doesn’t remember much from the weeks leading up to the surgery. Remote memories from his early life are more or less intact. On the other hand, Clive and KC have severe retrograde memory loss and can only remember vagaries of their lives before their respective brain damage. Clive, for example, remembers that he is married but cannot recall the date or the event. Both of them are severely lacking in the autobiographical department. Clive has it particularly hard because he cannot even remember being aware that he was conscious five minutes before! His journal is full of statements like—

10:47 am  I am aware for the first time. 

10:53 am  for the first time, I am conscious.

11:02 am  I am truly and fully aware for the first time!

Clive is cognizant of the fact that he cannot recall things, and it makes him very angry. He crosses out journal entries as he thinks someone else wrote them in his handwriting. However, he has intact skill memories. He is a renowned conductor and choral master and his professional skills are unimpaired. He can conduct an orchestra without incident.

KC is similarly without autobiographical knowledge, but is also unable to picture himself in future situations. He literally cannot conceive of himself in a problem-solving situation or doing something specific in the future. Clive’s and KC’s more widespread deficits likely are due to the fact that many more cortical regions, where remote memories are stored, have been compromised.

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Hollywood can get parts of amnesia right, but as always it puts its own spin on reality. Procedural memories and basic skills are generally unimpaired in amnesiacs. They can pop up out of nowhere, and can be a mechanism for some minimal new learning. But leave it to the movies and television to tack on a happy ending to any situation. Spontaneous recovery, a common plot device, is not likely to happen when a patient has severe damage, especially as both retrograde and anterograde memories become compromised. The hope of recovery is virtually nil.

Often the best anyone can hope for is the inability to understand how bad they have it.

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The Evil Monkey

Evil MonkeyThe Evil Monkey has a Ph.D. in the neurosciences and used to study memory in primates for a living.  He now resides in the Midwest and spends his professional time teaching at the local state university and community college.  When not teaching, he is often engaged in an endless cycle of home repair or playing with his stepkids or writing his irreverent neuroscience blog Neurotopia.