It’s also why people with amnesia have been such a valuable resource in reaching a deeper understanding of how memory works. Researching memory disorders helps us understand its structure and functioning. It just so happens that most of the research in this area has focused on amnesic syndrome, when a person’s episodic memory is damaged and doesn’t work properly.
So when we research amnesia and episodic memory we have to focus on what effect that damage has. In other words, there are two basic types of amnesia. First, there’s retrograde amnesia: when someone forgets anything that happened to them before their illness or accident. Then there’s anterograde amnesia: when someone can’t create new memories. In the next few sections we’re going to go more in depth of both types of amnesia.
Retrograde amnesia is characterized by an inability to access memories from before the moment when memory was damaged. The time period it affects varies a lot. Some people just forget a few days, and others forget an entire lifetime.
It’s also worth keeping in mind that someone can still create new memories with this type of amnesia. They’ll also still be able to use other kinds of memories, like implicit memory and procedural memory.
The root cause of this disorder is usually physical. In other words, it happens when there’s some kind of brain damage. It usually happens with injuries in or around the hippocampus, basal ganglia, or diencephalon. But there are times when retrograde amnesia happens without any kind of injury. This is what they call psychogenic, or functional amnesia.
Clive Wearing was one of the most widely studied patients with a serious case of amnesic syndrome. Clive couldn’t remember a single piece of information about what happened to him before his accident in 1985 that completely destroyed his hippocampus and heavily damaged his temporal lobes. Clive also had anterograde amnesia. He couldn’t create new memories either, so he was stuck in a constant present.
Anterograde is basically the opposite of retrograde amnesia. Anterograde amnesia means you can’t create new memories after your injury. Therefore, any information you stored before will still be there. Just like the other type of amnesia, most of the time this one comes from some kind of physical damage. The affected areas in this case are the hypothalamus and the temporal lobes.
One unique aspect of this disorder is that it only affects explicit memory coding. What this means is that even though someone with it can’t create new memories, they can still learn new skills, whether they’re procedural or implicit.
For example, if someone with anterograde amnesia plays the piano every day, they’ll get better as time goes on. Of course they won’t actually remember that they played, because every day will seem like their first.
One of the most famous cases of anterograde amnesia was the patient HM (which stands for Henry Molaison). He had surgery to address his epileptic seizures, but during surgery, they took out much of his hypothalamus. This meant he wouldn’t have any more seizures, but it also caused serious memory problems.
HM couldn’t remember anything new after his operation, but his procedural and executive memory functions still worked. If you wanted to have a conversation with him, he was always clear and coherent. It didn’t seem like there was anything wrong with him. But if you walked away from the conversation and came back, he wouldn’t remember any of it.
Earlier on we mentioned that the two types of amnesia are independent. In other words, you can have one without having the other. But it’s worth mentioning that amnesic syndrome is truly a much more complicated issue than that.
Normally these injuries affect more than one area of the brain, which leads to a mixed kind of amnesia. Still, it’s interesting to study “pure” types of amnesia and learn more about how memory works.