Prefer to read this post in the original Dutch? Check out Marlieke's personal blog.
When we think about memory disorders, we are usually thinking of dementia or temporary memory loss after an accident. These cases most often involve older memories that slowly or abruptly disappear. A much rarer type of memory disorder involves losing the ability to form new memories. This is called anterograde amnesia and the results are catastrophic, which is very nicely depicted in the movie “Memento”.
Patients suffering from complete anterograde amnesia are (luckily) very scarce, but their condition is very interesting as it helps us understand how memories are formed. As a consequence, these patients tend to become very famous in the scientific world because of the numerous experiments they participate in. A particularly famous example is Henry Molaison, who passed away in 2008. Henry was incapable of forming new memories because of an epilepsy operation. The circumstances of Henry’s subsequent amnesia led memory research to a strong focus on the hippocampus - the region that was removed from his brain. Over the years Henry’s participation in research led to many insights (and new questions) about the processes that underlie the fate of our beloved memories.
Because of all the attention Henry receives, in textbooks and classrooms alike, you might think that he was the only amnesiac that affected neuroscientific theories. This is not true. The recently deceased Kent Cochrane (earlier known through his initials K.C.) has meant a lot to memory research as well, because of his own particular amnesia.
Kent Cochrane was a Canadian who worked at a manufacturing plant. He was born in 1951, two years before Henry’s unfortunate surgery. In 1981, when he was 30 years old, Kent had a motorcycle accident that damaged both his hippocampi. Like you would expect from Henry’s case, Kent also suffered from complete anterograde amnesia; he could not construct new memories. He also lost some, but not all, of his earlier memories. When researchers – guided by Dr Endel Tulving – dug a bit deeper, they noticed that Kent’s remaining memories were easy to classify.
He appeared to have lost all specific personal experiences, but factual information – general information you can look up in an encyclopedia – was unaffected!
Different kinds of memories
Because of Henry Molaison, researchers already knew that our memory system can be divided into procedural memory (unconscious memory for procedures, like knowing how to bike) and declarative memory (conscious memory for declarable information, like a memory of a specific bike trip). Only the latter requires a functional hippocampus. To illustrate this, researchers showed that Henry could learn to write mirror reversed words, but did not have any conscious memory that he ever learned doing it.
Kent’s amnesia shed light on a further division of declarative memory, into episodic memories (specific personal experiences) and semantic memories (facts). Kent specifically lost his episodic memories, leading him to have conserved general knowledge about the world, but no specific, autobiographical memories.
Interestingly, Kent was able to learn new information, although it took him longer and these memories were only stored as general, semantic memories. This led him, for example, to be able to recognize the name Bill Clinton even though he only became president after Kent’s accident. However, Kent could not remember what Bill’s occupation was. This was a very interesting finding because up to then, scientists assumed that no declarative memories could be formed without a functional hippocampus.
Instead this assumption appeared to only be valid for episodic memories, something that was later confirmed in Henry’s case. In this way, both cases have mutually influenced each other, and our understanding of what memories are, and how they are formed.
Memories are precious
So, at the end of the day, what have these patients taught us? Both Kent and Henry’s cases showed that there are multiple types of memories, of which only some are dependent on the infamous hippocampus. Exactly how all these different types of memories are formed and stored, and how we use them during our daily lives, remains an area of active research. Because another thing that Kent and Henry taught us is that many questions about memory are still unanswered.
When legends in memory research pass away, it reminds me how lucky we should be that these people, despite their awful fate, spend their lives helping science proceed. At the same time, it reminds me how precious our memories are, and that we should cherish them more often. So, thank you, Henry and Kent! It is now up to us to use your invaluable scientific contributions to help other people suffering from similar fates, while at the same time deepening our understanding of our mysterious yet precious memories.