When you think about memory what comes to mind? Do you think about an important event from your past, or the list of groceries you need to pick up on your way home from work today? There are many different types of memory, which require different brain processes. Memories from important events in our past are called episodic memories, and remembering to pick up groceries on the way home requires prospective memory. Some memories are clearly tied to a specific point in our life, like our favorite birthday. But other memories aren’t tied to a time and place. For instance do you remember when you learnt what an apple is? Probably not. But you know that an apple is a fruit that grows on trees. Your knowledge of an apple is part of a different memory system called your semantic memory, or your memory for facts and information.
Some types of memory are made even without us being consciously aware of them through what’s called implicit learning. For example the more that we repeat an action more ingrained it becomes in our minds and the less effort is needed to use the memory. Think about learning to ride a bike. When you were young it took a lot of effort. You to think about staying balanced, peddling, keeping the handlebars straight, and so on. But as an adult even if you haven’t ridden a bike and over 10 years, once you get on a bike your body will remember how to ride. Just like the phrase “it’s just like riding a bike”, some skills once we learn them we have them for life, even if we don’t practice them regularly. Knowing how to ride a bike is a procedural memory, which is part of our implicit memory system. We know how to do it even without consciously thinking about the skills required.
Our knowledge that there are many different types of memory, and that certain brain regions are important for learning is relatively recent. Before the 1950s we thought memory was tied to intelligence and perception.
We have a famous patient named Henry Molaison (commonly referred to as patient HM), and a Canadian Neuropsychologist named Brenda Milner to thank for our modern understandings of memory.
Henry Molaison developed seizures when he was 10 years old. His seizures started small, but progressed to convulsive seizures before he was 16. He was able to work for a while as an adult, and worked on an assembly line. But in 1953, when Henry was 27 years old his seizures became so incapacitating he was no longer able to work or lead a normal life. Anticonvulsant medications weren’t helping Henry. A neurosurgeon named William Scoville offered Henry an experimental procedure that involved removal of parts of his bilateral temporal lobes (which are on the sides of your brain, from about your temples back over your ears). We now know that the temporal lobes, which include a structure called the hippocampus, play a critical role in our ability to make new memories. But we didn’t know this when Henry had his surgery.
After the surgery Henry’s epilepsy was controlled, but he developed severe memory impairment. Dr. Milner was asked to travel to his memory difficulties.
What Dr. Milner observed when she met Henry was someone who forgot daily events almost as fast as they happened. Henry described to Dr. Milner that he felt “like waking from a dream… every day is alone in itself”. He forgot names of people he was introduced to almost immediately, and after working with Henry for 50 years Dr. Milner had to introduce herself each time they met. He would also say his own age wrong because he was not able to remember that time had passed, and would incorrectly state the year.
Although Henry would forget information very quickly, other components of his thinking skills remained strong. His intelligence was preserved. Henry also had a remarkable ability to maintain attention and he could remember some information for a brief time after he heart it. As a result, he could carry on conversations, and he was able to remember information as long as he kept repeating it in his mind. But if he was distracted he completely forgot what he was trying to remember, and didn’t recall ever being asked to remember something in the first place!
Henry was also able to learn new skills, even if he didn’t remember ever doing the task before. For example Dr. Milner would have him perform repetitive motor tasks like copying a star that was reflected in a mirror. His performance would improve over time, and when asked to do the same task days later his performance would remain strong, even though he would insist he had never done the task before. This type of memory is called procedural memory, which is a type of non-declarative or implicit memory.
Henry could also remember things that had happened before his surgery. He remembered important events from his past, and famous people from before his surgery. When he was show pictures of people that became famous after his surgery, however, he did not recognize them, even if they were regularly in the news.
Henry died on December 2, 2008 at the age of 82. Although Henry did not remember the contributions he was making to neuroscience from day to day, his story forever changed how we understand memory and the brain. Thank you Henry Molaison and Dr. Milner for your contributions to science.