By: Gregory Brown, MD/PhD Candidate
Memory can be broken up into a few different types. We will discuss these types in detail, but a visual depiction is below:
Working Memory is trying to remember a phone number
First is working memory (sometimes called short-term memory), which can store a small amount of information for a short period of time. A rule of thumb is 7 +/- 2 items. Remembering a phone number falls into this category. Much of this information is stored in the neocortex, or “new brain,” which is the most recently evolved brain regions involved in higher-order brain functions.
As working memories become stronger, the brain begins using other brain regions (particularly the hippocampus, which is affected in Alzheimer’s Disease) to store this information. The distinction between memory and working memory is fluid, and many things considered as long-term memory will be forgotten. Things remembered for more than 3-5 minutes are considered long-term memories, which is a pretty short time interval. For this reason, science is switching to use the terms ‘working memory’ and ‘memory’ for short-term and long-term memory, respectively.
Declarative memories are facts that you can state
Memory can be broken into two types: declarative and nondeclarative. Declarative memory is the facts that you can state or declare. This can be further broken into two categories: semantic and episodic. Semantic memories are pieces of information that you have learned but did not experience. For example, knowing that the war of 1812 was in fact in 1812. You likely were not around during that war, and do not have an experience of this memory or where you learned that fact, but you know that piece of information. Episodic memory is remembering events: you can state what you had for breakfast this morning because you remember the entire episode, and you can picture in your mind eating the meal.
Nondeclarative memories are muscle or emotion based memories
The other type of long-term memory is nondeclarative memory, which cannot be stated as easily. Nondeclarative memories can be broken into three components: 1) skills and habits, such as golfing or shooting a free throw. This is often referred to as muscle memory but is actually stored in movement regions in the brain called the basal ganglia and cerebellum. 2) Emotional associations, such as how you felt when you first saw a certain romantic interest. That feeling that you get in the present about a past experience is an emotional memory. You can recognize and attribute words to that feeling, but that is a cognitive action in the present. The amygdala, which is an emotion center in the brain, is involved in emotional memory. Finally, conditioned reflexes are nondeclarative memories. The most common example of this is Pavlov’s dog, where the dog was given a treat every time Ivan Pavlov rang a bell. Eventually, the dog salivated on ringing the bell. This is a nondeclarative association, or memory, of ringing the bell leading to a treat.
The term ‘memory’ can refer to many different things and brain regions. Therefore, science and research studying memory is vast and often focuses on a certain type. Hopefully, this brief overview provides some interesting background the next time you wonder how you forgot where you placed your keys.