Memory

Memories: everyone has them, and probably takes them for granted. You think that you can always rely on them, although it does let you down occasionally (ever walked into a room and forgot why you went in there?!). So how reliable actually is it? Here’s my explanation of human memory.

You can separate memory into two different types: short term memory and long term memory. In this post, I’m going to focus on long term memory, just because I think it’s more interesting.

Here is a diagram explaining how human long term memory can be broken down into various different subsystems:

As you can see – there is more to long term memory than you’d expect. The two types of long term memory are declarative (memories you can vocalise) and non-declarative (memories you’d fine hard to talk about or explain). Examples of declarative memories include semantic memories (e.g. facts) and episodic memories (personal memories). These differ from non-declarative memories such as procedural memories (e.g. how to ride a bike) or emotional memories (e.g. feeling happy). These sub-systems of memory help researchers to classify memory deficits and measure how they are learnt.

Hopefully, you now know a bit more about the things which make up our long term memory. However, there is more to its different components than the memories they contain – these different types of memory are stored in different areas of the brain. Although the range of locations in the brain used to store memories can get quite confusing, I’ll aim to make it as simple as possible.

The hippocampus is a structure in the middle of the temporal lobe on both sides of the brain, and is vital for the storage and recollection of episodic and some semantic memories. This is known because of research on case studies, such as patient H.M. whose hippocampus was removed to cure his violent epilepsy. He was left unable to form new declarative memories (anterograde amnesia) and his declarative memories from the years just before his operation had also disappeared (retrograde amnesia). However, he was still able to carry out motor functions such as walking, in other words, his procedural memory was still intact. This therefore shows that procedural memories must be stored in another area of the brain.

As well as the hippocampus, it is also believed that the structures around it such as the mammillary bodies, fornix and perirhinal cortex are also important for remembering. The perirhinal cortex is thought to be especially important for semantic memory. The frontal lobe is also thought to play a part in memory, especially for memories which have been rehearsed several times, and perhaps date back years. It is thought that stronger traces are developed as each memory is recalled over and over, which means that it becomes less dependent on the hippocampus.

As for procedural memory, this is thought to be reliant on several structures in the brain that are involved in motor-learning, such as the basal ganglia and the cerebellum (the structure at the back of the brain). The amygdala are next to the hippocampus and are involved in processing emotional memory, something which is also done by the frontal cortex.

I hope you now understand human long term memory a bit better than you did before you read this post! As you can see, it relies upon many structures in the brain: there is no one area responsible for our memories. Check back for future posts about memory impairments, and how reliable our memory actually is 🙂