More on human memory, Patient H.M.

There are different types of memory. There is declarative memory (can be put into words) and non-declarative memory (cannot be put into words). Under declarative memory, there is semantic memory (deals with facts and common sense) and episodic memory (deals with bringing back sensory experiences from the past). Under non-delcarative memory, there is procedural memory (deals with muscle action skills without thinking about it - i.e. riding a bike) and priming memory (short-term, not conscious thinking). 

Since I was already on the topic of memory, I thought I might tell you a little bit about patient H.M. Patient H.M. developed epilepsy and agreed to undergo an experimental brain surgery. Surgeons took out his hippocampus (refer to picture on above)and his epilepsy went away, but obviously you can't just surgically remove a part of your brain and expect nothing to happen. So the problem H.M. ran into was that he could not form new memory nor could he remember anything that happened in the past year. However, he remember everything from more than 1 year ago (his childhood, 2 years ago, etc.). The loss of the ability to form new memories is called anterograde amnesia. In other words, H.M. could not form new declarative memories, but he could remember old episodic memories and learn new procedural memories and muscle movements.

Because of this experiment, H.M. led scientists to conclude that declarative memories are hippocampus dependent and non-declarative memories are hippocampus non-dependent. So basically, the hippocampus links memories together. But, as memories grow older, they get consolidated into the neuron network and no longer rely on the hippocampus. Therefore, patient H.M. was able to remember all his old memories but could not remember his more recent ones from the past year.