Every species of animal stores some form of memories in its brain, but how and where memories are stored is still poorly understood and hotly debated. Recent studies support the hypothesis that memories are sets of encoded neural connections and to recall a memory, one must simply stimulate the part of the brain where the neural connection for that memory occurs. As scientists learn more about how the human brain works, our understanding of how we remember events and learn new skills and information is enhanced.
We are constantly receiving sensory information from our environment. Some of this data is held for a few moments and then forgotten, while some is moved into short-term memory, and some is stored in our long-term memory. Short-term memory has a limited storage area - able to hold only about seven independent items at one time and for only about 30 seconds - making this area good for information we need now or will need in the near future. Information that may help us in the future is sent to our long-term memory, where it can last a lifetime.
There are two basic types of long-term memory: procedural and declarative. Each uses different parts of the brain. Procedural memories, also called implicit memories, are the "how-to" memories we use when riding a bike or playing an instrument. It is generally difficult for people to communicate their procedural memories because they are based in a non-conscious understanding. For instance, most people can easily characterize a face as "attractive," but they have a hard time explaining why and cannot provide a definition of attractive. So if you are trying to learn to play a musical instrument and it seems as if you are practicing the same thing over and over again with little to gain, a non-conscious understanding and memory are most likely developing.
Declarative memories are made up of the information we accumulate through repetitious practice as well as events and experiences. Declarative memories are so-called due to our ability to communicate them, and are best maintained through repetition or frequent recollection, as well as systematic methods such as mnemonic devices. There are two sub-categories of declarative memory: episodic and semantic. Semantic memories are based on knowledge and understanding and are not related to any one experience. Episodic memory is the memory of an event or experience and incorporates time, place and emotions during the experience. Most people who suffer from amnesia have deficits in episodic memory and studies have shown that episodic memories are more prevalent in women than men.
When trying to learn a skill, we now know that practicing something well is more important than doing more of it or doing it faster. The non-conscious absorption of a skill is recalled correctly when it has been practiced accurately. This is because non-conscious memories are stored as action sequences. If you mess up a sequence, it is better to continue through the mistake, rather than to start over because the memory of it will end when you resume the sequence.
Memories are not hardwired, meaning a memory can be replaced or imaginarily enhanced through suggestion very easily. This is why memories recalled through hypnotism can not be used in a court of law.
The next time you are trying to remember something, think of what type of memory it is. Is it procedural or declarative? If it's procedural, you may be able to simply start the activity, such as riding a bike, and the memories should return. If it's declarative, then you have to ask yourself if it's episodic or semantic. If it's episodic, recalling one aspect can usually bring back the entire experience. If it's declarative then you just might need some more repetition to get the answer right or you can find an episodic memory that relates to it. For example, if you had an emotional relationship around the time that you learned about the American Civil War, remembering details of that relationship might lead you to forgotten information about the Civil War.
Don't worry too much if you can't remember something because, ironically, stress can cause you to forget! Stay calm and logically work out what kind memory you are seeking.