The long-term memory
The long-term memory is a permanent knowledge store. It stores all the impressions, experiences, information, emotions, skills, words, dates and facts we have accumulated in our lifetime. It comprises the overall knowledge of a person. Unlike the short term memory its capacity is virtually unlimited. Information can be stored in long-term memory for minutes to years or even an entire lifespan.
Since the Canadian psychologist Endel Tulving published his studies in 1972, long-term memory has been divided into two main groups: declarative memory (explicit memory) and procedural memory (implicit memory).
Declarative Memory/Explicit Memory
Declarative memory can be divided again into two categories: episodic memory (or memory of personal experiences) and semantic memory (or general memory).
In episodic memory we store everyday events (like yesterday's lunch), which we only remember for a short period of time, as well as significant events, such as the birth of a child, cases of death, the first date, the honeymoon ... all these kind of events. All the memories in our life which are associated with events, are called episodic memories. They are associated with the experience with which they were stored, and can fade as time goes by, depending on the meaning we give to the event. The stronger the impression is and the more often we think about it, the more present the memory will be in our minds.
In semantic memory we store general knowledge, such as "Paris is the capital of France", "The Taj Mahal is in India" or "the Pythagorean theorem". It refers to all the memories or meanings of words and concepts, formula and facts, etc. The retrieval time is fantastic. We can tell almost immediately whether we know the first President of the United States or not. By the way, it was George Washington. But you have already received this information from your brain. It is amazing to have access to an unimaginable amount of data everyone has stored. You may not immediately be able to recall the name but you can tell very quickly whether you know it or not. Sometimes it happens that you say: "It is on the tip of my tongue". Then the brain is still looking for an information but it is already clear whether you know it or not.
Procedural Memory/Implicit Memory
Procedural memory refers to actions we perform automatically and which reside below the level of conscious awareness. This concerns, in particular, motor skills like walking, biking, roller skating, swimming, dancing or skiing. It is believed that skills we once acquired (referred to as procedural memories), can last a lifetime. If someone has not ridden a bike for years, he will be able to bike again without any problems after only a few minutes. Scientists have found out that complex skills like biking will last a lifetime because balance is strongly involved. However, the ability to play the piano or drive a car can decrease without regular practice.
Learned content can fade after a while. However, it can become conscious again in particular external conditions or events. You may have difficulties to remember the name of a school friend being on your own but being with other school friends at a class reunion and talking about the « good old times », the name will be present. If learned content is not used regularly, it can be forgotten. Fortunately, it can also be refreshed because unused neuronal connections are not completely eliminated but rather only regressed; you do not have to start again at zero.