Endel Tulving:

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Experiments conducted in the 1970s by the Canadian psychologist Endel Tulving and his associate Daniel Schacter proved that "intension" (such as concepts) and "extension" (such as episodes) are dealt with by two different types of memory. Episodic memory contains specific episodes of the history of the individual, while semantic memory contains general knowledge (both concepts and facts) applicable to different situations.

Episodic memory, which receives and stores information about temporally-dated episodes and spatiotemporal relations among them, is a faithful record of a person's experience. Semantic memory, instead, is organized knowledge about the world. Tulving believes these memory systems are physically distinct because their behavior is significantly different. In episodic memory, for example, the recall of a piece of information depends on the conditions ("cues") under which that piece of information has been learned (an explicit or implicit reference to it). There are at least two more aspects of memory that fall neither into the intensions or extensions.

Procedural memory allows us to learn new skills and acquire habits. William James had been particularly interested in this kind of memory, having realized how important "habits" are to determine our behavior. He reduced habits to a sequence of "reflexes", i.e. stimulus-response events. Basically, each stimulus-response pattern, once learned, becomes the building block for more complex patterns which are our "habits", each of which is in turn a building block to create more complex "habits". The French philosopher Henri Bergson explicitly separated the memory of habits from the memory of events.

Implicit memory is "unconscious" memory, memory without awareness: unlike other types of memories, retrieval cues do not bring about a recollection of them. Implicit memories are weakly encoded memories which can nonetheless affect conscious thought and behavior. Implicit memories are not lost: they just cannot be retrieved. Amnesia is the standard condition of human memory: most of what happens is not recorded in a form that can be retrieved. In the first years, because of incompletely developed brain structures, most memories are lost or warped. Nonetheless, memories of childhood are preserved without awareness of remembering. Implicit memory is the one activated in "priming" events, or in the identification of words and objects.

That makes a grand total of four different types of memory: procedural, semantic, episodic and implicit.

Tulving also devised a scheme by which memory can associate a new perception or thought to an old memory: the remembering of events always depends on the interaction (or compatibility) between encoding and retrieval conditions.

It now appears that the brain accomodates several different memory systems, each of them involving the cortex but each characterized by different "pathways" leading from the cortex to other areas of the brain. Studies on amnesia (particularly by Neal Cohen in 1980) show that there are at least two separate memory systems: "declarative memory" (the memory that one can consciously remember, which is forgotten in an amnesia) and "procedural memory" (the skills and procedures which are usually not forgotten, as people with amnesia can still perform most actions they have learned throughout their lives). It appears that the hippocampus is the key to declarative memory, or at least the key to linking together declarative memories. Procedural memory is realized by circuits that involve the motor areas of the cortex and two loops that spread through the striatum and the cerebellum: acquiring skills is, indeed, a complex phenomenon. Emotional memory, on the other hand, seems to depend on the working of the amygdala, i.e. on another separate memory system. These three memory systems are physically connected to the cortex along different pathways, which means that they can work in parallel.

Tulving summarized the relationship between remembered and rememberer in the "encoding specificity principle": remembering depends on the affinity between encoding and decoding. Memories are encoded in a way that depends on the circumstances when the event originally happened. The likelihood of recalling a memory (of decoding it) depends on recreating those circumstances, on reinstating the same psychological state. In other words, the way we feel about an event play an important role in the way that event can later be recalled. For example, the feeling that I feel when I read a sentence is going to be important for later recalling that sentence. That feeling has become part of the episode, as it is encoded in my memory.

Tulving's episodic memory packages different aspects of an event to give it the "autobiographical" feeling that makes it more than just a retrieval of information, it makes it a memory of something that happened in our life. In other words, an essential part of an episodic memory is the "rememberer". The rememberer does more than retrieve information about a past event: the rememberer experiences that event again. In fact, the episodic memory is more about the feeling of being there than about the event in itself: the feeling of the event is generally recalled in more accurate terms than the details of the event. In fact, it is easier to remember something that happened a long time ago but had a strong emotional impact on us than something that happened just minutes ago. I do not remember what I had for lunch two days ago, but I do remember episodes of my childhood that happened several decades ago (if I focus, I can even feel what I felt then). Ultimately, episodic memory is about the rememberer, not the remembered.

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