These are excerpts and elaborations from my book "The Nature of Consciousness"
Types of Memory Experiments performed 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 intension or extension. 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. As the French philosopher
Maine de Biran had already observed two centuries earlier (“The Influence of
Habit on the Faculty of Thinking”, 1804), habits rely on an “implicit” memory. 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 indeed 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, instead, 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 yet another
separate memory system. These three memory systems are physically connected to
the cortex along different pathways, which means that they can work in
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