Types of memory

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Of all human capacities, few spark our interest and inspire
speculation more than memory. This ability to acquire, store, and use
myriad types of information in countless ways is basic to our identity
and allows us to function meaningfully in the world. More than just a
stagnant repository of facts and miscellaneous knowledge, memory is a
highly interactive and dynamic collection of processes that reflect
and inform who we are as unique individuals.

The importance of memory becomes clearest when it is gone. The
capacity to learn and remember--among the many characteristics that
make us human--is eroded and ultimately destroyed when memory-robbing
diseases such as Alzheimer's disease and other dementias strike.

How does the brain learn and transform experiences and sensory input
into memories? Are there different types of memory? How do injury and
disease impair memory? Since the beginnings of modern neuroscience in
the 19th century, scientists in disciplines ranging from molecular
biology to psychology have explored these questions, and the advent of
new technologies has enhanced their ability to piece together the
cellular, molecular, and behavioral components of this remarkable
ability.

Types of memory
Most experts agree that at least several distinct memory systems exist--
sensory, short-term, working, and long-term--and that these systems
interact continuously to give rise to the phenomenon of memory as we
experience it in our everyday lives.

Sensory memory
As we move through the world, we encounter hundreds and thousands of
sensations. These are perceived through sight, sound, smell, taste,
and touch. Each of these senses is associated with a type of memory
that allows us briefly to retain impressions of specific experiences
after the specific sensory stimuli have ceased. Although these
perceptions flow into our sensory memory automatically, they decay
within seconds if we do not consciously attend to them. Sensory
memory, then, acts like a buffer that temporarily stores our sensory
experiences and rapidly jettisons all but those to which we
specifically direct our attention.

Short-term memory
Experts believe that the stimuli that receive sufficient conscious
attention pass selectively into short-term memory, a temporary holding
tank for information. As anyone who has tried to remember a new phone
number long enough to dial it knows, the storage capacity of this
system is very limited, and the retention of information in short-term
memory is easily disrupted by interference.

Working memory
Another distinct way in which the brain temporarily holds information
is through working memory. This type of memory is used to hold
information for a short time while the brain manipulates and processes
it. For example, working memory is used in processes that require
reasoning, such as retaining the meaning of several sentences to
understand an entire paragraph, or performing and retaining all of the
steps of a mental calculation to arrive at a final answer.

Long-term memory
Long-term memories, which experts say are those that endure for more
than 30 seconds, are classified as either declarative or procedural,
according to the type of information and learning processes involved.

Declarative memory involves facts and events learned through conscious
recall. Declarative memory is further subdivided into semantic and
episodic memory, once again according to the type of information
involved. Semantic memories are independent of context, such as time,
place, or circumstances. They consist of our abstract knowledge of the
world, such as the meanings of words; the size, shape, and color of
objects; our implicit understanding of social customs; or our
knowledge of our own time and place. Because we are repeatedly exposed
to such information throughout our lives, semantic memories are
rapidly and effortlessly recalled. Episodic memories, in contrast, are
highly contextual memories of the events that have occurred in our own
lives and include the time and place of these events as well as their
sensory and emotional associations.

Procedural memory involves "how to" knowledge, such as the specific
patterns of hand and finger movements required to play the piano or
the muscle actions necessary to ride a bicycle. Procedural memories
are acquired through repetition rather than active recall and can be
used without conscious effort.

Types of memory also can be distinguished according to their temporal
direction, that is, whether they are recollections of past learning or
events (retrospective memory) or involve content to be remembered for
the future (prospective memory), such as remembering your dentist
appointment next Thursday morning. Retrospective memory includes both
episodic and semantic memory.