| Memory
Memory is a function of the brain: the ability to retain information.
Memory is much studied by cognitive psychology and neuroscience.
There are multiple types of classifications for memory based on
duration, nature and retrieval of perceived items. The main stages
in the formation and retrieval of memory, (from an information processing
perspective) are:
- Encoding (processing and combining of received information)
- Storage (creation of a permanent record of the encoded information)
- Retrieval/Recall (calling back the stored information in response
to some cue for use in some process or activity)
Classification by duration
A basic and generally accepted classification of memory is based
on the duration of memory retention, and identifies three distinct
types of memory: sensory memory, short-term memory, and long-term
memory.
The sensory memory corresponds approximately to the initial moment
that an item is perceived. Some of this information in the sensory
area proceeds to the sensory store, which is referred to as short-term
memory. Sensory memory is characterized by the duration of memory
retention from milliseconds to seconds and short-term memory from
seconds to minutes. These stores are generally characterised as
of strictly limited capacity and duration, whereas in general stored
information can be retrieved in a period of time which ranges from
days to years; this type of memory is called long-term memory.
It may be that short-term memory is supported by transient changes
in neuronal communication, whereas long-term memories are maintained
by more stable and permanent changes in neural structure that are
dependent on protein synthesis. Some psychologists, however, argue
that the distinction between long- and short-term memories is arbitrary,
and is merely a reflection of differing levels of activation within
a single store.
If we are given a random seven-digit number, we may remember it
only for a few seconds and then forget (short-term memory). On the
other hand, we can remember telephone numbers for many years (assuming
we use them often enough). Those long-lasting memories are said
to be stored in long-term memory.
Additionally, the term working memory is used to refer to the short-term
store needed for certain mental tasks - it is not a synonym for
short-term memory, since it is defined not in terms of duration,
but rather in terms of purpose. Some theories consider working memory
to be the combination of short-term memory and some attentional
control. For instance, when we are asked to mentally multiply 45
by 4, we have to perform a series of simple calculations (additions
and multiplications) to arrive at the final answer. The ability
to store the information regarding the instructions and intermediate
results is what is referred to as working memory.
Classification by information type
Long-term memory, the largest part of any model, can be divided
into declarative (explicit) and procedural (implicit) memories.
Declarative memory requires conscious recall, in that some conscious
process must call back the information. It is sometimes called explicit
memory, since it consists of information that is explicitly stored
and retrieved.
Declarative memory can be further sub-divided into semantic memory,
which concerns facts taken independent of context; and episodic
memory, which concerns information specific to a particular context,
such as a time and place. Semantic memory allows the encoding of
abstract knowledge about the world, such as "Paris is the capital
of France". Episodic memory, on the other hand, is used for
more personal memories, such as the sensations, emotions, and personal
associations of a particular place or time. Autobiographical memory
- memory for particular events within one's own life - is generally
viewed as either equivalent to, or a subset of, episodic memory.
Visual memory is part of memory preserving some characteristics
of our senses pertaining to visual experience. We are able to place
in memory information that resembles objects, places, animals or
people in sort of a mental image.[1]
In contrast, procedural memory (or implicit memory) is not based
on the conscious recall of information, but on an implicit learning.
Procedural memory is primarily employed in learning motor skills
and should be considered a subset of implicit memory. It is revealed
when we do better in a given task due only to repetition - no new
explicit memories have been formed, but we are unconsciously accessing
aspects of those previous experiences. Procedural memory involved
in motor learning depends on the cerebellum and basal ganglia.
Classification by temporal direction
A further major way to distinguish different memory functions is
whether the content to be remembered is in the past, retrospective
memory, or whether the content is to be remembered in the future,
prospective memory. Thus, retrospective memory as a category includes
semantic memory and episodic/ autobiographical memory. In contrast,
prospective memory is memory for future intentions, or remembering
to remember (Winograd, 1988). Prospective memory can be further
broken down into event- and time-based prospective remembering.
Time-based prospective memories are triggered by a time-cue, such
as going to the doctor (action) at 4pm (cue). Event-based prospective
memories are intentions triggered by cues, such as remembering to
post a letter (action) after seeing a mailbox (cue). Cues do not
need to be related to the action (as the mailbox example is), and
lists, sticky-notes, knotted hankerchiefs, or string around the
finger (see box) are all examples of cues that are produced by people
as a strategy to enhance prospective memory.
Physiology
Overall, the mechanisms of memory are not well understood. Brain
areas such as the hippocampus, the amygdala, or the mammillary bodies
are thought to be involved in certain kinds of memory. For example,
the hippocampus is believed to be involved in spatial learning and
declarative learning. Damage to certain areas in patients and animal
models and subsequent memory deficits is a primary source of information.
However, rather than implicating a specific area, it could be that
damage to adjacent areas, or to a pathway traveling through the
area is actually responsible for the observed deficit. Further,
it is not sufficient to describe memory, and its counterpart, learning,
as solely dependent on specific brain regions. Learning and memory
are attributed to changes in neuronal synapses, thought to be mediated
by long-term potentiation and long-term depression.
Disorders
Much of the current knowledge of memory has come from studying
memory disorders. Loss of memory is known as amnesia. There are
many sorts of amnesia, and by studying their different forms, it
has become possible to observe apparent defects in individual sub-systems
of the brain's memory systems, and thus hypothesize their function
in the normally working brain.
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