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Overview
Memory is a broad term that encompasses several distinct functions, each depending
on different structures and circuits in the brain. In the most general
terms, memory can be classified into two types, Declarative and Nondeclarativememory,
based on whether the retrieval of stored information is conscious or unconscious.
An example of a nondeclarative memory would be a
basic understanding of the rules of chess or checkers. One can
know the rules of these games without knowing exactly where or
when one learned them. An example of a nondeclarative memory would
be a memory for a particular chess game with a particular person
at a particular time. Because declarative memory deals with episodes,
it is also called episodic memory. These two types of memory rely
on different systems in the brain, thus injury to the brain may
impair declarative memory, for example, but leave nondeclarative
memory intact (or visa-versa).
In addition to the terms declarative and nondeclarative,
other terms that have been used to refer to memory processes include:
We will briefly discuss the meanings of each of these
terms below and the regions of the brain that are particularly
important for these functions:
Working
Memory
The term working memory is used to describe the process where
one “holds
on” to small bits of recently learned information. An example of working
memory would be the ability to hold on to the digits of a phone number in the
short time between hearing them and dialing the number. Another example would
be the base runner in a baseball game that must keep track of the number of
outs and runners on base. The capacity of our working memory is limited, allowing
us to remember only a few bits of information at one time.
Anatomy
Working memory is thought to involve the Frontal Cortex and Parietal Lobe.
Episodic
Memory
Episodic memory is the memory of an event or “episode”. This type
of memory is declarative and is relevant both to recent and remote events.
Examples of episodic memory would be the ability to recount what you did today
and where you were when you heard shocking news. Another example would be remembering
where you parked your car. Episodic memory can be thought of as a process with
several different steps, each of which relies on a separate system of the brain.
The initial step in forming this type of memory is
called encoding. Encoding
is necessary for the acquisition of new factual knowledge. Encoding
is an effortful process, and more effort results in better encoding.
For instance, if one were to hear a list of 10 words and asked
to remember the list after a half an hour, one would likely remember
more words if one were to think of a definition for each word than
if one were to simply repeat the list a couple of times. The work
done with the list in the encoding phase results in better memory
later on. Another example of a frequently used encoding strategy
is the use of mnemonics, or associations between something to be
remembered and something one already knows. For instance, if one
were introduced to someone named Charlie, one might make a note
that this is the same name as one’s uncle.
The step by which the information is accessed and
brought back into consciousness is called retrieval.
This too is an effortful process. If someone were to ask a person
where they were for dinner four nights ago, the answer might not
readily come to mind. Rather, one might take a strategy to try
to provide the answer, like thinking about what day was four days
ago, say it was Tuesday, what one usually does for dinner on Tuesday,
and whether this was a typical Tuesday or not.
Research and clinical experience suggest that information
is not simply stored and retrieved, and that there may be an intermediate
step in this process, which we call consolidation.
Consolidation is the process by which recently learned information
becomes more strongly represented in the brain, and more resistant
to the effects of hippocampal injury (see below).
Anatomy
The parts of the brain that are thought to be necessary for the effortful/variable
aspects of encoding and retrieval are the frontal lobe and the lateral temporal
lobes. When these regions are injured, memory encoding is disorganized, but
can still occur. Memory retrieval will be disorganized as well, but can be
aided with cues to organize retrieval. For instance, in someone who has impaired
retrieval from frontal lobe damage, a cue such as where they met someone
will help with the recall of the person’s name.
Studies show that the hippocampus and
medial temporal lobes are critical for encoding. No matter how efficient the
encoding strategy, if the hippocampus is severly injured, encoding will not
occur. Because the encoding impairment is so profound in hippocampal damage,
events will essentially disappear from memory once they are not attended to,
and even hints or cues will not help to bring them back, because retrieval
of these lost memories is impossible. Episodic memory is frequently impaired
in Alzheimer’s disease because the hippocampus is very vulnerable to
the changes occurring in this disorder.
The figure below shows a “slice” from
an MRI scan of two patients, one from a normal older person and
the other from a patient with Alzheimer’s disease. The brains
are being viewed as if they are being cut about a third of the
way back from their front tip (like cutting into a loaf of bread)
and then looking at the cut surface. The hippocampi from these
two individuals are circled in yellow, and one can see that the
hippocampus is smaller in the Alzheimer’s disease patient.
The hippocampus is part of a network of regions in the brain important for
memory. Research suggests that the mediodorsal nucleus of the thalamus, amygdala,
and hippocampus may work in concert to regulate which information is consolidated
in memory.
Diseases affecting Episodic
Memory
Because encoding and retrieval are effortful processes, they can be affected
by any problem that impairs the ability to make efforts for memory. This can
occur from damage to several different parts of the brain, and can be associated
with medication use, as well as psychological problems such as depression,
or even physical problems such as pain. This is probably one reason why memory
problems are the most common cognitive complaint.
There are a wide range of neurologic diseases and
conditions that can affect episodic memory. These include, but
are not limited to subarachnoid hemorrhage, trauma, hydrocephalus,
tumors, metabolic conditions including Vitamin B1 deficiency, infectious
and inflammatory conditions, such as Hashimoto’s
Encephalopathy,
and neurodegenerative diseases such as Alzheimer’s disease.
Semantic
Memory
Semantic memory is strongly language-based and describes memory for facts and
words. This can be thought of as a form of non-declarative memory because the
knowledge is often not associated with a particular time or place. However,
semantic memory must be thought of as separate because damage to brain regions
that impair semantic memory do not always affect other forms of non-declarative
memory, and visa-versa.
Remote
Memory
Memory of events that occurred in the distant past is referred to as remote
memory.
Anatomy
The underlying anatomy of remote memory is poorly understood, in part because
testing this type of memory must be personalized to a patient’s autobiographical
past. What is known is that, like semantic memory, remote memory eventually
becomes independent of the hippocampus.
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© 2008 The Regents of the University of California
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