refers to the storage, retention and recall of information
including past experiences, knowledge and thoughts. Memory
for specific information can vary greatly according to the
individual and the individual's state of mind. It can also
vary according to the content of the information itself; thus
information which is novel or exciting tends to be better
remembered than information which is uninteresting or ordinary.
Failure of memory can normally result from failure to adequately
store the memory in the first place, failure to retain the
information (forgetting), and failure to retrieve the information
The precise biological mechanisms of memory
are not fully understood, but most scientists believe that
memory results from changes in connections or connection strengths
in the brain. One possible mechanism is long-term
potentiation (LTP). Roughly stated, LTP refers to a process
whereby if two neurons are usually active together, the connection
between them will be strengthened; over time, this means that
activity in one neuron will tend to produce activity in the
Categories of Memory Systems
Psychologists and memory researchers often
divide memory into categories defined by the duration for
which the memory is expected to last.
Sensory memory refers to the fact that,
after experiencing a stimulus, information about that stimulus
is briefly held in memory in the exact form it was received,
until it can be further processed. Typically, sensory memories
may last only a few seconds before decaying -- or being overwritten
by new, incoming information. But, while they last, sensory
memories contains detailed information: almost like an internal
"copy" of the stimulus, in perfect detail. For example, psychologists
have assumed that there is a memory area (called a "buffer")
where incoming visual information is stored as a picture or
icon. This is sometimes called iconic memory. While visual
information remains in iconic memory, an individual can answer
detailed questions, such as what is the third row of numbers
in a numerical display. Psychologists have assumed that there
is also an echoic memory for auditory information (stored
as an echo) and other buffers for information related to the
other senses: taste, smell and touch.
Short-term memory refers to memories which
last for a few minutes. Unlike sensory memory, which is stored
in the exact form it was experienced, short-term memory has
received some processing; thus, "A" is stored not as a visual
stimulus, but as an abstract concept of the letter "A". Short-term
memory is of limited capacity, usually 5-9 items ("7-plus-or-minus-two").
Beyond this capacity, new information can "bump" out other
items from short-term memory. This is one form of forgetting.
Objects in short-term memory can be of indefinite complexity:
thus short-term memory can hold several numbers, or several
words, or several complex concepts simultaneously. Thus, while
an individual may only be able to remember seven random digits,
it may be possible to remember more digits if they are "chunked"
into meaningful objects: thus, "1776-2001-1941" represents
twelve separate digits -- well beyond most people's capacity
-- but only three easily-remembered chunks.
Items can be maintained indefinitely in
short-term memory by rehearsal: e.g. by repeating the information
over and over again. An example would be a seven-digit phone
number, which is maintained in short-term memory by repetition
until the number is dialed, and then fades from short-term
memory once the conversation starts. Repetition may also increase
the probability that items in short-term memory will enter
permanent storage in long-term memory.
Intermediate-term or working memory is sometimes
considered a synonym for short-term memory. However, memory
researchers often consider this a specialized term referring
for information about the current task. Thus, even though
a specific phone number may occupy short-term memory, working
memory contains the information that lets you remember that
you are in the process of phoning the gas company to complain
about a recent billing error.
Long-term memory is memory that lasts for
years or longer. It contains everything we know about the
world, including semantic and factual information as well
as autobiographical experience. In general, long-term memory
is organized so that it is easy to reach a stored item by
a number of routes. For example, the concept "umbrella" may
be retrieved by seeing an umbrella, experiencing a rainstorm,
hearing the words to the song "Let a smile be your umbrella,"
and so on. Retrieval of an item also facilitates other related
items: so that retrieving information about a cat can lead
to retrieval of information about dogs, lions, specific instances
of cats (Grandmother's tabby), the Cheshire Cat from Alice
in Wonderland, and so on.
NOTE: Clinicians (e.g. neurologists) often
use a slightly different classification, in which short-term
memory is memory for events which occurred recently (e.g.
a few days or weeks ago) and long-term memory is memory for
events which occurred in the distant past (e.g. childhood).
Kinds of Long-term Memory
There are several different ways to classify
long-term memories according to their content.
Declarative memory is a term for information
which is available to conscious recollection and verbal retrieval
(i.e., it can be "declared"). Two subclasses of declarative
memory are episodic memory, which is autobiographical information,
and semantic memory, which is factual information about the
world (vocabulary items, knowledge of what a hammer is used
for, memory of multiplication tables, etc.).
Brain Structures involved in Long-Term
Most types of memory appear to be stored
in the cortex.
Different areas of cortex specialize in different kinds of
information, so that visual information about the Statue of
Liberty may be stored in one location (e.g., the inferior
temporal cortex), while information about its associations
to liberty and immigration might be stored in another (e.g.,
the frontal cortex). High linkage between these two areas
means that seeing a picture of the Statue of Liberty can retrieve
memory about its associations. At the same time, damage to
specific areas of cortex can produce specific memory deficits.
For example, damage to a specific region within the temporal
lobe can produce a memory deficit in which the patient loses
knowledge about "living things" (e.g. dogs, lions, birds)
but maintains knowledge about other categories (e.g. inanimate
objects such as furniture and utensils).
Formation of new declarative memories depends
on the hippocampus
and related structures in the medial
temporal lobe. When these structures are damaged, a condition
amnesia can result, in which older declarative memories
are largely spared, but few if any new declarative memories
are acquired. At this point, the process whereby the hippocampus
and other medial temporal lobe structures contribute to long-term
memory formation is still incompletely understood. Some researchers
believe that the hippocampus acts as a temporary store for
new information, which is then gradually transferred to permanent
storage in the cortex. Other researchers believe that the
hippocampus never actually stores information itself, but
is needed by the cortex in the process of developing new memories.
Another important structure is the amygdala,
which lies near the hippocampus in the medial temporal lobes.
The amygdala is critically involved in emotional memory; an
individual with damage to the amygdala may remember the details
of a traumatic (or joyful) event but not the emotional content
of that event.
Pathology of Memory
Memory can be impaired by various injuries
and diseases. Damage to the medial temporal lobe and hippocampus
can devastate the ability to acquire new declarative memory;
damage to the storage areas in cortex can disrupt retrieval
of old memories and interfere with acquisition of new memories
-- simply because there is nowhere to put them.
Another critical factor is attention. Items
are more likely to be remembered if they are attended to in
the first place; this is why novel or exciting items are more
likely to be remembered than dull or ordinary ones. Damage
to the frontal
lobes, which disrupts attention, may affect memory.
Various psychiatric disorders such as paranoia
and schizophrenia may affect memory adversely, either by disrupting
attention or by disrupting the biological bases of memory,
disease causes memory impairments from the early stages,
probably because of cell death in the basal
forebrain, an area that produces the chemical acetylcholine
which facilitates plasticity
(learning). Recent memories tend to be poorly remembered,
while there may be good memory for long-ago events.
Other conditions such as viral infections,
depression and use of drugs (including medication) can affect
memory by disrupting brain chemicals as well.
Although a mild memory impairment is a common
feature of old age, there is currently much debate over whether
memory loss is inevitable with aging, or whether it is a by-product
of conditions (such as Alzheimer's disease and cardiovascular
disease) which are more common in old age than in youth. (See
also: Age-associated Memory Impairment.)
Further reading: L. Squire & E. Kandel (2000).
Memory: From Mind to Molecules. New York: Scientific
by Catherine E. Myers. Copyright © 2006 Memory Loss and the Brain
R. Klatzky (1980). Human Memory: Structures and Processes,
2nd Edition. New York: WH Freeman.