Memory 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 later.
The precise biological mechanisms of memory are not fully understood, but most scientists believe that memory results from changes in connections or connection strengths between neurons 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 other neuron.
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 Memory
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 of anterograde 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, or both.
Alzheimer's 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 American Library;
R. Klatzky (1980). Human Memory: Structures and Processes. 2nd Edition. New York: WH Freeman.
by Catherine E. Myers. Copyright © 2006 Memory Loss and the Brain