Long-term memory encompasses all learning or knowledge that is stored in the mind for longer than a few seconds or minutes. It encompasses all forms of learning and memory: memory for past events, facts, motor skills, perceptual skills, and conditioning.
Long-term memory interacts with short-term memory (also called working memory). Short-term memory consists of information currently being held in mind—the current contents of consciousness. The interactions between short-term memory and long-term memory are described in the Modal model of memory. In this model, perceptual information from the world is first stored in sensory system-specific buffers; in vision, this is referred to as iconic memory, and in audition, echoic memory. Some information can then pass from sensory memory into short-term memory; the amount of information is limited by attentional resources and the size of the short-term memory store. Once in short-term memory, information can be rehearsed; some information in short-term memory is transferred to long-term memory, generally information that receives more attention or is processed more deeply.
If information is not transferred to long-term memory, it cannot be retrieved later. A good illustration of the interactions among sensory, short-term, and long-term memory is in the common situation of looking up and calling a telephone number. When the telephone number is viewed in the phone book, it enters sensory memory, along with all the other information on the page. However, only the attended phone number moves from sensory to short-term memory. Once in short-term memory, the number can be rehearsed over a period of several seconds, allowing time to go to the phone and dial the number. However, if distracted, the number is lost from short-term memory and must be looked up again. The telephone number is only likely to enter long-term memory, and be accessible later, if the person pays attention to the number and processes its meaning and patterns. This deeper processing can include the use of mnemonic strategies.
There are several qualitative distinctions made within the area of long-term memory based on the type of material being maintained. The primary distinction is between declarative memory and nondeclarative memory. Declarative memory consists of knowledge that can be verbally stated (declared) and can be further subdivided into two forms, semantic and episodic. Semantic memory refers to memory for facts and definitions that are isolated from the specific point in time in which they were acquired. For example, the meaning of the word astronaut or knowledge that George Washington was the first president of the United States would be semantic memory.
Episodic memory refers to memory for the events of one’s life, for example, remembering what happened at your 6th birthday party, or what you ate for breakfast this morning. Episodic memories tend to be complex memories in which specific persons and objects are related to each other and to time (what time of day, season, year) and place (what location). Episodic memory is typically assessed through tests of recall and recognition; in a recognition test, the person is presented with the memorized material and novel items and just has to indicate whether it was one of the memorized items or not; in recall, a person is given cues but must access the information from memory.
Nondeclarative memory, as its name implies, consists of all the forms of memory that are not declarative. These range across a wide variety of phenomena. One large group is motor skills, such as learning to ride a bicycle. Another is perceptual learning, such as learning to recognize new type scripts or being faster at reading a word when it is repeated. A third is simple forms of learning such as habituation, sensitization, and some forms of classical conditioning.
Cognitively, declarative and nondeclarative memories differ in several interesting ways. First, the information represented in declarative memory is more fully available to consciousness than that represented in nondeclarative memory. This has led to the alternative terms explicit and implicit memory being used to refer to declarative and nondeclarative memory, respectively. Declarative memory requires more attention for encoding and recall than nondeclarative memory; the latter can often be gained while subjects are simultaneously performing demanding tasks. Declarative memory is more prone to forgetting across time than nondeclarative; this is the basis of the common observation that one never forgets how to ride a bicycle.
The neural bases of declarative and nondeclarative memory also differ. Declarative memory requires structures in the medial temporal lobe (including the hippocampus), the fornix, and the mammillary bodies. These structures serve to encode new memories. People who sustain damage to these areas develop a condition known as global amnesia, in which they lose the ability to form new long-term memories. They can still access memories from before their brains were damaged, however. Common causes of global amnesia include stroke, anoxia, Korsakoff’s syndrome, and Alzheimer’s disease, although the damage in the latter case is not limited to the medial temporal lobe. The medial temporal lobe and related brain structures interact with other areas of the brain, particularly the lateral anterior and middle temporal lobes, and long-term declarative memories are primarily stored in the latter areas.
An important future research question is whether the different parts of the medial temporal lobe serve different functions. Some research indicates that the hippocampus proper is important for learning relationships between items (and thus is important for episodic memory because it involves learning relationships between the person, other elements of the memory, and the time and place of the memory). Memory for specific items involves areas in the parahippocampal gyrus surrounding the hippocampus.
Nondeclarative memory systems have in common that they do not require the medial temporal lobe, but otherwise differ in their neural substrates. Motor skill learning is generally thought to be reliant on changes in motor areas of the brain, including the basal ganglia, cerebellum, primary motor cortex, and premotor cortex. Perceptual learning involves changes in secondary perceptual cortexes. Classical conditioning is dependent on changes in synapses within circuits connecting perception of the conditioned stimulus and unconditioned response.
Nondeclarative memory is generally considered to have a relatively smooth developmental trajectory. It is present in roughly similar degrees across the entire life span, from infancy to old age. It is thought that much of the learning in prelinguistic children is attributable to nondeclarative memory processes. However, some nondeclarative memory tasks are impaired in older adults, particularly those requiring frontal lobe functioning, which is impaired in old age.
Declarative memory, on the other hand, shows decided changes across the life span. In general, memory is best in early and middle adulthood. One reason for the changes in declarative memory across the life span is developmental changes in the frontal lobes.
The frontal lobes are important in implementing strategies to recall information from long-term memory. The frontal lobes are also one of the last areas of the brain to develop fully (full maturation isn’t finished until early adulthood) and one of the first areas of the brain to show age-related decline in old age. Thus, older people are particularly impaired in recall and less impaired at recognition.
One interesting developmental phenomenon in memory is childhood amnesia, the finding that permanent long-term memories that can be recalled later in life are rare to nonexistent before an average age of 3.5 years. Young children do form long-term memories; even infants can remember a particularly interesting toy across a time delay of days. However, these memories are typically not accessible later in life. It is unclear what mechanism underlies this phenomenon. Freud postulated that it was due to repressing memories of trauma, although this theory is not generally accepted now. Others have hypothesized that memories become inaccessible as children move from prelinguistic to linguistic phases of development because the conceptual structures in the mind change qualitatively.
Another interesting developmental phenomenon is the distinction between memories of the distant past (e.g., one’s 6th birthday party) and memories of more recent events (e.g., breakfast this morning). Older adults often report more difficulty with recent events than events of the distant past. Both are considered to be long-term memory, but they may differ in some psychological and neural mechanisms; this is an important current area of research.
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