Long-term memory

Long-term memory (LTM) is memory that can last as little as 30 seconds or as long as decades. It differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 30 seconds. Biologically, short-term memory is a temporary potentiation of neural connections that can become long-term memory through the process of rehearsal and meaningful association. The proposed mechanism by which short-term memories move into LTM storage is via long-term potentiation, which leads to a physical change in the structure of neurons. Notably, the time scale involved at each level of memory processing remains under investigation.

As long-term memory is subject to fading in the natural forgetting process, several recalls/retrievals of memory may be needed for long-term memories to last for years, dependent also on the depth of processing. Individual retrievals can take place in increasing intervals in accordance with the principle of spaced repetition. This can happen quite naturally through reflection or deliberate recall, often dependent on the perceived importance of the material.

Sleep
Some theories consider sleep to be an important factor in establishing well-organized long-term memories. (See also sleep and learning.)

According to the theory of Tarnow, long term memories are stored in dream format (reminiscent of the Penfield & Rasmussen’s findings that electrical excitations of cortex give rise to experiences similar to dreams). During waking life an executive function interprets long term memory consistent with reality checking. At this point in time, this is not considered a generally accepted theory.

Types of memory
The brain does not store memories in one unified structure, as might be seen in a computer's hard disk drive. Instead, different types of memory are stored in different regions of the brain. LTM is typically divided up into two major headings: declarative memory and procedural memory.


 * 1) Declarative memory refers to all memories that are consciously available. These are encoded by the hippocampus, entorhinal cortex, and perirhinal cortex, but consolidated and stored elsewhere in the cortex.  The precise location of storage is unknown, but the temporal cortex has been proposed as a likely candidate.  Declarative memory also has two major subdivisions:
 * 2) *Episodic memory refers to memory for specific events in time
 * 3) *Semantic memory refers to knowledge about the external world, such as the function of a pencil.
 * 4) Procedural memory refers to the use of objects or movements of the body, such as how exactly to use a pencil or ride a bicycle. This type of memory is encoded and probably stored by the cerebellum and the striatum.
 * 5) Emotional memory is the memory for events that evoke a particularly strong emotion. This is not entirely accepted within the scientific community as its own type of memory and may be ultimately classified as either declarative or procedural, but in many ways is a combination of the two.  Emotional memories are consciously available, but elicit a powerful, unconscious physiological reaction.  They also have a unique physiological pathway that involes strong connections from the amygdala into the prefrontal cortex, but much weaker connections running back from the prefrontal cortex to the amgydala.

Disorders of memory
Minor everyday slips and lapses of memory are fairly commonplace, and may increase naturally with age, when ill, or when under stress (Reason J.). Some women may experience more memory lapses following the onset of the menopause. More serious problems with memory generally occur due to traumatic brain injury or neurodegenerative disease:

Traumatic Brain Injury
The majority of findings about memory have been the result of studies that lesioned specific brain regions in rats or primates, but some of the most important work has been the result of accidental or inadvertant brain trauma. The most famous case in memory studies is the case study of HM, who had parts of his hippocampus, parahippocampal cortices, and surrounding tissue removed in an attempt to cure his epilepsy. His subsequent total anterograde and partial retrograde amnesia provided the first evidence for the localization of memory function, and further clarified the differences between declarative and procedural memory.

Neurodegenerative Diseases
Very many neurodegenerative diseases can cause memory loss. Some of the most prevalent (and consequently, most intensely researched) include Alzheimer's Disease, Dementia, Huntington's Disease, Multiple Sclerosis, and Parkinson's Disease. None act specifically on memory; instead memory loss is often a casualty of generalized neuronal deterioration. Currently, these illnesses are irreversible, but research into stem cells, psychopharmacology, and genetic engineering hold much promise.