Long-term memory (LTM) is memory that can last as little as a few days or as long as decades. It differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 18 seconds (Peterson and Peterson, 1959). 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. Much is not known about the underlying biological mechanisms of long-term memory, but the process of long-term potentiation, which involves a physical change in the structure of neurons, has been proposed as the mechanism by which short-term memories move into long-term storage. 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 (a.k.a. recapitulation), often dependent on the perceived importance of the material.
Long term memory encodes information semantically for storage, as researched by Baddeley. However, there is some evidence that proves that long term memory also encodes by sound for storage. An example of this is when one experiences the "tip of the tongue" state in which one cannot remember a particular word, but they know it sounds similar to another word. The target word is said to be on the tip of the tongue. It is almost grasped, but not totally. This has nothing to do with meaning but with what the word sounds like.
According to Tarnow's theory, 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 (Tarnow 2003).
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 implicit memory (or procedural memory).. Computer programs store information similarly with a separate data section and code section.
Emotional memory, the memory for events that evoke a particularly strong emotion, is another. Emotion and memory is a domain that can involve both declarative and procedural memory processes. Emotional memories are consciously available, but elicit a powerful, unconscious physiological reaction. They also have a unique physiological pathway that involves strong connections from the amygdala into the prefrontal cortex, but much weaker connections running back from the prefrontal cortex to the amgydala.
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:
The everyday experience of memory problems is the problem of failed recall, forgetting. The tip-of-the-tongue phenomenon is particularly frustrating because the person trying to remember feels that the memory is available. In physical terms your neurons are firing, but your receptors aren't catching. Failing to remember something in the situation in which it would have been useful leads to regret.
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 inadvertent brain trauma. The most famous case in recent 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 amnesia and partial retrograde amnesia provided the first evidence for the localization of memory function, and further clarified the differences between declarative and procedural memory.
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, Parkinson's Disease, and Schizophrenia. 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 holds much promise.
Long term memory, unlike short term memory, is dependent upon the construction of new proteins. This occurs within the cellular body, and concerns particularly transmitters, receptors, and new synapse pathways that reinforce the communicative strength between neurons. The production of new proteins devoted to synapse reinforcement is triggered after the release of certain signaling substances (such as calcium within hippocampal neurons) in the cell. In the case of hippocampal cells, this release is dependent upon the expulsion of magnesium (a binding molecule) that is expelled after significant and repetitive synaptic signaling. The temporary expulsion of magnesium frees NMDA receptors to release calcium in the cell, a signal that leads to gene transcription and the construction of reinforcing proteins. For more information, see long-term potentiation (LTP).
One of the newly synthesized proteins in LTP is also critical for maintaining long-term memory. This protein is an autonomously active form of the enzyme protein kinase C (PKC), known as PKMζ. PKMζ maintains the activity-dependent enhancement of synaptic strength and inhibiting PKMζ erases established long-term memories, without affecting short-term memory or, once the inhibitor is eliminated, the ability to encode and store new long-term memories is restored.