Long-term memory (LTM) is the stage of the
Atkinson–Shiffrin memory model in which informative knowledge is held indefinitely. It is defined in contrast to
short-term and
working memory
Working memory is a cognitive system with a limited capacity that can hold information temporarily. It is important for reasoning and the guidance of decision-making and behavior. Working memory is often used synonymously with short-term memory, ...
, which persist for only about 18 to 30 seconds. Long-term memory is commonly labelled as
explicit memory
Explicit memory (or declarative memory) is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and con ...
(
declarative), as well as
episodic memory
Episodic memory is the memory of everyday events (such as times, location geography, associated emotions, and other contextual information) that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred ...
,
semantic memory
Semantic memory refers to general world knowledge that humans have accumulated throughout their lives. This general knowledge (word meanings, concepts, facts, and ideas) is intertwined in experience and dependent on culture. We can learn about n ...
,
autobiographical memory
Autobiographical memory is a memory system consisting of episodes recollected from an individual's life, based on a combination of episodic (personal experiences and specific objects, people and events experienced at particular time and place) a ...
, and
implicit memory
In psychology, implicit memory is one of the two main types of long-term human memory. It is acquired and used unconsciously, and can affect thoughts and behaviours. One of its most common forms is procedural memory, which allows people to perfo ...
(
procedural memory
Procedural memory is a type of implicit memory ( unconscious, long-term memory) which aids the performance of particular types of tasks without conscious awareness of these previous experiences.
Procedural memory guides the processes we perform ...
).
Dual-store memory model
According to
Miller
A miller is a person who operates a mill, a machine to grind a grain (for example corn or wheat) to make flour. Milling is among the oldest of human occupations. "Miller", "Milne" and other variants are common surnames, as are their equivalent ...
, whose paper in 1956 popularized the theory of the "magic number seven", short-term memory is limited to a certain number of chunks of information, while long-term memory has a limitless store.
Atkinson–Shiffrin memory model
According to the dual store memory model proposed by
Richard C. Atkinson and
Richard Shiffrin
Richard Shiffrin (born March 13, 1942) is an American psychologist, professor of cognitive science in the Department of Psychological and Brain Sciences at Indiana University, Indiana University, Bloomington. Shiffrin has contributed a number of t ...
in 1968, memories can reside in the short-term "buffer" for a limited time while they are simultaneously strengthening their associations in long-term memory. When items are first presented, they enter short-term memory for approximately twenty to thirty seconds, but due to its limited space, as new items enter, older ones are pushed out. The limit of items that can be held in the short-term memory is an average between four and seven, yet, with practice and new skills that number can be increased. However, each time an item in short-term memory is rehearsed, it is strengthened in long-term memory. Similarly, the longer an item stays in short-term memory, the stronger its association becomes in long-term memory.
Baddeley's model of working memory
In 1974
Baddeley Baddeley is a surname, and may refer to:
* Aaron Baddeley, Australian-American golfer
* Alan Baddeley, English professor of psychology
* Angela Baddeley, English actress
* Gavin Baddeley, English reverend and journalist
* Herbert Baddeley, Englis ...
and
Hitch proposed an alternative theory of short-term memory:
Baddeley's model of working memory
Baddeley's model of working memory is a model of human memory proposed by Alan Baddeley and Graham Hitch in 1974, in an attempt to present a more accurate model of primary memory (often referred to as short-term memory). Working memory splits pri ...
. According to this theory, short-term memory is divided into different slave systems for different types of input items, and there is an executive control supervising what items enter and exit those systems. The slave systems include the phonological loop, the visuo-spatial sketchpad, and the episodic buffer (later added by Baddeley).
Encoding of information
Long-term memory encodes information semantically for storage, as researched by
Baddeley Baddeley is a surname, and may refer to:
* Aaron Baddeley, Australian-American golfer
* Alan Baddeley, English professor of psychology
* Angela Baddeley, English actress
* Gavin Baddeley, English reverend and journalist
* Herbert Baddeley, Englis ...
. In vision, the information needs to enter working memory before it can be stored into long-term memory. This is evidenced by the fact that the speed with which information is stored into long-term memory is determined by the amount of information that can be fit, at each step, into visual working memory. In other words, the larger the capacity of working memory for certain stimuli, the faster will these materials be learned.
Synaptic consolidation is the process by which items are transferred from short-term to long-term memory. Within the first minutes or hours after acquisition, the
engram (memory trace) is encoded within synapses, becoming resistant (though not immune) to interference from outside sources.
As long-term memory is subject to fading in the natural
forgetting
Forgetting or disremembering is the apparent loss or modification of information already encoded and stored in an individual's short or long-term memory. It is a spontaneous or gradual process in which old memories are unable to be recalled from ...
process,
maintenance rehearsal (several recalls/retrievals of memory) may be needed to preserve long-term memories. Individual retrievals can take place in increasing intervals in accordance with the principle of
spaced repetition
Spaced repetition is an evidence-based learning technique that is usually performed with flashcards. Newly introduced and more difficult flashcards are shown more frequently, while older and less difficult flashcards are shown less frequently i ...
. This can happen quite naturally through reflection or deliberate
recall
Recall may refer to:
* Recall (bugle call), a signal to stop
* Recall (information retrieval), a statistical measure
* ''ReCALL'' (journal), an academic journal about computer-assisted language learning
* Recall (memory)
* ''Recall'' (Overwatch ...
(also known as recapitulation), often dependent on the perceived importance of the material. Using testing methods as a form of recall can lead to the
testing effect The testing effect (also known as retrieval practice, active recall, practice testing, or test-enhanced learning) suggests long-term memory is increased when some of the learning period is devoted to retrieving information from memory. It is diff ...
, which aids long-term memory through information retrieval and feedback.
In long term memory, brain cells fire in specific patterns. When someone experiences something in the world, the brain responds by creating a pattern of specific nerves firing in a specific way to represent the experience. This is called distributed representation. Distributed representation can be explained through a scientific calculator. At the top of the calculator is an opening in which the numbers typed in show up. This small slot is compiled by many blocks that light up to show a specific number. In that sense, certain blocks light up when prompted to show the number 4, but other blocks light up to show the number 5. There may be overlap in the blocks used, but ultimately, these blocks are able to generate different patterns for each specific situation. The encoding of specific episodic memories can be explained through distributed representation. When you try to remember an experience, perhaps your friend's birthday party a year ago, your brain is activating a certain pattern of neurons. If you try to remember your mother's birthday party, another pattern of neurons are fired but there may be overlap because they are both birthday parties. This kind of remembering is the idea of retrieval because it involves recalling the specific distributed representation created during the encoding of the experience.
Sleep
Some theories consider
sleep
Sleep is a sedentary state of mind and body. It is characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity and reduced interactions with surroundings. It is distinguished from wakefulness by a de ...
to be an important factor in establishing well-organized long-term memories. ''(See also
sleep and learning
Multiple hypotheses explain the possible connections between sleep and learning in humans. Research indicates that sleep does more than allow the brain to rest; it may also aid the consolidation of long-term memories.
REM sleep and slow-wave slee ...
.)'' Sleep plays a key function in the consolidation of new memories.
According to Tarnow's theory, long-term memories are stored in
dream
A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. Humans spend about two hours dreaming per night, and each dream lasts around 5 to 20 minutes, althou ...
format (reminiscent of Penfield & Rasmussen's findings that electrical excitations of the cortex give rise to experiences similar to dreams). During waking life an executive function interprets long-term memory consistent with reality checking . It is further proposed in the theory that the information stored in memory, no matter how it was learned, can affect performance on a particular task without the subject being aware that this memory is being used. Newly acquired declarative memory traces are believed to be reactivated during NonREM sleep to promote their hippocampo-neocortical transfer for long-term storage. Specifically, new declarative memories are better remembered if recall follows Stage II non-rapid eye movement sleep. The reactivation of memories during sleep can lead to lasting synaptic changes within certain neural networks. It is the high spindle activity, low oscillation activity, and delta wave activity during NREM sleep that helps to contribute to declarative memory consolidation. In learning before sleep, spindles are redistributed to neuronally active up-states within slow oscillations during NREM sleep.
Sleep spindles are thought to induce synaptic changes and thereby contribute to memory consolidation during sleep. Here, we examined the role of sleep in the object-place recognition task, a task closely comparable to tasks typically applied for testing human declarative memory: It is a one-trial task, hippocampus-dependent, not stressful and can be repeated within the same animal. Sleep deprivation reduces vigilance or arousal levels, affecting the efficiency of certain cognitive functions such as learning and memory.
The theory that sleep benefits memory retention is not a new idea. It has been around since Ebbinghaus's experiment on forgetting in 1885. More recently studies have been done by Payne and colleagues and Holtz and colleagues.
In Payne and colleague's
experiment participants were randomly selected and split into two groups. Both groups were given semantically related or unrelated word pairs, but one group was given the information at 9 am and the other group received theirs at 9 pm. Participants were then tested on the word pairs at one of three intervals 30 minutes, 12 hours, or 24 hours later. It was found that participants who had a period of sleep between the learning and testing sessions did better on the memory tests. This information is similar to other results found by previous experiments by Jenkins and Dallenbach (1924). It has also been found that many domains of declarative memory are affected by sleep such as emotional memory, semantic memory, and direct encoding.
Holtz
found that not only does sleep affect consolidation of declarative memories, but also procedural memories. In this experiment, fifty adolescent participants were taught either word pairs (which represents declarative memory) and a finger tapping task (procedural memory) at one of two different times of day. What they found was that the procedural finger tapping task was best encoded and remembered directly before sleep, but the declarative word pairs task was better remembered and encoded if learned at 3 in the afternoon.
Divisions
The
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
does not store memories in one unified structure. Instead, different types of memory are stored in different regions of the brain. Long-term memory is typically divided up into two major headings:
explicit memory
Explicit memory (or declarative memory) is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and con ...
and
implicit memory
In psychology, implicit memory is one of the two main types of long-term human memory. It is acquired and used unconsciously, and can affect thoughts and behaviours. One of its most common forms is procedural memory, which allows people to perfo ...
.
Explicit memory
Explicit memory
Explicit memory (or declarative memory) is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and con ...
(or declarative memory) refers to all memories that are consciously available. These are encoded by the
hippocampus
The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
,
entorhinal cortex
The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in the ...
, and
perirhinal cortex
The perirhinal cortex is a cortical region in the medial temporal lobe that is made up of Brodmann areas 35 and 36. It receives highly processed sensory information from all sensory regions, and is generally accepted to be an important region f ...
, but consolidated and stored elsewhere. The precise location of storage is unknown, but the temporal cortex has been proposed as a likely candidate. Research by Meulemans and Van der Linden (2003) found that
amnesiac patients with damage to the
medial temporal lobe
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe is involved in pro ...
performed more poorly on explicit learning tests than did healthy controls. However, these same amnesiac patients performed at the same rate as healthy controls on implicit learning tests. This implies that the medial temporal lobe is heavily involved in explicit learning, but not in implicit learning.
Declarative memory has three major subdivisions:
Episodic memory
Episodic memory
Episodic memory is the memory of everyday events (such as times, location geography, associated emotions, and other contextual information) that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred ...
refers to memory for specific events in time, as well as supporting their formation and retrieval. Some examples of episodic memory would be remembering someone's name and what happened at your last interaction with each other.
Experiments conducted by Spaniol and colleagues indicated that older adults have worse episodic memories than younger adults because episodic memory requires
context dependent memory.
Semantic memory
Semantic memory
Semantic memory refers to general world knowledge that humans have accumulated throughout their lives. This general knowledge (word meanings, concepts, facts, and ideas) is intertwined in experience and dependent on culture. We can learn about n ...
refers to knowledge about factual information, such as the meaning of words. Semantic memory is independent information such as information remembered for a test.
In contrast with episodic memory, older adults and younger adults do not show much of a difference in semantic memory, presumably because semantic memory does not depend on context memory.
Autobiographical memory
Autobiographical memory
Autobiographical memory is a memory system consisting of episodes recollected from an individual's life, based on a combination of episodic (personal experiences and specific objects, people and events experienced at particular time and place) a ...
refers to knowledge about events and personal experiences from an individual's own life. Though similar to
episodic memory
Episodic memory is the memory of everyday events (such as times, location geography, associated emotions, and other contextual information) that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred ...
, it differs in that it contains only those experiences which directly pertain to the individual, from across their lifespan. Conway and Pleydell-Pearce (2000) argue that this is one component of the self-memory system.
Implicit memory
Implicit memory
In psychology, implicit memory is one of the two main types of long-term human memory. It is acquired and used unconsciously, and can affect thoughts and behaviours. One of its most common forms is procedural memory, which allows people to perfo ...
(
procedural memory
Procedural memory is a type of implicit memory ( unconscious, long-term memory) which aids the performance of particular types of tasks without conscious awareness of these previous experiences.
Procedural memory guides the processes we perform ...
) refers to the use of objects or movements of the body, such as how exactly to use a pencil, drive a car, or ride a bicycle. This type of memory is encoded and it is presumed stored by the
striatum
The striatum, or corpus striatum (also called the striate nucleus), is a nucleus (a cluster of neurons) in the subcortical basal ganglia of the forebrain. The striatum is a critical component of the motor and reward systems; receives glutamate ...
and other parts of the
basal ganglia
The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an extern ...
. The basal ganglia is believed to mediate procedural memory and other brain structures and is largely independent of the hippocampus. Research by Manelis, Hanson, and Hanson (2011) found that the reactivation of the
parietal and
occipital regions was associated with implicit memory. Procedural memory is considered non-declarative memory or unconscious memory which includes
priming
Priming may refer to:
* Priming (agriculture), a form of seed planting preparation, in which seeds are soaked before planting
* Priming (immunology), a process occurring when a specific antigen is presented to naive lymphocytes causing them to d ...
and
non-associative learning.
The first part of nondeclarative memory (implicit memory) involves priming. Priming occurs when you do something faster after you have already done that activity, such as writing or using a fork.
Other categories of memory may also be relevant to the discussion of long-term memory. For example:
Emotional memory
Emotion can have a powerful effect on humans and animals. Numerous studies have shown that the most vivid autobiographical memories tend to be of emotional events, which are likely to be recalled more often and with more clarity and detail than ...
, the memory for events that evoke a particularly strong emotion, is a domain that can involve both declarative and procedural memory processes. Emotional memories are consciously available, but elicit a powerful, unconscious physiological reaction. Research indicates that the
amygdala
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verte ...
is extremely active during emotional situations, and acts with the
hippocampus
The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
and
prefrontal cortex
In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, BA13, BA14, BA24, BA25, BA32, BA44, BA45, BA46 ...
in the encoding and consolidation of emotional events.
Working memory
Working memory is a cognitive system with a limited capacity that can hold information temporarily. It is important for reasoning and the guidance of decision-making and behavior. Working memory is often used synonymously with short-term memory, ...
is not part of long-term memory, but is important for long-term memory to function. Working memory holds and manipulates information for a short period of time, before it is either forgotten or encoded into long-term memory. Then, in order to remember something from long-term memory, it must be brought back into working memory. If working memory is overloaded it can affect the encoding of long-term memory. If one has a good working memory they may have a better long-term memory encoding.
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. Some women may experience more memory lapses following the onset of the menopause.
In general, more serious problems with memory occur due to
traumatic brain injury
A traumatic brain injury (TBI), also known as an intracranial injury, is an injury to the brain caused by an external force. TBI can be classified based on severity (ranging from mild traumatic brain injury TBI/concussionto severe traumatic b ...
or
neurodegenerative disease
A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...
.
Traumatic brain injury
The majority of findings on memory have been the result of studies that
lesion
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classif ...
ed 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
Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrica ...
. His subsequent total
anterograde amnesia
In neurology, anterograde amnesia is the inability to create new memories after the event that caused amnesia, leading to a partial or complete inability to recall the recent past, while long-term memories from before the event remain intact ...
and partial
retrograde amnesia
In neurology, retrograde amnesia (RA) is a loss of memory-access to events that occurred or information that was learned in the past. It is caused by an injury or the onset of a disease. It tends to negatively affect episodic, autobiographical, ...
provided the first evidence for the localization of memory function, and further clarified the differences between declarative and procedural memory.
Neurodegenerative diseases
Many neurodegenerative diseases can cause memory loss. Some of the most prevalent (and, as a consequence, most intensely researched) include
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...
,
dementia
Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
,
Huntington's disease
Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an uns ...
,
multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
, and
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
. 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.
Those with Alzheimer's disease generally display symptoms such as getting momentarily lost on familiar routes, placing possessions in inappropriate locations and distortions of existing memories or completely forgetting memories. Researchers have often used the
Deese–Roediger–McDermott paradigm
The Deese–Roediger–McDermott (DRM) paradigm is a procedure in cognitive psychology used to study false memory in humans. The procedure was pioneered by James Deese in 1959, but it was not until Henry L. Roediger III and Kathleen McDermott e ...
(DRM) to study the effects of Alzheimer's disease on memory. The DRM paradigm presents a list of words such as doze, pillow, bed, dream, nap, etc., with a theme word that is not presented. In this case, the theme word would have been sleep. Alzheimer's disease patients are more likely to recall the theme word as being part of the original list than healthy adults. There is a possible link between longer encoding time and increased
false memory
In psychology, a false memory is a phenomenon where someone recalls something that did not happen or recalls it differently from the way it actually happened. Suggestibility, activation of associated information, the incorporation of misinformat ...
in LTM. The patients end up relying on the gist of information instead of the specific words themselves. Alzheimer's leads to an uncontrolled inflammatory response brought on by extensive amyloid deposition in the brain, which leads to cell death in the brain. This gets worse over time and eventually leads to cognitive decline, after the loss of memory. Pioglitazone may improve cognitive impairments, including memory loss and may help protect long-term and visuospatial memory from neurodegenerative disease.
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
patients have problems with cognitive performance; these issues resemble what is seen in frontal lobe patients and can often lead to dementia. It is thought that Parkinson's disease is caused by degradation of the dopaminergic mesocorticolimbic projection originating from the ventral tegmental area. It has also been indicated that the hippocampus plays an important role in episodic and spatial (parts of LTM) memory and Parkinson's disease patients have abnormal hippocampuses resulting in abnormal functioning of LTM. L-dopa injections are often used to try to relieve Parkinson's disease symptoms as well as behavioral therapy.
Schizophrenia patients have trouble with attention and executive functions which in turn affects long-term memory consolidation and retrieval. They cannot encode or retrieve temporal information properly, which causes them to select inappropriate social behaviors. They cannot effectively use the information they possess. The prefrontal cortex, where schizophrenia patients have structural abnormalities, is involved with the temporal lobe and also affects the hippocampus, which causes their difficulty in encoding and retrieving temporal information (including long-term memory).
Biological underpinnings at the cellular level
Long-term memory, unlike short-term memory, is dependent upon the synthesis of new proteins. This occurs within the cellular body, and concerns the particular 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
In neuroscience, long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons ...
(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.
Also,
BDNF
Brain-derived neurotrophic factor (BDNF), or abrineurin, is a protein found in the and the periphery. that, in humans, is encoded by the ''BDNF'' gene. BDNF is a member of the neurotrophin family of growth factors, which are related to the canon ...
is important for the persistence of long-term memories.
The long-term stabilization of synaptic changes is also determined by a parallel increase of pre- and postsynaptic structures such as
synaptic boutons,
dendritic spine
A dendritic spine (or spine) is a small membranous protrusion from a neuron's dendrite that typically receives input from a single axon at the synapse. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical s ...
s, and
postsynaptic density
The postsynaptic density (PSD) is a protein dense ''specialization'' attached to the postsynaptic membrane. PSDs were originally identified by electron microscopy as an electron-dense region at the membrane of a postsynaptic neuron. The PSD is in ...
.
[
]
On the molecular level, an increase of the postsynaptic scaffolding proteins
PSD-95
PSD-95 (postsynaptic density protein 95) also known as SAP-90 (synapse-associated protein 90) is a protein that in humans is encoded by the ''DLG4'' (discs large homolog 4) gene.
PSD-95 is a member of the membrane-associated guanylate kinase (MA ...
and
HOMER1
Homer protein homolog 1 or Homer1 is a neuronal protein that in humans is encoded by the ''HOMER1'' gene. Other names are Vesl and PSD-Zip45.
Structure
Homer1 protein has an N-terminal EVH1 domain, involved in protein interaction, and a C-te ...
c has been shown to correlate with the stabilization of synaptic enlargement.
The cAMP response element-binding protein (
CREB
CREB-TF (CREB, cAMP response element-binding protein) is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements (CRE), thereby increasing or decreasing the transcription of the genes. CREB was first de ...
) is a
transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
which is believed to be important in consolidating short-term to long-term memories, and which is believed to be downregulated in
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...
.
DNA methylation
DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts t ...
and
demethylation Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen at ...
Rats exposed to an intense
learning
Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machine learning, machines ...
event may retain a life-long
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, ...
of the event, even after a single training session. The long-term memory of such an event appears to be initially stored in the
hippocampus
The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
, but this storage is transient. Much of the long-term storage of the memory seems to take place in the
anterior cingulate cortex
In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33.
It is involved ...
. When such an exposure was experimentally applied, more than 5,000 differently methylated DNA regions appeared in the
hippocampus
The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
al
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
of the rats at one and at 24 hours after training. These alterations in methylation pattern occurred at many
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s that were
down-regulated, often due to the formation of new
5-methylcytosine sites in CpG rich regions of the genome. Furthermore, many other genes were
upregulated
In the biological context of organisms' production of gene products, downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external stimulus. The complementary pro ...
, likely often due to hypomethylation. Hypomethylation often results from the removal of methyl groups from previously existing
5-methylcytosines in DNA. Demethylation is carried out by several proteins acting in concert, including
TET enzymes
The TET enzymes are a family of ten-eleven translocation (TET) methylcytosine dioxygenases. They are instrumental in DNA demethylation. 5-Methylcytosine (see first Figure) is a methylated form of the DNA base cytosine (C) that often regulates ge ...
as well as enzymes of the DNA
base excision repair
Base excision repair (BER) is a cellular mechanism, studied in the fields of biochemistry and genetics, that repairs damaged DNA throughout the cell cycle. It is responsible primarily for removing small, non-helix-distorting base lesions from t ...
pathway (see
Epigenetics in learning and memory
While the cellular and molecular mechanisms of learning and memory have long been a central focus of neuroscience, it is only in recent years that attention has turned to the epigenetic mechanisms behind the dynamic changes in gene transcription r ...
). The pattern of induced and repressed genes in brain neurons subsequent to an intense learning event likely provides the molecular basis for a long-term memory of the event.
Contradictory evidence
A couple of studies have had results that contradict the dual-store memory model. Studies showed that in spite of using distractors, there was still both a
recency effect
Serial-position effect is the tendency of a person to recall the first and last items in a series best, and the middle items worst. The term was coined by Hermann Ebbinghaus through studies he performed on himself, and refers to the finding that re ...
for a list of items and a
contiguity
Contiguity or contiguous may refer to:
*Contiguous data storage, in computer science
*Contiguity (probability theory)
*Contiguity (psychology)
*Contiguous distribution of species, in biogeography
*Geographic contiguity of territorial land
*Contigu ...
effect.
Another study revealed that how long an item spends in short-term memory is not the key determinant in its strength in long-term memory. Instead, whether the participant actively tries to remember the item while elaborating on its meaning determines the strength of its store in long-term memory.
Single-store memory model
An alternative theory is that there is only one memory store with associations among items and their contexts. In this model, the context serves as a cue for retrieval, and the recency effect is greatly caused by the factor of context. Immediate and delayed free-recall will have the same recency effect because the relative similarity of the contexts still exists. Also, the contiguity effect still occurs because contiguity also exists between similar contexts.
See also
*
Intermediate-term memory Intermediate-term memory (ITM) is a stage of memory distinct from sensory memory, working memory/short-term memory, and long-term memory. While sensory memory persists for several milliseconds, working memory persists for up to thirty seconds, and l ...
*
De novo protein synthesis theory of memory formation
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Memory and aging
Age-related memory loss, sometimes described as "normal aging" (also spelled "ageing" in British English), is qualitatively different from memory loss associated with types of dementia such as Alzheimer's disease, and is believed to have a differ ...
*
Neurogenesis
Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). It occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs) ...
*
Short-term memory
*
Sensory memory
During every moment of an organism's life, sensory information is being taken in by sensory receptors and processed by the nervous system. Sensory information is stored in sensory memory just long enough to be transferred to short-term memory. Hu ...
Footnotes
References
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Further reading
The role of testing-effect in a long-term memory
{{DEFAULTSORT:Long-Term Memory
Memory
is:Langtímaminni