Memory consolidation
Encyclopedia
Memory consolidation is a category of processes that stabilize a memory trace after the initial acquisition. Consolidation is distinguished into two specific processes, synaptic consolidation, which occurs within the first few hours after learning, and system consolidation, where hippocampus-dependent
memories become independent of the hippocampus
over a period of weeks to years. Recently, a third process has become the focus of research, reconsolidation, in which previously consolidated memories can be made labile
again through reactivation of the memory trace.
teacher of rhetoric
Quintillian. He noted the “curious fact… that the interval of a single night will greatly increase the strength of the memory,” and presented the possibility that “… the power of recollection .. undergoes a process of ripening and maturing during the time which intervenes.” The process of consolidation was later proposed based on clinical data illustrated in 1882 by Ribot’s
Law of Regression, “progressive destruction advances progressively from the unstable to the stable”. This idea was elaborated on by William H. Burnham a few years later in a paper on amnesia integrating findings from experimental psychology and neurology. Coining of the term “consolidation” is credited to the German researchers Müller
and Alfons Pilzecker who rediscovered the concept that memory takes time to fixate or undergo “Konsolidierung” in their studies conducted between 1892 and 1900.
Systematic studies of retrograde amnesia
started to emerge in the 1960s and 1970s. These were accompanied by the creation of animal models of human amnesia
in an effort to identify brain substrates critical for slow consolidation. Meanwhile, neuropharmacological
studies of selected brain areas began to shed light on the molecules possibly responsible for fast consolidation. In recent decades, advancements in cellular
preparations, molecular biology
, and neurogenetics
have revolutionized the study of consolidation.
, when discussed in the arena of synaptic consolidation, is memory
that lasts for at least 24 hours. An exception to this 24-hour rule is long-term potentiation
, or LTP, a model of synaptic plasticity
related to learning
, in which an hour is thought to be sufficient. Synaptic consolidation is achieved faster than systems consolidation, within only minutes to hours of learning. LTP, one of the best understood forms of synaptic plasticity, is thought to be a possible underlying process in synaptic consolidation.
are achieved through activating intracellular transduction cascades
. These molecular cascades trigger transcription factors that lead to changes in gene expression
. The result of the gene expression is the lasting alteration of synaptic proteins, as well as synaptic remodeling and growth. In a short time-frame immediately following learning, the molecular cascade, expression and process of both transcription factors and immediate early genes, are susceptible to disruptions. Disruptions caused by specific drugs, antibodies and gross physical trauma can block the effects of synaptic consolidation.
production and receptor
sensitivity, lasting minutes to even days. The process of LTP
is regarded as a contributing factor to synaptic plasticity
and in the growth of synaptic strength, which are suggested to underlie memory formation. LTP is also considered to be an important mechanism in terms of maintaining memories within brain regions, and therefore is thought to be involved in learning. There is compelling evidence that LTP is critical for Pavlovian fear conditioning in rats suggesting that it mediates learning
and memory
in mammals. Specifically, NMDA-receptor antagonists
appear to block the induction of both LTP and fear conditioning
and that fear conditioning increases amygdaloidal synaptic transmission that would result in LTP.
, and as such is considered the ‘fast’ type of consolidation. As soon as six hours after training, memories become impervious to interferences that disrupt synaptic consolidation and the formation of long-term memory.
has been found to enhance memory consolidation, specifically for relational memory. Experimental results suggest that distributing learning over the course of 24 hours decreases the rate of forgetting compared to massed learning
, and enhances relational memory consolidation. When interpreted in the context of synaptic consolidation, mechanisms of synaptic strengthening may depend on the spacing of memory reactivation to allow sufficient time for protein synthesis to occur, and thereby strengthen long-term memory.
s administered after learning
, weaken memory, suggesting that protein synthesis is required for memory consolidation. Additionally, reports have suggested that the effects of protein synthesis inhibitors also inhibit LTP
. However, it should be noted that other results have shown that protein synthesis may not in fact be necessary for memory consolidation, as it has been found that the formation of memories can withstand vast amounts of protein synthesis inhibition
, suggesting that this criterion of protein synthesis as necessary for memory consolidation is not unconditional.
, therefore affecting memory. Specifically, it was found that dietary-derived flavanoids might protect neurons, enhance neuronal function, and stimulate neuronal regeneration
. Additionally, these dietary phytochemicals interact with several neuronal signaling cascade pathways
that are responsible for alterations in LTP, and consequently, learning and human memory. Flavanoids may trigger certain events, including the activation of the CREB
transcription factor
, which is important to the enhancement of short-term
and long-term memory
. This activation then triggers the synthesis of important proteins related to LTP, ultimately leading to synapse growth
and eventually long-term memory.
consolidation. It is a reorganization process in which memories from the hippocampal region where memories are first encoded are moved to the neo-cortex in a more permanent form of storage. System consolidation is a slow dynamic process that can take from one to two decades to be fully formed in humans, unlike synaptic consolidation that only takes minutes to hours for new information to stabilize into memories.
information is originally encoded and registered, memory of these new stimuli
becomes retained in both the hippocampus
and cortical regions. Later the hippocampus’ representations of this information become active in explicit
(conscious) recall or implicit
(unconscious) recall like in sleep
and ‘offline’ processes.
Memory is retained in the hippocampus
for up to one week after initial learning
, representing the hippocampus-dependent stage. During this stage the hippocampus is ‘teaching’ the cortex more and more about the information and when the information is recalled it strengthens the cortico-cortical connection thus making the memory hippocampus-independent. Therefore from one week and beyond the initial training experience, the memory is slowly transferred to the neo-cortex where it becomes permanently stored.
consolidation, semantic memory
and episodic memory
need to be treated as different types of memory. This additional distinction expands the Standard Model by Frankland, which does not consider the types of memory as separate. Evidence from extensive neuro-imaging research on the different function of cortical and hippocampus
memory traces have found that the hippocampus provides temporal and spatial context, whereas the cortical traces are primarily context-free. Episodic memory, no matter whether new or old, relies on hippocampus-cortical networks whereas remote semantic memories can be retrieved independent of the hippocampus.
(MTT) builds on the distinction between semantic memory
and episodic memory
, arguing that semantic memories are created by multiple traces left in the neo-cortex in the process of consolidation, separate from the hippocampus. Hence, while proper hippocampus functioning is necessary for the retention and retrieval of episodic memories, it is less necessary for semantic memories.
has been implicated in the overnight learning
in humans by the re-organization of novel information in the hippocampal and cortical regions of the brain
. REM sleep elicits an increase in neuronal activity following an enriched or novel waking experience, thus increasing neuronal plasticity and therefore playing an essential role in the consolidation of memories.
In particular studies have been done on sensory
and motor
related tasks. In one study testing finger-tapping, people were split into two groups and tested post-training with or without intervening sleep; results concluded that sleep post-training increases both speed and accuracy in this particular task, while increasing the activation of both cortical and hippocampal regions; whereas the post-training awake group had no such improvements.
is an Immediate Early Gene
(IEG) thought to be involved in neuroplasticity by an up-regulation of the transcription factor during REM sleep after pre-exposure to an enriched environment. Results from studies testing the effects of zif268 on mice brains postmortem, suggest that a waking experience prior to sleep can have an enduring effect in the brain, due to an increase of neuroplasticity.
. Once memories undergo the process of consolidation and become part of long-term memory, they are thought of as stable. However, the retrieval of a memory trace can cause another labile phase that then requires an active process to make the memory
stable after retrieval is complete. It is believed that post-retrieval stabilization is different and distinct from consolidation, despite its overlap in function (e.g. storage
) and its mechanisms (e.g. protein synthesis). Memory modification needs to be demonstrated in the retrieval in order for this independent process to be valid.
on rats found that a consolidated fear memory can return to a labile state, with immediate amygdala
infusions of the protein synthesis inhibitor anisomycin
, but not infusions made six hours afterwards. It was concluded that consolidated fear memory, when reactivated, enters a changeable state that requires de novo
protein synthesis for new consolidation or reconsolidation of the old memory. Since these break through studies many more have been done testing the theory of reconsolidation. Studies have been done on numerous subjects including; crabs, chicks
, honeybees, medaka fish
, lymnaea
, humans and rodents.
, certain forms of memory reactivation could actually represent new extinction
learning
rather than activation of an old memory trace. Under this possibility, traditional disruptions of reconsolidation might actually maintain the original memory trace but preventing the consolidation of extinction
learning
.
Reconsolidation experiments are more difficult to run than typical consolidation experiments as disruption of a previously consolidated memory must be shown to be specific to the reactivation of the original memory trace. Furthermore, it is important to demonstrate that the vulnerability
of reactivation occurs in a limited time frame, which can be assessed by delaying infusion
till six hours after reactivation. It is also useful to show that the behavioral measure used to assess disruption of memory
is not just due to task impairment caused by the procedure, which can be demonstrated by testing control groups in absence of the original learning
. Finally, it is important to rule out alternative explanations, such as extinction
learning
by lengthening the reactivation phase.
inhibitor anisomycin
) and both require the transcription factor
CREB
. However, recent amygdala research suggests that BDNF is required for consolidation (but not reconsolidation) whereas the transcription factor
and immediate early gene Zif268
is required for reconsolidation but not consolidation. A similar double dissociation between Zif268
for reconsolidation and BDNF for consolidation was found in the hippocampus
for fear conditioning
. However not all memory
tasks show this double dissociation, such as object recognition
memory.
Declarative memory
Declarative memory is one of two types of long term human memory. It refers to memories which can be consciously recalled such as facts and knowledge. Its counterpart is known as non-declarative or Procedural memory, which refers to unconscious memories such as skills...
memories become independent of the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
over a period of weeks to years. Recently, a third process has become the focus of research, reconsolidation, in which previously consolidated memories can be made labile
Lability
Lability refers to something that is constantly undergoing change or something that is likely to undergo change.-Chemistry:The term is used to describe a relatively unstable and transient chemical species...
again through reactivation of the memory trace.
History
Memory consolidation was first referred to in the writings of the renowned RomanAncient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....
teacher of rhetoric
Rhetoric
Rhetoric is the art of discourse, an art that aims to improve the facility of speakers or writers who attempt to inform, persuade, or motivate particular audiences in specific situations. As a subject of formal study and a productive civic practice, rhetoric has played a central role in the Western...
Quintillian. He noted the “curious fact… that the interval of a single night will greatly increase the strength of the memory,” and presented the possibility that “… the power of recollection .. undergoes a process of ripening and maturing during the time which intervenes.” The process of consolidation was later proposed based on clinical data illustrated in 1882 by Ribot’s
Théodule-Armand Ribot
Théodule-Armand Ribot , French psychologist, was born at Guingamp, and was educated at the Lycée de St Brieuc.In 1856 he began to teach, and was admitted to the École Normale Supérieure in 1862...
Law of Regression, “progressive destruction advances progressively from the unstable to the stable”. This idea was elaborated on by William H. Burnham a few years later in a paper on amnesia integrating findings from experimental psychology and neurology. Coining of the term “consolidation” is credited to the German researchers Müller
Georg Elias Müller
Georg Elias Müller was a significant early German experimental psychologist.-Biography:Born in Grimma, Saxony, he completed his doctorate in 1873 under the supervision of Hermann Lotze in Göttingen. He was also educated at the universities of Leipzig and Berlin...
and Alfons Pilzecker who rediscovered the concept that memory takes time to fixate or undergo “Konsolidierung” in their studies conducted between 1892 and 1900.
Systematic studies of retrograde amnesia
Retrograde amnesia
Retrograde amnesia is a loss of access to events that occurred, or information that was learned, before an injury or the onset of a disease....
started to emerge in the 1960s and 1970s. These were accompanied by the creation of animal models of human amnesia
Amnesia
Amnesia is a condition in which one's memory is lost. The causes of amnesia have traditionally been divided into categories. Memory appears to be stored in several parts of the limbic system of the brain, and any condition that interferes with the function of this system can cause amnesia...
in an effort to identify brain substrates critical for slow consolidation. Meanwhile, neuropharmacological
Neuropsychopharmacology
Neuropsychopharmacology is an interdisciplinary science related to psychopharmacology and fundamental neuroscience...
studies of selected brain areas began to shed light on the molecules possibly responsible for fast consolidation. In recent decades, advancements in cellular
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
preparations, molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
, and neurogenetics
Neurogenetics
Neurogenetics studies the role of genetics in the development and function of the nervous system. It considers neural characteristics as phenotypes , and is mainly based on the observation that the nervous systems of individuals, even of those belonging to the same species, may not be identical...
have revolutionized the study of consolidation.
Synaptic Consolidation
Synaptic consolidation is one form of memory consolidation seen across all species and long-term memory tasks. Long-term memoryLong-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...
, when discussed in the arena of synaptic consolidation, is memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
that lasts for at least 24 hours. An exception to this 24-hour rule is long-term potentiation
Long-term potentiation
In neuroscience, long-term potentiation is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them synchronously. It is one of several phenomena underlying synaptic plasticity, the ability of chemical synapses to change their strength...
, or LTP, a model of synaptic plasticity
Synaptic plasticity
In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...
related to learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
, in which an hour is thought to be sufficient. Synaptic consolidation is achieved faster than systems consolidation, within only minutes to hours of learning. LTP, one of the best understood forms of synaptic plasticity, is thought to be a possible underlying process in synaptic consolidation.
Standard Model
The standard model of synaptic consolidation suggests that alterations of synaptic protein synthesis and changes in membrane potentialMembrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
are achieved through activating intracellular transduction cascades
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
. These molecular cascades trigger transcription factors that lead to changes in gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
. The result of the gene expression is the lasting alteration of synaptic proteins, as well as synaptic remodeling and growth. In a short time-frame immediately following learning, the molecular cascade, expression and process of both transcription factors and immediate early genes, are susceptible to disruptions. Disruptions caused by specific drugs, antibodies and gross physical trauma can block the effects of synaptic consolidation.
Long-term Potentiation
LTP can be thought of as the prolonged strengthening of synaptic transmission, and is known to produce increases in the neurotransmitterNeurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
production and receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
sensitivity, lasting minutes to even days. The process of LTP
LTP
- Science and technology :* Lunar Transient Phenomena, a short-lived change in appearance of Earth's moon* Long-tailed pair, a differential pair amplifier* Lightweight Telephony Protocol, a signaling protocol...
is regarded as a contributing factor to synaptic plasticity
Synaptic plasticity
In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...
and in the growth of synaptic strength, which are suggested to underlie memory formation. LTP is also considered to be an important mechanism in terms of maintaining memories within brain regions, and therefore is thought to be involved in learning. There is compelling evidence that LTP is critical for Pavlovian fear conditioning in rats suggesting that it mediates learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
and memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
in mammals. Specifically, NMDA-receptor antagonists
Receptor antagonist
A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses...
appear to block the induction of both LTP and fear conditioning
Fear conditioning
Fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus is associated with a particular neutral context or neutral stimulus , resulting in the expression of fear responses to the originally neutral...
and that fear conditioning increases amygdaloidal synaptic transmission that would result in LTP.
Timeline of Consolidation
Synaptic consolidation, when compared to systems consolidation (which is said to take weeks to months to years to be accomplished), is considerably faster. There is evidence to suggest that synaptic consolidation takes place within minutes to hours of memory encoding or learningLearning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
, and as such is considered the ‘fast’ type of consolidation. As soon as six hours after training, memories become impervious to interferences that disrupt synaptic consolidation and the formation of long-term memory.
Spacing Effect
Distributed learningDistributed learning
Distributed learning is defined as opposed to massed learning. Reviews of material separated by a long period of time yield more learning than reviews separated by a shorter period of time ; this is called the spacing effect. Review of material increases long-term memory best when there is more...
has been found to enhance memory consolidation, specifically for relational memory. Experimental results suggest that distributing learning over the course of 24 hours decreases the rate of forgetting compared to massed learning
Spacing effect
In psychology, the spacing effect refers to the fact that humans and animals more easily remember or learn items in a list when they are studied a few times over a long period of time , rather than studied repeatedly in a short period time .The phenomenon was first identified by Hermann Ebbinghaus;...
, and enhances relational memory consolidation. When interpreted in the context of synaptic consolidation, mechanisms of synaptic strengthening may depend on the spacing of memory reactivation to allow sufficient time for protein synthesis to occur, and thereby strengthen long-term memory.
Protein Synthesis
Protein synthesis has been suggested to play a critical role in the formation of new memories. Studies have shown that protein synthesis inhibitorProtein synthesis inhibitor
A protein synthesis inhibitor is a substance that stops or slows the growth or proliferation of cells by disrupting the processes that lead directly to the generation of new proteins....
s administered after learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
, weaken memory, suggesting that protein synthesis is required for memory consolidation. Additionally, reports have suggested that the effects of protein synthesis inhibitors also inhibit LTP
LTP
- Science and technology :* Lunar Transient Phenomena, a short-lived change in appearance of Earth's moon* Long-tailed pair, a differential pair amplifier* Lightweight Telephony Protocol, a signaling protocol...
. However, it should be noted that other results have shown that protein synthesis may not in fact be necessary for memory consolidation, as it has been found that the formation of memories can withstand vast amounts of protein synthesis inhibition
Protein synthesis inhibitor
A protein synthesis inhibitor is a substance that stops or slows the growth or proliferation of cells by disrupting the processes that lead directly to the generation of new proteins....
, suggesting that this criterion of protein synthesis as necessary for memory consolidation is not unconditional.
Dietary Flavanoids
There is evidence to suggest that dietary flavanoids have effects on encouraging LTP and synaptic plasticitySynaptic plasticity
In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...
, therefore affecting memory. Specifically, it was found that dietary-derived flavanoids might protect neurons, enhance neuronal function, and stimulate neuronal regeneration
Nerve regeneration
Neuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Neuroregeneration differs between the peripheral nervous system and the central nervous system by the functional...
. Additionally, these dietary phytochemicals interact with several neuronal signaling cascade pathways
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
that are responsible for alterations in LTP, and consequently, learning and human memory. Flavanoids may trigger certain events, including the activation of the CREB
CREB
CREB is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements , thereby increasing or decreasing the transcription of the downstream genes....
transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
, which is important to the enhancement of short-term
Short-term memory
Short-term memory is the capacity for holding a small amount of information in mind in an active, readily available state for a short period of time. The duration of short-term memory is believed to be in the order of seconds. A commonly cited capacity is 7 ± 2 elements...
and long-term memory
Long-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...
. This activation then triggers the synthesis of important proteins related to LTP, ultimately leading to synapse growth
Synaptogenesis
Synaptogenesis is the formation of synapses. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development...
and eventually long-term memory.
System Consolidation
System Consolidation is the second form of memoryMemory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
consolidation. It is a reorganization process in which memories from the hippocampal region where memories are first encoded are moved to the neo-cortex in a more permanent form of storage. System consolidation is a slow dynamic process that can take from one to two decades to be fully formed in humans, unlike synaptic consolidation that only takes minutes to hours for new information to stabilize into memories.
Standard Model
The Standard model of systems consolidation has been summarized by Squire and Alvarez (1995); it states that when novelNovel
A novel is a book of long narrative in literary prose. The genre has historical roots both in the fields of the medieval and early modern romance and in the tradition of the novella. The latter supplied the present generic term in the late 18th century....
information is originally encoded and registered, memory of these new stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
becomes retained in both the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
and cortical regions. Later the hippocampus’ representations of this information become active in explicit
Explicit memory
Explicit memory is the conscious, intentional recollection of previous experiences and information. People use explicit memory throughout the day, such as remembering the time of an appointment or recollecting an event from years ago....
(conscious) recall or implicit
Implicit memory
Implicit memory is a type of memory in which previous experiences aid in the performance of a task without conscious awareness of these previous experiences. Evidence for implicit memory arises in priming, a process whereby subjects show improved performance on tasks for which they have been...
(unconscious) recall like in sleep
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...
and ‘offline’ processes.
Memory is retained in the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
for up to one week after initial learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
, representing the hippocampus-dependent stage. During this stage the hippocampus is ‘teaching’ the cortex more and more about the information and when the information is recalled it strengthens the cortico-cortical connection thus making the memory hippocampus-independent. Therefore from one week and beyond the initial training experience, the memory is slowly transferred to the neo-cortex where it becomes permanently stored.
Semantic vs. Episodic Memory
Nadel and Moscovitch argued that when studying the structures and systems involved in memoryMemory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
consolidation, semantic memory
Semantic memory
Semantic memory refers to the memory of meanings, understandings, and other concept-based knowledge unrelated to specific experiences. The conscious recollection of factual information and general knowledge about the world is generally thought to be independent of context and personal relevance...
and episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
need to be treated as different types of memory. This additional distinction expands the Standard Model by Frankland, which does not consider the types of memory as separate. Evidence from extensive neuro-imaging research on the different function of cortical and hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
memory traces have found that the hippocampus provides temporal and spatial context, whereas the cortical traces are primarily context-free. Episodic memory, no matter whether new or old, relies on hippocampus-cortical networks whereas remote semantic memories can be retrieved independent of the hippocampus.
Multiple Trace Theory
Multiple Trace TheoryMultiple trace theory
Multiple Trace Theory is a memory consolidation model advanced as an alternative model to strength theory. It posits that each time some information is presented to a person, it is neurally encoded in a unique memory trace composed of a combination of its attributes...
(MTT) builds on the distinction between semantic memory
Semantic memory
Semantic memory refers to the memory of meanings, understandings, and other concept-based knowledge unrelated to specific experiences. The conscious recollection of factual information and general knowledge about the world is generally thought to be independent of context and personal relevance...
and episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
, arguing that semantic memories are created by multiple traces left in the neo-cortex in the process of consolidation, separate from the hippocampus. Hence, while proper hippocampus functioning is necessary for the retention and retrieval of episodic memories, it is less necessary for semantic memories.
REM Sleep
Rapid eye movement (REM) sleepSleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...
has been implicated in the overnight learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
in humans by the re-organization of novel information in the hippocampal and cortical regions of the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
. REM sleep elicits an increase in neuronal activity following an enriched or novel waking experience, thus increasing neuronal plasticity and therefore playing an essential role in the consolidation of memories.
In particular studies have been done on sensory
Sensory
Sensory may refer to:relating to senses or smellIn biology:* Sensory preference* Sensory system, part of the nervous system of organisms* Sensory neuron, nerve cell responsible for transmitting information about external stimuli...
and motor
Motor skill
A motor skill is a learned sequence of movements that combine to produce a smooth, efficient action in order to master a particular task. The development of motor skill occurs in the motor cortex, the region of the cerebral cortex that controls voluntary muscle groups.- Development of motor skills...
related tasks. In one study testing finger-tapping, people were split into two groups and tested post-training with or without intervening sleep; results concluded that sleep post-training increases both speed and accuracy in this particular task, while increasing the activation of both cortical and hippocampal regions; whereas the post-training awake group had no such improvements.
Zif268 & REM Sleep
Zif268Zif268
EGR-1 also known as Zif268 or NGFI-A is a protein that in humans is encoded by the EGR1 gene....
is an Immediate Early Gene
Immediate early gene
Immediate early genes are genes which are activated transiently and rapidly in response to a wide variety of cellular stimuli. They represent a standing response mechanism that is activated at the transcription level in the first round of response to stimuli, before any new proteins are synthesized...
(IEG) thought to be involved in neuroplasticity by an up-regulation of the transcription factor during REM sleep after pre-exposure to an enriched environment. Results from studies testing the effects of zif268 on mice brains postmortem, suggest that a waking experience prior to sleep can have an enduring effect in the brain, due to an increase of neuroplasticity.
Reconsolidation
Memory reconsolidation is the process of previously consolidated memories being recalled and actively consolidated. It is a distinct process that serves to maintain, strengthen and modify memories that are already stored in the long-term memoryLong-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...
. Once memories undergo the process of consolidation and become part of long-term memory, they are thought of as stable. However, the retrieval of a memory trace can cause another labile phase that then requires an active process to make the memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
stable after retrieval is complete. It is believed that post-retrieval stabilization is different and distinct from consolidation, despite its overlap in function (e.g. storage
Storage (memory)
Storage in human memory is one of three core process of memory, along with Recall and Encoding. It refers to the retention of information, which has been achieved through the encoding process, in brain for prolonged period of time until it is accessed by the recall process...
) and its mechanisms (e.g. protein synthesis). Memory modification needs to be demonstrated in the retrieval in order for this independent process to be valid.
History
The theory of reconsolidation has been debated for many years and has become quite controversial. Reconsolidation was first conceptualized after studies were done on elimination of phobias with electroconvulsive shock therapy; the disruption of the consolidated fear memory after shock administration led to further investigation into the concept. In many early studies electroconvulsive shock therapy was used to test for reconsolidation, as it was a known amnesic agent, and lead to memory loss if administered directly after the retrieval of a memory. Later research using Pavlovian fear conditioningFear conditioning
Fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus is associated with a particular neutral context or neutral stimulus , resulting in the expression of fear responses to the originally neutral...
on rats found that a consolidated fear memory can return to a labile state, with immediate amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
infusions of the protein synthesis inhibitor anisomycin
Anisomycin
Anisomycin, also known as flagecidin is an antibiotic produced by Streptomyces griseolus which inhibits protein synthesis. Partial inhibition of DNA synthesis occurs at anisomycin concentrations that effect 95% inhibition of protein synthesis...
, but not infusions made six hours afterwards. It was concluded that consolidated fear memory, when reactivated, enters a changeable state that requires de novo
De novo
In general usage, de novo is a Latin expression meaning "from the beginning," "afresh," "anew," "beginning again." It is used in:* De novo transcriptome assembly, the method of creating a transcriptome without a reference genome...
protein synthesis for new consolidation or reconsolidation of the old memory. Since these break through studies many more have been done testing the theory of reconsolidation. Studies have been done on numerous subjects including; crabs, chicks
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...
, honeybees, medaka fish
Oryzias latipes
Oryzias latipes, also known as Medaka and Japanese killifish, is a member of genus Oryzias , the only genus in the subfamily Oryziinae. This rather small...
, lymnaea
Lymnaea
Lymnaea is a genus of small to large-sized air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the family Lymnaeidae, the pond snails.Lymnaea is the type genus of the family Lymnaeidae.-Species:...
, humans and rodents.
Criticisms
Some studies have supported this theory, while others have failed to demonstrate disruption of consolidated memory after retrieval. It is important to note that negative results may be examples of conditions where memories are not susceptible to a permanent disruption, thus a determining factor of reconsolidation. After much debate and a detailed review of this field it had been concluded that reconsolidation was a real phenomenon. More recently Tronson and Taylor compiled a lengthy summary of multiple reconsolidation studies, noting a number of studies were unable to show memory impairments due to blocked reconsolidation. However the need for standardized methods was underscored as in some learning tasks such as fear conditioningFear conditioning
Fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus is associated with a particular neutral context or neutral stimulus , resulting in the expression of fear responses to the originally neutral...
, certain forms of memory reactivation could actually represent new extinction
Extinction (psychology)
Extinction is the conditioning phenomenon in which a previously learned response to a cue is reduced when the cue is presented in the absence of the previously paired aversive or appetitive stimulus.-Fear conditioning:...
learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
rather than activation of an old memory trace. Under this possibility, traditional disruptions of reconsolidation might actually maintain the original memory trace but preventing the consolidation of extinction
Extinction (psychology)
Extinction is the conditioning phenomenon in which a previously learned response to a cue is reduced when the cue is presented in the absence of the previously paired aversive or appetitive stimulus.-Fear conditioning:...
learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
.
Reconsolidation experiments are more difficult to run than typical consolidation experiments as disruption of a previously consolidated memory must be shown to be specific to the reactivation of the original memory trace. Furthermore, it is important to demonstrate that the vulnerability
Vulnerability
Vulnerability refer to the susceptibility of a person, group, society, sex or system to physical or emotional injury or attack. The term can also refer to a person who lets their guard down, leaving themselves open to censure or criticism...
of reactivation occurs in a limited time frame, which can be assessed by delaying infusion
Infusion pump
An infusion pump infuses fluids, medication or nutrients into a patient's circulatory system. It is generally used intravenously, although subcutaneous, arterial and epidural infusions are occasionally used....
till six hours after reactivation. It is also useful to show that the behavioral measure used to assess disruption of memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
is not just due to task impairment caused by the procedure, which can be demonstrated by testing control groups in absence of the original learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
. Finally, it is important to rule out alternative explanations, such as extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
by lengthening the reactivation phase.
Distinctions from Consolidation
Questions arose if reconsolidation was a unique process or merely another phase of consolidation. Both consolidation and reconsolidation can be disrupted by pharmacological agents (e.g. the protein synthesisTranslation (genetics)
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
inhibitor anisomycin
Anisomycin
Anisomycin, also known as flagecidin is an antibiotic produced by Streptomyces griseolus which inhibits protein synthesis. Partial inhibition of DNA synthesis occurs at anisomycin concentrations that effect 95% inhibition of protein synthesis...
) and both require the transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
CREB
CREB
CREB is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements , thereby increasing or decreasing the transcription of the downstream genes....
. However, recent amygdala research suggests that BDNF is required for consolidation (but not reconsolidation) whereas the transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
and immediate early gene Zif268
Zif268
EGR-1 also known as Zif268 or NGFI-A is a protein that in humans is encoded by the EGR1 gene....
is required for reconsolidation but not consolidation. A similar double dissociation between Zif268
Zif268
EGR-1 also known as Zif268 or NGFI-A is a protein that in humans is encoded by the EGR1 gene....
for reconsolidation and BDNF for consolidation was found in the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
for fear conditioning
Fear conditioning
Fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus is associated with a particular neutral context or neutral stimulus , resulting in the expression of fear responses to the originally neutral...
. However not all memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
tasks show this double dissociation, such as object recognition
Object recognition
Object recognition in computer vision is the task of finding a given object in an image or video sequence. Humans recognize a multitude of objects in images with little effort, despite the fact that the image of the objects may vary somewhat in different view points, in many different sizes / scale...
memory.
See also
- Atkinson-Shiffrin memory model
- Coherence therapyCoherence therapyCoherence therapy is a system of psychotherapy based in the theory that symptoms of mood, thought and behavior are produced coherently according to the person's current models of reality, most of which are implicit and unconscious. It was founded by Bruce Ecker and Laurel Hulley in the 1990s...
- EngramEngram (neuropsychology)Engrams are a hypothetical means by which memory traces are stored as biophysical or biochemical changes in the brain in response to external stimuli....
- Patient HM
- Multiple trace theoryMultiple trace theoryMultiple Trace Theory is a memory consolidation model advanced as an alternative model to strength theory. It posits that each time some information is presented to a person, it is neurally encoded in a unique memory trace composed of a combination of its attributes...