Declarative memory
Encyclopedia
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 (e.g. learning to ride a bicycle). Declarative memory can be divided into two categories: episodic memory
which stores specific personal experiences and semantic memory
which stores factual information.
major treatise, On the Soul
, in which he compares the human mind
to a blank slate
. He theorized that all humans are born free of any knowledge and are the sum of their experiences. It wasn’t until the late 1800s, however, that a young German philosopher by the name of Herman Ebbinghaus developed the first scientific approach to studying memory. While some of his findings have endured and remain relevant to this day (Learning Curve
), his greatest contribution to the field of memory research was demonstrating that memory can be studied scientifically. In 1972, Endel Tulving
proposed the distinction between episodic and semantic memory. This was quickly adopted and is now widely accepted. Following this, in 1985, Daniel Schacter
proposed a more general distinction between explicit (declarative) and implicit (procedural) memory With the recent advances in neuroimaging
technology, there have been a multitude of findings linking specific brain areas to declarative memory. Despite these advances in Cognitive psychology
, there is still much to be discovered in terms of the operating mechanisms of declarative memory. It is unclear whether declarative memory is mediated by a particular “memory system” or if it is more accurately classified as a “type of knowledge” and it is not known how or why declarative memory evolved to begin with.
and surrounding structures appear to be most important in declarative memory specifically. The ability to retain and recall episodic memories is highly dependent on the hippocampus, whereas the formation of new declarative memories relies on both the hippocampus and parahippocampus
Other studies have found that the parahippocampal cortices were related to superior Recognition Memory
.
The Three Stage Model was developed by Eichenbaum, et. Al (2001), and proposes that the hippocampus does three things with episodic memory:
To support this model, a version of Piaget’s Transitive Inference Task
was used to show that the hippocampus is in fact used as the memory space.
When experiencing an event for the first time, a link is formed in the hippocampus allowing us to recall that event in the future. Separate links are also made for features related to that event. For example, when you meet someone new, a unique link is created for them. More links are then connected to that person’s link so you can remember what colour their shirt was, what the weather was like when you met them, etc. Specific episodes are made easier to remember and recall by repeatedly exposing oneself to them (which strengthens the links in the memory space) allowing for faster retrieval when remembering.
Hippocampal cells (neurons) are activated depending on what information one is exposed to at that moment. Some cells are specific to spatial information, certain stimuli (smells, etc.), or behaviours as has been shown in a Radial Maze Task. It is therefore the hippocampus that allows us to recognize certain situations, environments, etc. as being either distinct or similar to others. However, the Three Stage Model does not incorporate the importance of other cortical structures in memory.
(PFC) is essential for remembering contextual details of an experience rather than for memory formation. The PFC is also more involved with episodic memory than semantic memory, although it does play a small role in semantics.
Using PET studies and word stimuli, Endel Tulving
found that remembering is an automatic process. It is also well documented that a hemispheric asymmetry occurs in the PFC: When encoding memories, the Left Dorsolateral PFC (LPFC) is activated, and when retrieving memories, activation is seen in the Right Dorsolateral PFC (RPFC).
Studies have also shown that the PFC is extremely involved with autonoetic consciousness (See Tulving's theory
). This is responsible for humans’ recollective experiences and ‘mental time travelling’ abilities (characteristics of episodic memory).
is believed to be involved in the encoding and retrieval of emotionally charged memories. Much of the evidence for this has come from research on a phenomenon known as flashbulb memories
. These are instances in which memories of powerful emotional events are more highly detailed and enduring than regular memories (e.g. September 11 attacks, assassination of JFK
). These memories have been linked to increased activation in the amygdala. Recent studies of patients with damage to the amygdala suggest that it is involved in memory for general knowledge, and not for specific information.
have shown brain activation when a remote memory is being recovered and the Occipital lobe
, Ventral Temporal lobe
, and Fusiform gyrus
all play a role in memory formation.
studies are commonly used in cognitive neuroscience research. Lesions can occur naturally through trauma or disease, or they can be surgically induced by researchers. In the study of declarative memory, the hippocampus and the amygdala are two structures frequently examined using this technique.
The Morris water navigation task tests spatial learning in rats. In this test rats learn to escape from a pool by swimming toward a platform submerged just below the surface of the water. Visual cues that surround the pool (i.e. Chair or window) help the rat to locate the platform on subsequent trials. The rats' use of specific events, cues and places are all forms of declarative memory. Two groups of rats are observed: a control group with no lesions and an experimental group with hippocampal lesions. In this task created by Morris, et al., rats are placed in the pool at the same position for 12 trials. Each trial is timed and the path taken by the rats is recorded. Rats with hippocampal lesions successfully learn to find the platform. If the starting point is moved, the rats with hippocampal lesions typically fail to locate the platform. The control rats, however, are able to find the platform using the cues acquired during the learning trials. This demonstrates the involvement of the hippocampus in declarative memory.
The Odor-odor Recognition Task, devised by Bunsey and Eichenbaum, involves a social encounter between two rats (a "subject" and a "demonstrator"). The demonstrator, after eating a specific type of food, interacts with the subject rat, who then smells the food odor on the other's breath. The experimenters then present the subject rat with a decision between two food options; the food previously eaten by the demonstrator, and a novel food. The researchers found that when there was no time delay, both control rats and rats with lesions chose the familiar food. After 24 hours, however, the rats with hippocampal lesions were just as likely to eat both types of food, while control rats chose the familiar food. This can be attributed to the inability to form episodic memories due to lesions in the hippocampus. The effects of this study can be observed in humans with amnesia, indicating the role of the hippocampus in developing episodic memories that can be generalized to similar situations.
Patient H.M.
had parts of his medial temporal lobe removed which resulted in the loss of the ability to form new memories. The long-term declarative memory was crucially affected when the structures from the medial temporal lobe were removed, including the ability to form of new semantic knowledge and memories. The dissociation in H.M. between the acquisition of declarative memory and other kinds of learning was seen initially in motor learning. H.M.'s declarative memory was not functioning, as was seen when H.M. completed the task of repetition priming
. His performance does improve over trials, however, his scores were inferior to those of control participants. In the condition of H.M. the same results from this priming task are reflected when looking at the other basic memory functions like remembering, recall and recognizing. Lesions should not be interpreted as an all-or-nothing condition, in the case of H.M. not all memory and recognition is lost, although the declarative memory is severely damaged he still has a sense of self and memories that were developed before the lesion occurred.
The emotionally arousing slide (slide 7) was remembered no better by the bilateral damage participants than any of the other slides. All other participants notably remembered the seventh slide the best and in most detail out of all the other slides. This shows that the amygdala is necessary to facilitate encoding of declarative knowledge regarding emotionally arousing stimuli, but is not required for encoding knowledge of emotionally neutral stimuli.
has a very large impact on the formation of declarative memories. Lupien, et al. completed a study that had 3 phases for participants to take part in. Phase 1 involved memorizing a series of words, phase 2 entailed either a stressful (public speaking) or non-stressful situation (an attention task), and phase 3 required participants to recall the words they learned in phase 1. A declarative memory was formed in phase 1 if the words shown to participants were remembered. There were signs of decreased declarative memory performance in the participants that had to complete the stressful situation after learning the words. This showed that the stress of the situation impaired participants’ ability to form concrete declarative knowledge. In the non-stressful situation, participants could easily remember the words learned from phase 1.
Posttraumatic stress disorder (PTSD) emerges after exposure to a traumatic event eliciting fear, horror or helplessness that involves bodily injury, the threat of injury, or death to one’s self or another person The chronic stress in PTSD contributes to an observed decrease in hippocampal volume and declarative memory deficits.
is one of the most common GC’s in the human body, and hydrocortisone (a derivative of cortisol) decreases brain activity in the above areas during declarative memory retrieval.
Elevations in cortisol occur during stress, and long-term stress impairs declarative memory this way. A study done by Damoiseaux, et. Al evaluated the effect of glucocorticoids on MTL and PFC activation in young men. They found that GC’s given to participants 1 hour before retrieval of information impairs free recall of words, yet when administered before or after learning they had no effect. Although it is not known exactly how GC’s influence memory, there are Glucocorticoid receptor
s in the hippocampus and PFC that tell us these structures are targets for the circulating hormone. However, it is known that cortisone impairs memory function by reducing the blood flow in the right parahippocampal gyrus, left visual cortex, and the Cerebellum
.
Note: This study only involved male subjects which may be significant as sex steroids may have different effects in the responses to cortisol administration. Men and women also respond differently to emotional stimuli and this may affect cortisol levels. Also, this study was the first Functional magnetic resonance imaging
(fMRI) study to be done involving GC's and more research is necessary to support these findings.
plays an active role in consolidation of declarative memory. Specifically, sleep’s unique properties enhance memory consolidation
, such as the reactivation of newly learned memories during sleep. For example, it has been suggested that the central mechanism for consolidation of declarative memory during sleep is the reactivation of hippocampal memory representations. Specifically, this reactivation transfers information to neocortical networks where it is integrated into long-term representations. For instance, studies on rats involving maze learning found that hippocampal neuronal assemblies that are used in the encoding of spatial information are reactivated in the same temporal order. Similarly, positron emission tomography (PET) has shown reactivation of the hippocampus
in slow-wave sleep (SWS) after spatial learning. Together these studies show that newly learned memories are reactivated during sleep and through this, help consolidate new memory traces. In addition, researchers have identified three types of sleep (SWS, Sleep Spindle and REM) in which declarative memory is consolidated.
Slow-Wave Sleep
, often referred to as deep sleep, plays the most important role in consolidation of declarative memory and there is a large amount of evidence to support this claim. For instance, one study found that the first 3.5 hours of sleep offer the greatest performance enhancement on memory recall tasks, because the first couple of hours are dominated by SWS. Yet, additional hours of sleep do not add to the initial level of performance. Thus, this study suggests that full sleep may not be important for optimal performance of memory. Another study shows that people who experience SWS during the first half of their sleep cycle compared to subjects who did not, showed better recall of information. However this is not the case for subjects who were tested for the second half of their sleep cycle, as they experience less SWS.
Another key piece of evidence regarding SWS’s involvement in declarative memory consolidation is a finding that people with pathological conditions of sleep, such as insomnia, exhibit both reduction in Slow-Wave Sleep
and also have impaired consolidation of declarative memory during sleep. Yet, another study found that middle aged people compared to young group had a worse retrieval of memories. This in turn indicated that SWS is associated with poor declarative memory consolidation but not with age itself. In sum, the above studies and numerous others demonstrate that SWS indeed plays a big role in consolidation of declarative memory.
Some researchers believe that Sleep Spindle
, a burst of brain activity occurring during stage 2 sleep, plays a role in boosting consolidation of declarative memories. Some critics point out that spindle activity is positively correlated with intelligence. Nevertheless, Schabus and Gruber point out that sleep-spindle activity only relates to performance on newly learned memories, but not to absolute performance. This supports the hypothesis that sleep spindle helps to consolidate recent memory traces but not memory performance in general.
The relationship between sleep spindles and declarative memory consolidation is not yet fully understood.
There is a relatively small, yet growing body of evidence that supports the idea that REM sleep helps consolidate highly emotional declarative memories. For instance Wagner, et al. compared memory retention for emotional versus neutral text over two instances; early sleep that is dominated by SWS and late sleep that is dominated by REM phase. He found that sleep improved memory retention of emotional text only during late sleep phase, which was primarily REM. Similarly Hu & Stylos-Allen, et al. did a study with emotional versus neutral pictures and came to the same conclusion that REM sleep facilitates consolidation of emotional declarative memories.
However, the view that sleep plays an active role in declarative memory consolidation is not shared by all of the researchers. For instance Ellenbogen, et al. argue that sleep actively protects declarative memory from associative interference. Furthermore, Wixted believes that the sole role of sleep in declarative memory consolidation is nothing more but creating ideal conditions for memory consolidation. For example, when awake, people are bombarded with mental activity which interferes with effective consolidation. However, during sleep, when interference is minimal, memories can be consolidated without any obstacles. In sum, this view suggests that sleep provides ideal conditions for declarative memory consolidation but does not actively enhance memory consolidation. However more research is needed to make a definite statement whether sleep creates favourable conditions for consolidation or it actively enhances declarative memory consolidation.
cs are frequently portrayed in television and movies. Some of the better known examples include:
In the romantic comedy 50 First Dates
(2004), Adam Sandler plays veterinarian Henry Roth, who falls for Lucy Whitmore, played by Drew Barrymore. Having lost her short term memory in a car crash, Lucy can only remember the current day's events until she falls asleep. When she wakes up the next morning, she has no recollection of the previous day's experiences. These experiences would normally be transferred into declarative knowledge, allowing them to be recalled in the future. Although this movie is not the most accurate representation of a true amnesic patient, it is useful for informing viewers of the detrimental effects of amnesia.
Memento (2000) a film inspired by the case of Henry Molaison (H.M.). Guy Pearce
plays an ex-insurance investigator suffering from severe anterograde amnesia
caused by a head injury. Unlike most amnesiacs, Leonard retains his identity and the memories of events that occurred before the injury, but loses all ability to form new memories. This loss of ability to form new memories indicates that the head injury affected the medial temporal lobe of the brain resulting in the inability for Leonard to form declarative memory.
Finding Nemo
features a reef fish named Dory with an inability to develop declarative memory. This prevents her from learning or retaining any new information such as names or directions. The exact origin of Dory's impairment is not mentioned in the film, but her memory loss accurately portrays the difficulties facing amnesiacs.
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....
. It refers to memories which can be consciously recalled such as facts and knowledge. Its counterpart is known as non-declarative or Procedural memory
Procedural memory
Procedural memory is memory for how to do things. Procedural memory guides the processes we perform and most frequently resides below the level of conscious awareness. When needed, procedural memories are automatically retrieved and utilized for the execution of the integrated procedures involved...
, which refers to unconscious memories such as skills (e.g. learning to ride a bicycle). Declarative memory can be divided into two categories: 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...
which stores specific personal experiences and 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...
which stores factual information.
Types
There are two types of declarative memory. Semantic memories are those that store general factual knowledge that is independent of personal experience. Examples include types of food, capital cities, lexical knowledge (vocabulary), etc. Episodic memories are those that store specific events such as attending a class or flying to France. Retrieval of these memories can be thought of as mentally reliving the past events they concern. Episodic memory is believed the system that provides the basic support for semantic memory. For example, remembering being in watching a horror movie recently that can be grouped according to some basic characteristics that are present in the film is an example of semantic memory. Episodic and semantic memories are intertwine and support each other in many areas. A perfect example of their relationship would be a autobiographical memory.History
The study of human memory stretches back over the last 2000 years. An early attempt to understand memory can be found in Aristotle’sAristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...
major treatise, On the Soul
On the Soul
On the Soul is a major treatise by Aristotle on the nature of living things. His discussion centres on the kinds of souls possessed by different kinds of living things, distinguished by their different operations...
, in which he compares the human mind
Mind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...
to a blank slate
Tabula rasa
Tabula rasa is the epistemological theory that individuals are born without built-in mental content and that their knowledge comes from experience and perception. Generally proponents of the tabula rasa thesis favour the "nurture" side of the nature versus nurture debate, when it comes to aspects...
. He theorized that all humans are born free of any knowledge and are the sum of their experiences. It wasn’t until the late 1800s, however, that a young German philosopher by the name of Herman Ebbinghaus developed the first scientific approach to studying memory. While some of his findings have endured and remain relevant to this day (Learning Curve
Learning curve
A learning curve is a graphical representation of the changing rate of learning for a given activity or tool. Typically, the increase in retention of information is sharpest after the initial attempts, and then gradually evens out, meaning that less and less new information is retained after each...
), his greatest contribution to the field of memory research was demonstrating that memory can be studied scientifically. In 1972, Endel Tulving
Endel Tulving
Endel Tulving is an experimental psychologist and cognitive neuroscientist whose research on human memory has influenced generations of psychological scientists, neuroscientists, and clinicians...
proposed the distinction between episodic and semantic memory. This was quickly adopted and is now widely accepted. Following this, in 1985, Daniel Schacter
Daniel Schacter
Daniel Lawrence Schacter is an American psychologist. He is a Professor of Psychology at Harvard University. His research has focused on psychological and biological aspects of human memory and amnesia, with a particular emphasis on the distinction between conscious and nonconscious forms of...
proposed a more general distinction between explicit (declarative) and implicit (procedural) memory With the recent advances in neuroimaging
Neuroimaging
Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...
technology, there have been a multitude of findings linking specific brain areas to declarative memory. Despite these advances in Cognitive psychology
Cognitive psychology
Cognitive psychology is a subdiscipline of psychology exploring internal mental processes.It is the study of how people perceive, remember, think, speak, and solve problems.Cognitive psychology differs from previous psychological approaches in two key ways....
, there is still much to be discovered in terms of the operating mechanisms of declarative memory. It is unclear whether declarative memory is mediated by a particular “memory system” or if it is more accurately classified as a “type of knowledge” and it is not known how or why declarative memory evolved to begin with.
Hippocampus
Although many psychologists believe that the entire brain is involved with memory, the hippocampusHippocampus
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 surrounding structures appear to be most important in declarative memory specifically. The ability to retain and recall episodic memories is highly dependent on the hippocampus, whereas the formation of new declarative memories relies on both the hippocampus and parahippocampus
Parahippocampal gyrus
The parahippocampal gyrus is a grey matter cortical region of the brain that surrounds the hippocampus. This region plays an important role in memory encoding and retrieval....
Other studies have found that the parahippocampal cortices were related to superior Recognition Memory
Recognition memory
Recognition memory is a subcategory of declarative memory Essentially, recognition memory is the ability to recognize previously encountered events, objects, or people...
.
The Three Stage Model was developed by Eichenbaum, et. Al (2001), and proposes that the hippocampus does three things with episodic memory:
- Mediates the recording of episodic memories
- Identifies common features between episodes
- Links these common episodes in a memory space.
To support this model, a version of Piaget’s Transitive Inference Task
Jean Piaget
Jean Piaget was a French-speaking Swiss developmental psychologist and philosopher known for his epistemological studies with children. His theory of cognitive development and epistemological view are together called "genetic epistemology"....
was used to show that the hippocampus is in fact used as the memory space.
When experiencing an event for the first time, a link is formed in the hippocampus allowing us to recall that event in the future. Separate links are also made for features related to that event. For example, when you meet someone new, a unique link is created for them. More links are then connected to that person’s link so you can remember what colour their shirt was, what the weather was like when you met them, etc. Specific episodes are made easier to remember and recall by repeatedly exposing oneself to them (which strengthens the links in the memory space) allowing for faster retrieval when remembering.
Hippocampal cells (neurons) are activated depending on what information one is exposed to at that moment. Some cells are specific to spatial information, certain stimuli (smells, etc.), or behaviours as has been shown in a Radial Maze Task. It is therefore the hippocampus that allows us to recognize certain situations, environments, etc. as being either distinct or similar to others. However, the Three Stage Model does not incorporate the importance of other cortical structures in memory.
The Conservation of Mammalian Hippocampus
The anatomy of the hippocampus is largely conserved across mammals. The role of these areas in declarative memory are conserved across species as well. The organization and neural pathways of the hippocampus are very similar in humans and other mammal species. In all mammals, a cross-section of the hippocampus shows the dense cell layers of the CA fields and dentate gyrus. The intrinsic connectivity of these areas are also conserved.Prefrontal cortex
The lateral Prefrontal cortexPrefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...
(PFC) is essential for remembering contextual details of an experience rather than for memory formation. The PFC is also more involved with episodic memory than semantic memory, although it does play a small role in semantics.
Using PET studies and word stimuli, Endel Tulving
Endel Tulving
Endel Tulving is an experimental psychologist and cognitive neuroscientist whose research on human memory has influenced generations of psychological scientists, neuroscientists, and clinicians...
found that remembering is an automatic process. It is also well documented that a hemispheric asymmetry occurs in the PFC: When encoding memories, the Left Dorsolateral PFC (LPFC) is activated, and when retrieving memories, activation is seen in the Right Dorsolateral PFC (RPFC).
Studies have also shown that the PFC is extremely involved with autonoetic consciousness (See Tulving's theory
Remember versus know judgements
There is evidence suggesting that different processes are involved in remembering something versus knowing whether it is familiar or not. It appears that "remembering" and "knowing" represent relatively different characteristics of memory as well as reflect different ways of using memory.To...
). This is responsible for humans’ recollective experiences and ‘mental time travelling’ abilities (characteristics of episodic memory).
Amygdala
The amygdalaAmygdala
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.-...
is believed to be involved in the encoding and retrieval of emotionally charged memories. Much of the evidence for this has come from research on a phenomenon known as flashbulb memories
Flashbulb memory
A flashbulb memory is a highly detailed, exceptionally vivid 'snapshot' of the moment and circumstances in which a piece of surprising and consequential news was heard. Flashbulb memory is an appropriate name for the phenomenon in that it suggests surprise, an indiscriminate illumination, and...
. These are instances in which memories of powerful emotional events are more highly detailed and enduring than regular memories (e.g. September 11 attacks, assassination of JFK
John F. Kennedy assassination
John Fitzgerald Kennedy, the thirty-fifth President of the United States, was assassinated at 12:30 p.m. Central Standard Time on Friday, November 22, 1963, in Dealey Plaza, Dallas, Texas...
). These memories have been linked to increased activation in the amygdala. Recent studies of patients with damage to the amygdala suggest that it is involved in memory for general knowledge, and not for specific information.
Other structures involved
The regions of the DiencephalonDiencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...
have shown brain activation when a remote memory is being recovered and the Occipital lobe
Occipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
, Ventral Temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
, and Fusiform gyrus
Fusiform gyrus
The fusiform gyrus is part of the temporal lobe in Brodmann Area 37. It is also known as the occipitotemporal gyrus. Other sources have the fusiform gyrus above the occipitotemporal gyrus and underneath the parahippocampal gyrus....
all play a role in memory formation.
Lesion studies
LesionLesion
A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...
studies are commonly used in cognitive neuroscience research. Lesions can occur naturally through trauma or disease, or they can be surgically induced by researchers. In the study of declarative memory, the hippocampus and the amygdala are two structures frequently examined using this technique.
Hippocampal lesion studies
The Odor-odor Recognition Task, devised by Bunsey and Eichenbaum, involves a social encounter between two rats (a "subject" and a "demonstrator"). The demonstrator, after eating a specific type of food, interacts with the subject rat, who then smells the food odor on the other's breath. The experimenters then present the subject rat with a decision between two food options; the food previously eaten by the demonstrator, and a novel food. The researchers found that when there was no time delay, both control rats and rats with lesions chose the familiar food. After 24 hours, however, the rats with hippocampal lesions were just as likely to eat both types of food, while control rats chose the familiar food. This can be attributed to the inability to form episodic memories due to lesions in the hippocampus. The effects of this study can be observed in humans with amnesia, indicating the role of the hippocampus in developing episodic memories that can be generalized to similar situations.
Patient H.M.
HM (patient)
Henry Gustav Molaison , famously known as HM or H.M., was an American memory disorder patient who was widely studied from late 1957 until his death...
had parts of his medial temporal lobe removed which resulted in the loss of the ability to form new memories. The long-term declarative memory was crucially affected when the structures from the medial temporal lobe were removed, including the ability to form of new semantic knowledge and memories. The dissociation in H.M. between the acquisition of declarative memory and other kinds of learning was seen initially in motor learning. H.M.'s declarative memory was not functioning, as was seen when H.M. completed the task of repetition priming
Priming (psychology)
Priming is an implicit memory effect in which exposure to a stimulus influences a response to a later stimulus. It can occur following perceptual, semantic, or conceptual stimulus repetition...
. His performance does improve over trials, however, his scores were inferior to those of control participants. In the condition of H.M. the same results from this priming task are reflected when looking at the other basic memory functions like remembering, recall and recognizing. Lesions should not be interpreted as an all-or-nothing condition, in the case of H.M. not all memory and recognition is lost, although the declarative memory is severely damaged he still has a sense of self and memories that were developed before the lesion occurred.
Amygdala lesion studies
Adolph, Cahill and Schul completed a study showing that emotional arousal facilitates the encoding of material into long term declarative memory. They selected two subjects with bilateral damage to the amygdala, as well as six control subjects and six subjects with brain damage. All subjects were shown a series of twelve slides accompanied by a narrative. The slides varied in the degree to which they evoked emotion - slides 1 through 4 and slides 9 through 12 contain non-emotional content. Slides 5 through 8 contain emotional material, and the seventh slide contained the most emotionally arousing image and description (a picture of surgically repaired legs of a car crash victim).The emotionally arousing slide (slide 7) was remembered no better by the bilateral damage participants than any of the other slides. All other participants notably remembered the seventh slide the best and in most detail out of all the other slides. This shows that the amygdala is necessary to facilitate encoding of declarative knowledge regarding emotionally arousing stimuli, but is not required for encoding knowledge of emotionally neutral stimuli.
Stress
StressStress (biology)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...
has a very large impact on the formation of declarative memories. Lupien, et al. completed a study that had 3 phases for participants to take part in. Phase 1 involved memorizing a series of words, phase 2 entailed either a stressful (public speaking) or non-stressful situation (an attention task), and phase 3 required participants to recall the words they learned in phase 1. A declarative memory was formed in phase 1 if the words shown to participants were remembered. There were signs of decreased declarative memory performance in the participants that had to complete the stressful situation after learning the words. This showed that the stress of the situation impaired participants’ ability to form concrete declarative knowledge. In the non-stressful situation, participants could easily remember the words learned from phase 1.
Posttraumatic stress disorder (PTSD) emerges after exposure to a traumatic event eliciting fear, horror or helplessness that involves bodily injury, the threat of injury, or death to one’s self or another person The chronic stress in PTSD contributes to an observed decrease in hippocampal volume and declarative memory deficits.
Neurochemical factors of stress on the brain
In the brain, Glucocorticoids (GC's) modulate the ability of the hippocampus and PFC to process memories. CortisolCortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
is one of the most common GC’s in the human body, and hydrocortisone (a derivative of cortisol) decreases brain activity in the above areas during declarative memory retrieval.
Elevations in cortisol occur during stress, and long-term stress impairs declarative memory this way. A study done by Damoiseaux, et. Al evaluated the effect of glucocorticoids on MTL and PFC activation in young men. They found that GC’s given to participants 1 hour before retrieval of information impairs free recall of words, yet when administered before or after learning they had no effect. Although it is not known exactly how GC’s influence memory, there are Glucocorticoid receptor
Glucocorticoid receptor
The glucocorticoid receptor also known as NR3C1 is the receptor to which cortisol and other glucocorticoids bind....
s in the hippocampus and PFC that tell us these structures are targets for the circulating hormone. However, it is known that cortisone impairs memory function by reducing the blood flow in the right parahippocampal gyrus, left visual cortex, and the Cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
.
Note: This study only involved male subjects which may be significant as sex steroids may have different effects in the responses to cortisol administration. Men and women also respond differently to emotional stimuli and this may affect cortisol levels. Also, this study was the first Functional magnetic resonance imaging
Functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...
(fMRI) study to be done involving GC's and more research is necessary to support these findings.
Declarative memory consolidation during sleep
It is believed by many researchers that 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...
plays an active role in consolidation of declarative memory. Specifically, sleep’s unique properties enhance memory consolidation
Memory consolidation
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...
, such as the reactivation of newly learned memories during sleep. For example, it has been suggested that the central mechanism for consolidation of declarative memory during sleep is the reactivation of hippocampal memory representations. Specifically, this reactivation transfers information to neocortical networks where it is integrated into long-term representations. For instance, studies on rats involving maze learning found that hippocampal neuronal assemblies that are used in the encoding of spatial information are reactivated in the same temporal order. Similarly, positron emission tomography (PET) has shown reactivation 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...
in slow-wave sleep (SWS) after spatial learning. Together these studies show that newly learned memories are reactivated during sleep and through this, help consolidate new memory traces. In addition, researchers have identified three types of sleep (SWS, Sleep Spindle and REM) in which declarative memory is consolidated.
Slow-Wave Sleep
Slow-wave sleep
Slow-wave sleep , often referred to as deep sleep, consists of stages 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3 and 4 now...
, often referred to as deep sleep, plays the most important role in consolidation of declarative memory and there is a large amount of evidence to support this claim. For instance, one study found that the first 3.5 hours of sleep offer the greatest performance enhancement on memory recall tasks, because the first couple of hours are dominated by SWS. Yet, additional hours of sleep do not add to the initial level of performance. Thus, this study suggests that full sleep may not be important for optimal performance of memory. Another study shows that people who experience SWS during the first half of their sleep cycle compared to subjects who did not, showed better recall of information. However this is not the case for subjects who were tested for the second half of their sleep cycle, as they experience less SWS.
Another key piece of evidence regarding SWS’s involvement in declarative memory consolidation is a finding that people with pathological conditions of sleep, such as insomnia, exhibit both reduction in Slow-Wave Sleep
Slow-wave sleep
Slow-wave sleep , often referred to as deep sleep, consists of stages 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3 and 4 now...
and also have impaired consolidation of declarative memory during sleep. Yet, another study found that middle aged people compared to young group had a worse retrieval of memories. This in turn indicated that SWS is associated with poor declarative memory consolidation but not with age itself. In sum, the above studies and numerous others demonstrate that SWS indeed plays a big role in consolidation of declarative memory.
Some researchers believe that Sleep Spindle
Sleep spindle
A sleep spindle is a burst of brain activity visible on an EEG that occurs during stage 2 sleep. It consists of 12–14 Hz waves that occur for at least 0.5 seconds.-Function:...
, a burst of brain activity occurring during stage 2 sleep, plays a role in boosting consolidation of declarative memories. Some critics point out that spindle activity is positively correlated with intelligence. Nevertheless, Schabus and Gruber point out that sleep-spindle activity only relates to performance on newly learned memories, but not to absolute performance. This supports the hypothesis that sleep spindle helps to consolidate recent memory traces but not memory performance in general.
The relationship between sleep spindles and declarative memory consolidation is not yet fully understood.
There is a relatively small, yet growing body of evidence that supports the idea that REM sleep helps consolidate highly emotional declarative memories. For instance Wagner, et al. compared memory retention for emotional versus neutral text over two instances; early sleep that is dominated by SWS and late sleep that is dominated by REM phase. He found that sleep improved memory retention of emotional text only during late sleep phase, which was primarily REM. Similarly Hu & Stylos-Allen, et al. did a study with emotional versus neutral pictures and came to the same conclusion that REM sleep facilitates consolidation of emotional declarative memories.
However, the view that sleep plays an active role in declarative memory consolidation is not shared by all of the researchers. For instance Ellenbogen, et al. argue that sleep actively protects declarative memory from associative interference. Furthermore, Wixted believes that the sole role of sleep in declarative memory consolidation is nothing more but creating ideal conditions for memory consolidation. For example, when awake, people are bombarded with mental activity which interferes with effective consolidation. However, during sleep, when interference is minimal, memories can be consolidated without any obstacles. In sum, this view suggests that sleep provides ideal conditions for declarative memory consolidation but does not actively enhance memory consolidation. However more research is needed to make a definite statement whether sleep creates favourable conditions for consolidation or it actively enhances declarative memory consolidation.
In popular culture
AmnesiaAmnesia
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...
cs are frequently portrayed in television and movies. Some of the better known examples include:
In the romantic comedy 50 First Dates
50 First Dates
50 First Dates is a 2004 American romantic comedy film directed by Peter Segal and written by George Wing. The film stars Adam Sandler as a woman-chasing veterinarian and Drew Barrymore as an amnesiac, along with Rob Schneider, Sean Astin, Lusia Strus, Blake Clark, and Dan Aykroyd.Most of the film...
(2004), Adam Sandler plays veterinarian Henry Roth, who falls for Lucy Whitmore, played by Drew Barrymore. Having lost her short term memory in a car crash, Lucy can only remember the current day's events until she falls asleep. When she wakes up the next morning, she has no recollection of the previous day's experiences. These experiences would normally be transferred into declarative knowledge, allowing them to be recalled in the future. Although this movie is not the most accurate representation of a true amnesic patient, it is useful for informing viewers of the detrimental effects of amnesia.
Memento (2000) a film inspired by the case of Henry Molaison (H.M.). Guy Pearce
Guy Pearce
Guy Edward Pearce is an English-born Australian actor and musician, known for his roles as Leonard Shelby in Christopher Nolan's Memento, Lieutenant Ed Exley in L.A...
plays an ex-insurance investigator suffering from severe anterograde amnesia
Anterograde amnesia
Anterograde amnesia is a loss of the ability to create new memories after the event that caused the amnesia, leading to a partial or complete inability to recall the recent past, while long-term memories from before the event remain intact. This is in contrast to retrograde amnesia, where memories...
caused by a head injury. Unlike most amnesiacs, Leonard retains his identity and the memories of events that occurred before the injury, but loses all ability to form new memories. This loss of ability to form new memories indicates that the head injury affected the medial temporal lobe of the brain resulting in the inability for Leonard to form declarative memory.
Finding Nemo
Finding Nemo
Finding Nemo is a 2003 American comi-drama animated film written by Andrew Stanton, directed by Andrew Stanton and Lee Unkrich and produced by Pixar. It tells the story of the overly protective clownfish Marlin who, along with a regal tang called Dory , searches for his abducted son Nemo...
features a reef fish named Dory with an inability to develop declarative memory. This prevents her from learning or retaining any new information such as names or directions. The exact origin of Dory's impairment is not mentioned in the film, but her memory loss accurately portrays the difficulties facing amnesiacs.