Declarative memory (sometimes referred to as Explicit memory) is one of two types of long term human memory. It refers to memories which can be consciously recalled such as facts and events. [1] 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 distinct categories: Semantic and Episodic memory. [2]
There are two types of declarative memory:
The study of human memory stretches back over the last 2000 years. An early attempt to understand memory can be found in Aristotle’s major treatise, On the Soul, in which he compares the human mind to a blank slate [3]. 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. [4] 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 [2]. 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 [5] 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 [6]. 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 [6].
Although many psychologists believe that the entire brain is involved with memory, the hippocampus and surrounding structures appear to be most important in declarative memory specifically [7]. The ability to retain and recall episodic memories is highly dependent on the hippocampus [7], whereas the formation of new declarative memories relies on both the hippocampus and parahippocampus [8] Other studies have found that the parahippocampal cortices were related to superior Recognition Memory [8].
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 [7].
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 [7].
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 [7]. 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 lateral Prefrontal cortex (PFC) is essential for remembering contextual details of an experience rather than for memory formation [8]. The PFC is also more involved with episodic memory than semantic memory, although it does play a small role in semantics. [9]
Using PET studies and word stimuli, Endel Tulving found that remembering is an automatic process [10]. 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) [10].
Studies have also shown that the PFC is extremely involved with autonoetic consciousness (See Tulving's theory) [11].This is responsible for humans’ recollective experiences and ‘mental time travelling’ abilities (characteristics of episodic memory).
The amygdala 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. attack on the World Trade Centre, assassination of JFK). These memories have been linked to increased activation in the amygdala. [12] Recent studies of patients with damage to the amygdala suggest that it is involved in memory for general knowledge, and not for specific information. [13] [14]
The regions of the Diencephalon have shown brain activation when a remote memory is being recovered [9] and the Occipital lobe, Ventral Temporal lobe, and Fusiform gyrus all play a role in memory formation [8].
Lesion 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.
Stress 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 [25]. 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 [26] The chronic stress in PTSD contributes to an observed decrease in hippocampal volume and declarative memory deficits [27].
In the brain, Glucocorticoids (GC's) modulate the ability of the hippocampus and PFC to process memories [28]. Cortisol 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 [28].
Elevations in cortisol occur during stress, and long-term stress impairs declarative memory this way [28]. 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 [28]. Although it is not known exactly how GC’s influence memory, there are Glucocorticoid receptors in the hippocampus and PFC that tell us these structures are targets for the circulating hormone [28]. 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 [28].
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 [28].
It is believed by many researchers that sleep 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 [29]. 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 [30]. Similarly, positron emission tomography (PET) has shown reactivation of the hippocampus in slow-wave sleep (SWS) after spatial learning [31]. Together these studies show that newly learned memories are reactivated during sleep and through this, help consolidate new memory traces [32]. In addition, researchers to date have pinpointed three types of sleep (SWS, Sleep Spindle and REM) which they believe contribute to declarative memory consolidation.
More research is needed to fully understand the relationship between sleep spindles and declarative memory consolidation.
However, the view that sleep play’s an active role in declarative memory consolidation is not shared by all of the researchers. For instance Ellenbogen, et al. [42] argues 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 [43]. 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 [32].
Amnesiacs are frequently portrayed in television and movies. Some of the better known examples include:
Declarative memory (sometimes referred to as Explicit memory) is one of two types of long term human memory. It refers to memories which can be consciously recalled such as facts and events. [1] 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 distinct categories: Semantic and Episodic memory. [2]
There are two types of declarative memory:
The study of human memory stretches back over the last 2000 years. An early attempt to understand memory can be found in Aristotle’s major treatise, On the Soul, in which he compares the human mind to a blank slate [3]. 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. [4] 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 [2]. 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 [5] 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 [6]. 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 [6].
Although many psychologists believe that the entire brain is involved with memory, the hippocampus and surrounding structures appear to be most important in declarative memory specifically [7]. The ability to retain and recall episodic memories is highly dependent on the hippocampus [7], whereas the formation of new declarative memories relies on both the hippocampus and parahippocampus [8] Other studies have found that the parahippocampal cortices were related to superior Recognition Memory [8].
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 [7].
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 [7].
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 [7]. 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 lateral Prefrontal cortex (PFC) is essential for remembering contextual details of an experience rather than for memory formation [8]. The PFC is also more involved with episodic memory than semantic memory, although it does play a small role in semantics. [9]
Using PET studies and word stimuli, Endel Tulving found that remembering is an automatic process [10]. 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) [10].
Studies have also shown that the PFC is extremely involved with autonoetic consciousness (See Tulving's theory) [11].This is responsible for humans’ recollective experiences and ‘mental time travelling’ abilities (characteristics of episodic memory).
The amygdala 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. attack on the World Trade Centre, assassination of JFK). These memories have been linked to increased activation in the amygdala. [12] Recent studies of patients with damage to the amygdala suggest that it is involved in memory for general knowledge, and not for specific information. [13] [14]
The regions of the Diencephalon have shown brain activation when a remote memory is being recovered [9] and the Occipital lobe, Ventral Temporal lobe, and Fusiform gyrus all play a role in memory formation [8].
Lesion 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.
Stress 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 [25]. 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 [26] The chronic stress in PTSD contributes to an observed decrease in hippocampal volume and declarative memory deficits [27].
In the brain, Glucocorticoids (GC's) modulate the ability of the hippocampus and PFC to process memories [28]. Cortisol 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 [28].
Elevations in cortisol occur during stress, and long-term stress impairs declarative memory this way [28]. 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 [28]. Although it is not known exactly how GC’s influence memory, there are Glucocorticoid receptors in the hippocampus and PFC that tell us these structures are targets for the circulating hormone [28]. 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 [28].
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 [28].
It is believed by many researchers that sleep 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 [29]. 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 [30]. Similarly, positron emission tomography (PET) has shown reactivation of the hippocampus in slow-wave sleep (SWS) after spatial learning [31]. Together these studies show that newly learned memories are reactivated during sleep and through this, help consolidate new memory traces [32]. In addition, researchers to date have pinpointed three types of sleep (SWS, Sleep Spindle and REM) which they believe contribute to declarative memory consolidation.
More research is needed to fully understand the relationship between sleep spindles and declarative memory consolidation.
However, the view that sleep play’s an active role in declarative memory consolidation is not shared by all of the researchers. For instance Ellenbogen, et al. [42] argues 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 [43]. 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 [32].
Amnesiacs are frequently portrayed in television and movies. Some of the better known examples include: