Explicit memory
Explicit memory (or declarative memory) is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and concepts.[1] This type of memory is dependent upon three processes: acquisition, consolidation, and retrieval.[2][3]
Explicit memory can be divided into two categories: episodic memory, which stores specific personal experiences, and semantic memory, which stores factual information.[4] Explicit memory requires gradual learning, with multiple presentations of a stimulus and response.
The type of knowledge that is stored in explicit memory is called declarative knowledge, the counterpart to explicit memory is known as implicit memory, refers to memories acquired and used unconsciously such as skills (e.g. knowing how to get dressed) or perception. Unlike explicit memory, implicit memory learns rapidly, even from a single stimulus, and it is influenced by other mental systems.
Sometimes a distinction is made between explicit memory and declarative memory. In such cases, explicit memory relates to any kind of conscious memory, and declarative memory relates to any kind of memory that can be described in words; however, if it is assumed that a memory cannot be described without being conscious and vice versa, then the two concepts are identical.
History[edit]
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.[12] He theorized that all humans are born free of any knowledge and are the sum of their experiences. It was only in the late 1800s, however, that a young German philosopher by the name of Herman Ebbinghaus developed the first scientific approach to studying memory.[13] 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.[4] 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[14]
With the recent advances in neuroimaging technology, there have been a multitude of findings linking specific brain areas to declarative memory. Despite those advances in cognitive psychology, there is still much to be discovered in terms of the operating mechanisms of declarative memory.[15] 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. Also it is unknown how or why declarative memory evolved in the first place.[15]
Consolidation during sleep[edit]
It is believed 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. This reactivation transfers information to neocortical networks where it is integrated into long-term representations.[44] 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.[45] Similarly, positron emission tomography (PET) has shown reactivation of the hippocampus in slow-wave sleep (SWS) after spatial learning.[46] Together these studies show that newly learned memories are reactivated during sleep and through this process new memory traces are consolidated.[47] 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. 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. 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.[48] 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.[49]
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.[50] 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.[51]
Some researchers suggest that sleep spindle, a burst of brain activity occurring during stage 2 sleep, plays a role in boosting consolidation of declarative memories.[52] Critics point out that spindle activity is positively correlated with intelligence.[53] In contrast, Schabus and Gruber point out that sleep spindle activity only relates to performance on newly learned memories and not to absolute performance. This supports the hypothesis that sleep spindle helps to consolidate recent memory traces but not memory performance in general.[54] The relationship between sleep spindles and declarative memory consolidation is not yet fully understood.[54]
There is a relatively small 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.[55] This study found that sleep improved memory retention of emotional text only during late sleep phase, which was primarily REM. Similarly, Hu & Stylos-Allen, et al. performed a study with emotional versus neutral pictures and concluded that REM sleep facilitates consolidation of emotional declarative memories.[56]
The view that sleep plays an active role in declarative memory consolidation is not shared by all researchers. For instance Ellenbogen, et al. argue that sleep actively protects declarative memory from associative interference.[57] Furthermore, Wixted believes that the sole role of sleep in declarative memory consolidation is nothing more but creating ideal conditions for memory consolidation.[58] 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 associative interference. More research is needed to make a definite statement whether sleep creates favourable conditions for consolidation or it actively enhances declarative memory consolidation.[47]
Encoding and retrieval[edit]
The encoding of explicit memory depends on conceptually driven, top-down processing, in which a subject reorganizes the data to store it.[59] The subject makes associations with previously related stimuli or experiences.[60] This was termed deep encoding by Fergus Craik and Robert Lockhart.[61] This way a memory persists longer and will be remembered well. The later recall of information is thus greatly influenced by the way in which the information was originally processed.[59]
The depth-of-processing effect is the improvement in subsequent recall of an object about which a person has given thought to its meaning or shape. Simply put: To create explicit memories, you have to do something with your experiences: think about them, talk about them, write them down, study them, etc. The more you do, the better you will remember. Testing of information while learning has also shown to improve encoding in explicit memory. If a student reads a text book and then tests themselves afterward, their semantic memory of what was read is improved. This study – test method improves encoding of information. This Phenomenon is referred to as the Testing Effect.[62]
Retrieval: Because a person has played an active role in processing explicit information, the internal cues that were used in processing it can also be used to initiate spontaneous recall.[59] When someone talks about an experience, the words they use will help when they try to remember this experience at a later date. The conditions in which information is memorized can affect recall. If a person has the same surroundings or cues when the original information is presented, they are more likely to remember it. This is referred to as encoding specificity and it also applies to explicit memory. In a study where subjects were asked to perform a cued recall task participants with a high working memory did better than participants with a low working memory when the conditions were maintained. When the conditions were changed for recall both groups dropped. The subjects with higher working memory declined more.[63] This is thought to happen because matching environments activates areas of the brain known as the left inferior frontal gyrus and the hippocampus.[64]
Neural structures involved[edit]
Several neural structures are proposed to be involved in explicit memory. Most are in the temporal lobe or closely related to it, such as the amygdala, the hippocampus, the rhinal cortex in the temporal lobe, and the prefrontal cortex.[59] Nuclei in the thalamus also are included, because many connections between the prefrontal cortex and temporal cortex are made through the thalamus.[59] The regions that make up the explicit memory circuit receive input from the neocortex and from brainstem systems, including acetylcholine, serotonin, and noradrenaline systems.[65]
Traumatic brain injury[edit]
While the human brain is certainly regarded for its plasticity, there is some evidence that shows traumatic brain injury (TBI) in young children can have negative effects on explicit memory. Researchers have looked at children with TBI in early childhood (i.e. infancy) and late childhood. Findings showed that children with severe TBI in late childhood experienced impaired explicit memory while still maintaining implicit memory formation. Researchers also found that children with severe TBI in early childhood had both increased chance of having both impaired explicit memory and implicit memory. While children with severe TBI are at risk for impaired explicit memory, the chances of impaired explicit memory in adults with severe TBI is much greater.[66]
Memory loss[edit]
Alzheimer's disease has a profound effect on explicit memory. Mild cognitive impairment is an early sign of Alzheimer's disease. People with memory conditions often receive cognitive training. When an fMRI was used to view brain activity after training, it found increased activation in various neural systems that are involved with explicit memory.[67] People with Alzheimer's have problems learning new tasks. However, if the task is presented repeatedly they can learn and retain some new knowledge of the task. This effect is more apparent if the information is familiar. The person with Alzheimer's must also be guided through the task and prevented from making errors.[68] Alzheimer's also has an effect on explicit spatial memory. This means that people with Alzheimer's have difficulty remembering where items are placed in unfamiliar environments.[69] The hippocampus has been shown to become active in semantic and episodic memory.[70]
The effects of Alzheimer's disease are seen in the episodic part of explicit memory. This can lead to problems with communication. A study was conducted where Alzheimer's patients were asked to name a variety of objects from different periods. The results shown that their ability to name the object depended on frequency of use of the item and when the item was first acquired.[71] This effect on semantic memory also has an effect on music and tones. Alzheimer's patients have difficulty distinguishing between different melodies they have never heard before. People with Alzheimer's also have issues with picturing future events. This is due to a deficit in episodic future thinking.[72] There are many other reasons why adults and others may begin to have memory loss.
In popular culture[edit]
Amnesia is 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.[73] Those experiences would normally be transferred into declarative knowledge and allow them to be recalled in the future. The movie is not the most accurate representation of a true amnesic patient, but it is useful to inform viewers of the detrimental effects of amnesia.
Memento (2000) a film inspired by the case of Henry Molaison (H.M.).[74] Guy Pearce plays a former insurance investigator suffering from severe anterograde amnesia, which was caused by a head injury. Unlike most other amnesiacs, Leonard retains his identity and the memories of events that occurred before the injury but has lost all ability to form new memories. That loss of ability indicates that the head injury affected the medial temporal lobe of the brain, which has resulted in his inability to form declarative memory.
Finding Nemo features a reef fish named Dory with an inability to develop declarative memory. That 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.[73]