Episodic memory
Episodic memory is the memory of everyday events (such as times, location geography, associated emotions, and other contextual information) that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred at particular times and places; for example, the party on one's 7th birthday.[1] Along with semantic memory, it comprises the category of explicit memory, one of the two major divisions of long-term memory (the other being implicit memory).[2]
The term "episodic memory" was coined by Endel Tulving in 1972, referring to the distinction between knowing and remembering: knowing is factual recollection (semantic) whereas remembering is a feeling that is located in the past (episodic).[3]
One of the main components of episodic memory is the process of recollection, which elicits the retrieval of contextual information pertaining to a specific event or experience that has occurred. Tulving seminally defined three key properties of episodic memory recollection as:
Aside from Tulving, others named additional aspects of recollection, including visual imagery, narrative structure, retrieval of semantic information and feelings of familiarity.[4]
Events that are recorded into episodic memory may trigger episodic learning, i.e. a change in behavior that occurs as a result of an event,[5][6] such as a fear of dogs after being bitten by a dog.
Cognitive neuroscience[edit]
The formation of new episodic memories requires the medial temporal lobe, a structure that includes the hippocampus. Without the medial temporal lobe, one is able to form new procedural memories (such as playing the piano) but cannot remember the events during which they happened (See the hippocampus and memory).
The prefrontal cortex (and in particular the right hemisphere) is also involved in the formation of new episodic memories (also known as episodic encoding). Patients with damage to the prefrontal cortex can learn new information, but tend to do so in a disordered fashion. For example, they might show normal recognition of an object they had seen in the past, but fail to recollect when or where it had been viewed.[8] Some researchers believe that the prefrontal cortex helps organize information for more efficient storage, drawing upon its role in executive function. Others believe that the prefrontal cortex underlies semantic strategies which enhance encoding, such as thinking about the meaning of the study material or rehearsing it in working memory.[9]
Other work has shown that portions of the inferior parietal lobe play a role in episodic memory, potentially acting as an accumulator to support the subjective feeling that something is "old", or perhaps supporting mental imagery which allows you a sense of the vividness of memories. Indeed, bilateral damage to the inferior parietal lobe results in episodic memory that is largely intact, however it lacks details [10] and lesion patients report low levels of confidence in their memories.[11]
Researchers do not agree about how long episodic memories are stored in the hippocampus. Some researchers believe that episodic memories always rely on the hippocampus. Others believe the hippocampus only stores episodic memories for a short time, after which the memories are consolidated to the neocortex. The latter view is strengthened by recent evidence that neurogenesis in the adult hippocampus may ease the removal of old memories and increase the efficiency of forming new memories.[12]
Relationship to semantic memory[edit]
Endel Tulving originally described episodic memory as a record of a person's experience that held temporally dated information and spatio-temporal relations.[13] A feature of episodic memory that Tulving later elaborates on is that it allows an agent to imagine traveling back in time.[14] A current situation may cue retrieval of a previous episode, so that context that colours the previous episode is experienced at the immediate moment. The agent is provided with a means of associating previous feelings with current situations. Semantic memory, on the other hand, is a structured record of facts, concepts, and skills that we have acquired. Semantic information is derived from accumulated episodic memory. Episodic memory can be thought of as a "map" that ties together items in semantic memory. For example, all encounters with how a "dog" looks and sounds will make up the semantic representation of that word. All episodic memories concerning a dog will then reference this single semantic representation of "dog" and, likewise, all new experiences with the dog will modify the single semantic representation of that dog.
Together, semantic and episodic memory make up our declarative memory.[15] They each represent different parts of context to form a complete picture. As such, something that affects episodic memory can also affect semantic memory. For example, anterograde amnesia, from damage of the medial temporal lobe, is an impairment of declarative memory that affects both episodic and semantic memory operations.[16] Originally, Tulving proposed that episodic and semantic memory were separate systems that competed with each other in retrieval. However, this theory was rejected when Howard and Kahana completed experiments on latent semantic analysis (LSA) that supported the opposite. Instead of an increase in semantic similarity when there was a decrease in the strength of temporal associations, the two worked together so semantic cues on retrieval were strongest when episodic cues were strong as well.[17]
Relationship to emotion[edit]
The relationship between emotion and memory is complex, but generally, emotion tends to increase the likelihood that an event will be remembered later and that it will be remembered vividly. Flashbulb memory is one example of this. Flashbulb memory is event-specific, which consists of depictions of personal experiences. For example, saying "I remember seeing Grandma smile when I gave her the present", or remembering the detailed events of the tragedy of 9/11. This idea of flashbulb memory was proposed by R. Brown and Kulik (1977), in which they stated that this idea revolves around remembering an event or unexpected circumstance due to emotional arousal. They referred to this memory as "photographic vividness". However, whether the vividness of the flashbulb memory is due to a virtual "flash" that occurs because of the emotional experience has been hotly contested. Flashbulb memories may occur because of our propensity to rehearse and retell those highly emotional events, which strengthens the memory. R. Brown and Kulik represented that these memories contain information that falls under the categories: place, ongoing activity, informant, own affect, and aftermath. Flashbulb memory is usually perceived as highly accurate and consistent over time and are presented with great confidence, even if sometimes they are inaccurate. Authors Brown, Kulik, and Conway argued that these special memories involve the limbic system, specifically, the amygdala. There is an abundancy of research that shows the amygdala involvement regarding retrieval of emotional memories, for example, research using brain imaging techniques.
Pharmacological enhancement[edit]
In healthy adults, longterm visual episodic memory can be enhanced specifically[20] through administration of the Acetylcholine esterase inhibitor Donepezil, whereas verbal episodic memory can be improved in persons with the val/val genotype of the val158met polymorphism through administration of the CNS penetrant specific catecholamine-O-methyltransferase inhibitor Tolcapone.[21] Furthermore, episodic memory is enhanced through AZD3480, a selective agonist at the neuronal alpha4beta2 nicotinic receptor, which is developed by the company Targacept.[22] Currently, there are several other products developed by several companies—including new catecholamine-O-methyltransferase inhibitors with fewer side effects—that aim for improving episodic memory. A recent placebo controlled study found that DHEA, which is a functional cortisol antagonist, improves episodic memory in healthy young men (Alhaj et al. 2006).[23]
A 2015 meta-analysis of high quality evidence found that therapeutic doses of amphetamine and methylphenidate improve performance on working memory, episodic memory, and inhibitory control tests in normal healthy adults.[24]
Autobiographical memory[edit]
An autobiographical memory is a personal representation of general or specific events and personal facts. Additionally, it also refers to the memory of a person's history. An individual does not remember exactly everything that has happened in one's past. Memory is constructive, where previous experience affects how we remember events and what we end up recalling from memory. Similarly, autobiographical memory is constructive and reconstructed as an evolving process of history. A person's autobiographical memory is fairly reliable, although the reliability of autobiographical memories is questionable because of memory distortions.[38]
Autobiographical memories can differ for special periods of life. For instance, people recall a few personal events from the first years of their lives. The loss of these first events is called childhood or infantile amnesia. Also, people tend to recall many personal events from adolescence and early adulthood. This effect is called the reminiscence bump. Additionally, people recall many personal events from their previous few years. For adolescents and young adults, the reminiscence bump and the recent events can coincide.[39]
It is known that autobiographical memories initially are stored as episodic memories, but it is currently unknown if autobiographical memories are the same as episodic memories or if the autobiographical memories become converted to semantic memories with time.[40]