Explicit memory is the conscious, intentional recollection of previous experiences and information. This information is about a specific event that has occurred at a specific time and place, both in your personal life and in the world at large.
Explicit memory involves conscious recollection, compared with implicit memory which is an unconscious, nonintentional form of memory. Remembering a specific driving lesson is an example of explicit memory, while improving your driving skills during the lesson is an example of implicit memory.
Encoding: Explicit memory depends on conceptually driven, top-down processing, in which a subject reorganizes the data to store it. The subject makes associations with previously related stimuli or experiences. The later recall of information is thus greatly influenced by the way in which the information was originally processed. 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.
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. Simply put: When you've talked about a certain experience, the words that you used, will help you when you half a year later try to remember this experience.
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. Nuclei in the thalamus also are included, because many connections between the prefrontal cortex and temporal cortex are made through the thalamus.
The regions that make up the explicit memory circuit receive input from the neocortex and from brainstem systems, including acetylcholine, serotonin, and noradrenaline systems.
Episodic memory, also called autobiographical memory, consists of the recollection of singular events in the life of a person. It is the memory of life experiences centered on yourself.
Episodic memory is necessary for "time traveling": remembering your past and imagining your future. It is considered a uniquely human quality that depends on maturaton and therefore won't be found in babies and young children.
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.
The prefrontal cortex (and in particular the left 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 (Janowsky et al., 1989). 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 the study material or rehearsing it in working memory (Gabrieli et al., 1998).
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 (Deisseroth et al 2004).
According to Brain activation during episodic memory retrieval: sex differences, women tend to outperform men on episodic memory tasks.
Activation of specific brain areas (mostly the hippocampus) seems to be different between young and older people upon episodic memory retrieval, as shown by Maguire and Frith 2003. Older people tend to activate both left and right hippocampus, while young people activate only the left one.
Semantic memory consists of all explicit memory that is not autobiographical. Examples of semantic memory is knowledge of historical events and figures; the ability to recognize friends and acquaintances; and information learned in school, such as specialized vocabularies and reading, writing and mathematics.
Semantic memory includes generalized knowledge that does not involve memory of a specific event. For instance, you can answer a question like "Are wrenches pets or tools?" without remembering any specific event in which you learned that wrenches are tools.
Location of semantic memory in the brain
The cognitive neuroscience of semantic memory is a somewhat controversial issue with two dominant views. On the one hand, many researchers and clinicians believe that semantic memory is stored by the same brain systems involved in episodic memory. These include the medial temporal lobes (MTL) and hippocampal formation. In this system, the hippocampal formation "encodes" memories, or makes it possible for memories to form at all, and the cortex stores memories after the initial encoding process is completed.
Recently, new evidence has been presented in support of a more precise interpretation of this hypothesis. The hippocampal formation includes, among other structures: the hippocampus itself, the entorhinal cortex, and the perirhinal cortex. These latter two make up the "parahippocampal cortices". Amnesics with damage to the hippocampus but some spared parahippocampal cortex were able to demonstrate some degree of intact semantic memory despite a total loss of episodic memory. This strongly suggests that encoding of information leading to semantic memory does not have its physiological basis in the hippocampus. (Vargha-Kadem et al.)
Other researchers believe the hippocampus is only involved in episodic memory and spatial cognition. This then raises the question where semantic memory may be located. Some believe semantic memory lives in temporal neocortex. Others believe that semantic knowledge is widely distributed across all brain areas. To illustrate this latter view, consider your knowledge of dogs. Researchers holding the 'distributed semantic knowledge' view believe that your knowledge of the sound a dog makes exists in your auditory cortex, whilst your ability to recognize and imagine the visual features of a dog resides in your visual cortex. Perhaps all these representations are indexed by the left temporal pole, a region particularly vulnerable to damage in semantic dementia.
The neural basis of episodic and semantic memory is not yet known today. However, some scientists suggest that episodic memory might be dependent on the right hemisphere, and semantic memory on the left hemisphere.
The relationship of episodic memory to semantic memory
Episodic memory is thought of as being a "one-shot" learning mechanism. You only need one exposure to an episode to remember it. Semantic memory, on the other hand, can take into consideration multiple exposures to each referent - the semantic representation is updated on each exposure.
Episodic memory can be thought of as a "map" that ties together items in semantic memory. For example, semantic memory will tell you what a "dog" looks and sounds like. All episodic memories concerning your dog will reference this single semantic representation of "dog" and, likewise, all new experiences with your dog will modify your single semantic representation of your dog.
Some researchers believe that episodic memories are refined into semantic memories over time. In this process, most of the episodic information about a particular event is generalized and the context of the specific events is lost. One modification of this view is that episodic memories which are recalled often are remembered as a kind of monologue. If you tell and re-tell a story repeatedly, you may feel that you no longer remember the event, but that what you're recalling is a kind of pre-written story.
Others believe that you always remember episodic memories as episodic memories. Of course, episodic memories do inform semantic knowledge and episodic memories are reliant upon semantic knowledge. The point is that some people do not believe that all episodic memories will inevitably distill away into semantic memory.
Explicit Memory Wikipedia
SCREEN MEMORIES, ALIEN AND FAERY ABDUCTIONS
MEMORY (CONSCIOUSNESS) FROZEN IN TIME METAPHYSICS
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