Type of long-term human 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. This type of memory is dependent upon three processes: acquisition, consolidation, and retrieval.
Explicit memory can be divided into two categories: episodic memory, which stores specific personal experiences, and semantic memory, which stores factual information. 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. Its counterpart, known as implicit memory, refers to memories acquired and used unconsciously, such as skills (e.g. knowing how to get dressed) or perceptions. 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.
Types.
Episodic memory.
Episodic memory consists of the storage and recollection of observational information attached to specific life-events. These can be memories that happened to the subject directly or just memories of events that happened around them. Episodic memory is what people generally think of when they talk about memory. Episodic memory allows for recalling various contextual and situational details of one's previous experiences.
Some examples of episodic memory include the memory of entering a specific classroom for the first time, the memory of storing your carry-on baggage while boarding a plane, headed to a specific destination on a specific day and time, the memory of being notified that one is being terminated from one's job, or the memory of notifying a subordinate that they are being terminated from their job. The retrieval of these episodic memories can be thought of as the action of mentally reliving in detail the past events that they concern. Episodic memory is believed to be the system that provides the basic support for semantic memory.
Semantic memory.
Semantic memory refers to general world knowledge (facts, ideas, meaning and concepts) that can be articulated and is independent of personal experience. This includes world knowledge, object knowledge, language knowledge, and conceptual priming. Semantic memory is distinct from episodic memory, which is the memory of experiences and specific events that occur during people's lives, from which they can recreate at any given point. For instance, semantic memory might contain information about what a cat is, whereas episodic memory might contain a specific memory of petting a particular cat. Humans can learn about new concepts by applying their knowledge learned from things in the past.
Other examples of semantic memory include types of food, capital cities of a geographic region, facts about people, dates, and the lexicon of flowers; a language, such as a one's vocabulary or a person's final vocabulary both exemplify semantic memory.
Hybrid types.
"Autobiographical memory" is a memory system consisting of episodes recollected from an individual's life, based on a combination of episodic (personal experiences and specific objects, people and events experienced at particular time and place) and semantic (general knowledge and facts about the world) memory.
"Spatial memory" is the part of memory responsible for recording information about one's environment and its spatial orientation. For example, a person's spatial memory is required in order to navigate around a familiar city, just as a rat's spatial memory is needed to learn the location of food at the end of a maze. It is often argued that in both humans and animals, spatial memories are summarized as a cognitive map. Spatial memory has representations within working, short-term and long-term memory. Research indicates that there are specific areas of the brain associated with spatial memory. Many methods are used for measuring spatial memory in children, adults, and animals.
The model of language.
Declarative and procedural memory fall into two categories of human language. Declarative memory system is used by the lexicon. Declarative memory stores all arbitrary, unique word-specific knowledge, including word meanings, word sounds, and abstract representations such as word category. In other words, declarative memory is where random bits and pieces of knowledge about language that are specific and unpredictable are stored. Declarative memory includes representations of simple words (e.g. cat), bound morphemes (morphemes that have to go together), irregular morphological forms, verb complements, and idioms (or non-compositional semantic units). Irregular morphological structures fall into the declarative system; the irregularities (such as "went" being the past form of "go" or idioms) are what we have to memorize.
Declarative memory supports a superposition associative memory, which allows for generalizations across representations. For example, the memorization of phonologically similar stem-irregular past tense pairs (e.g. spring-sprung, sing-sang) may allow for memory-based generalization to new irregularities, either from real words (bring-brought) or from novel ones (spring-sprung). This ability to generalize could underlie some degree of productivity within the memory system.
While declarative memory deals with irregularities of morphology, procedural memory uses regular phonology and regular morphology. Procedural memory system is used by grammar, where grammar is defined by the building of a rule governed structure. Language's ability to use grammar comes from procedural memory, making grammar like another procedure. It underlies the learning of new, and already learned, rule-based procedures that oversee the regularities of language, particularly those procedures related to combining items into complex structures that have precedence and hierarchical relations- precedence in the sense of left to right and hierarchical in the sense of top to bottom. Procedural memory builds rule-governed structure (merging or series) of forms and representations into complex structures such as:
Broca and Wernicke's Brain Region.
Broca's area is important to procedural memory, because, "Broca's area is involved in the expressive aspects of spoken and written language (production of sentences constrained by the rules of grammar and syntax)." Broca's area corresponds to parts of the inferior frontal gyrus, presumably Brodmann's area 44 and 45. Procedural memory is affected by Broca's aphasia. Agrammatism is apparent in Broca's aphasia patients, where a lack of fluency and omission of morphology and function words occur. While those with Broca's aphasia are still able to understand or comprehend speech, they have difficulty producing it. Speech production becomes more difficult when sentences are complex; for example, the passive voice is a grammatically complex structure that is harder for those with Broca's aphasia to comprehend. Wernicke's area is crucial for language development, focusing on the comprehension of speech, rather than speech production. Wernicke's aphasia affects declarative memory. Opposite of Broca's aphasia, paragrammatism is apparent, which causes normal or excessive fluency and use of inappropriate words (neologisms). Those with Wernicke's aphasia struggle to understand the meaning of words and may not recognize their mistakes in speech.
History.
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. 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. 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 those 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. Also it is unknown how or why declarative memory evolved in the first place.
Neuropsychology.
Normal brain function.
Hippocampus.
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. 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 the 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.
The anatomy of the hippocampus is largely conserved across mammals, and 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 humans and other mammals, a cross-section of the hippocampus shows the dentate gyrus as well as the dense cell layers of the CA fields. The intrinsic connectivity of these areas are also conserved.
Results from an experiment by Davachi, Mitchell, and Wagner (2003) and subsequent research (Davachi, 2006) shows that activation in the hippocampus during encoding is related to a subject's ability to recall prior events or later relational memories. These tests did not differentiate between individual test items later seen and those forgotten.
Prefrontal cortex.
The lateral Prefrontal cortex (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).
Amygdala.
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. 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.
Other structures involved.
The regions of the diencephalon 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.
Lesion studies.
Lesion