Semantic memory
Semantic memory refers to general world knowledge that humans have accumulated throughout their lives. This general knowledge is intertwined in experience and dependent on culture. New concepts are learned by applying knowledge learned from things in the past.
Semantic memory is distinct from episodic memory—the memory of experiences and specific events that occur in one's life that can be recreated 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 stroking a particular cat.
Semantic memory and episodic memory are both types of explicit memory, or memory of facts or events that can be consciously recalled and "declared". The counterpart to declarative or explicit memory is implicit memory.
History
The idea of semantic memory was first introduced following a conference in 1972 between Endel Tulving and W. Donaldson on the role of organization in human memory. Tulving constructed a proposal to distinguish between episodic memory and what he termed semantic memory. He was mainly influenced by the ideas of Reiff and Scheers, who in 1959 made the distinction between two primary forms of memory. One form was titled remembrances, and the other memoria. The remembrance concept dealt with memories that contained experiences of an autobiographic index, whereas the memoria concept dealt with memories that did not reference experiences having an autobiographic index.Semantic memory reflects the knowledge of the world, and the term general knowledge is often used. It holds generic information that is more than likely acquired across various contexts and is used across different situations. According to Madigan in his book titled Memory, semantic memory is the sum of all knowledge one has obtained—vocabulary, understanding of math, or all the facts one knows. In his book titled Episodic and Semantic Memory, Tulving adopted the term semantic from linguists to refer to a system of memory for "words and verbal symbols, their meanings and referents, the relations between them, and the rules, formulas, or algorithms for influencing them".
The use of semantic memory differs from episodic memory: semantic memory refers to general facts and meanings one shares with others, while episodic memory refers to unique and concrete personal experiences. Tulving's proposal of this distinction was widely accepted, primarily because it allowed the separate conceptualization of world knowledge. Tulving discusses conceptions of episodic and semantic memory in his book titled Précis of Elements of Episodic Memory, in which he states that several factors differentiate between episodic memory and semantic memory in ways that include
- the characteristics of their operations,
- the kind of information they process, and
- their application to the real world as well as the memory laboratory.
Recent research has focused on the idea that when people access a word's meaning, sensorimotor information that is used to perceive and act on the concrete object the word suggests is automatically activated. In the theory of grounded cognition, the meaning of a particular word is grounded in the sensorimotor systems. For example, when one thinks of a pear, knowledge of grasping, chewing, sights, sounds, and tastes used to encode episodic experiences of a pear are recalled through sensorimotor simulation.
A grounded simulation approach refers to context-specific re-activations that integrate the important features of episodic experience into a current depiction. Such research has challenged previously utilized amodal views. The brain encodes multiple inputs such as words and pictures to integrate and create a larger conceptual idea by using amodal views. Instead of being representations in modality-specific systems, semantic memory representations had previously been viewed as redescriptions of modality-specific states. Some accounts of category-specific semantic deficits that are amodal remain even though researchers are beginning to find support for theories in which knowledge is tied to modality-specific brain regions. The concept that semantic representations are grounded across modality-specific brain regions can be supported by episodic and semantic memory appearing to function in different yet mutually dependent ways. The distinction between semantic and episodic memory has become a part of the broader scientific discourse. For example, researchers speculate that semantic memory captures the stable aspects of our personality while episodes of illness may have a more episodic nature.
Empirical evidence
Jacoby and Dallas (1981)
This study was not created to solely provide evidence for the distinction of semantic and episodic memory stores. However, they did use the experimental dissociation method which provides evidence for Tulving's hypothesis.In the first part, subjects were presented with a total of 60 words and were asked different questions.
- Some questions asked were to cause the subject to pay attention to the visual appearance: Is the word typed in bold letters?
- Some questions caused the participants to pay attention to the sound of the word: Does the word rhyme with ball?
- Some questions caused the subjects to pay attention to the meaning of the word: Does the word refer to a form of communication?
- Half of the questions were "no" answers and the other half "yes"
The subjects were given one of two tasks:
- Perceptual identification task: The words were flashed on a video-screen for 35 milliseconds and the subjects were required to say what the word was.
- Episodic recognition task: Subjects were presented with each word and had to decide whether they had seen the word in the previous stage of the experiment.
Models
Semantic memory's contents are not tied to any particular instance of experience, as in episodic memory. Instead, what is stored in semantic memory is the "gist" of experience, an abstract structure that applies to a wide variety of experiential objects and delineates categorical and functional relationships between such objects. There are numerous sub-theories related to semantic memory that have developed since Tulving initially posited his argument on the differences between semantic and episodic memory; an example is the belief in hierarchies of semantic memory, in which different information one has learned with specific levels of related knowledge is associated. According to this theory, brains are able to associate specific information with other disparate ideas despite not having unique memories that correspond to when that knowledge was stored in the first place. This theory of hierarchies has also been applied to episodic memory, as in the case of work by William Brewer on the concept of autobiographical memory.Network models
of various sorts play an integral part in many theories of semantic memory. Generally speaking, a network is composed of a set of nodes connected by links. The nodes may represent concepts, words, perceptual features, or nothing at all. The links may be weighted such that some are stronger than others or, equivalently, have a length such that some links take longer to traverse than others. All these features of networks have been employed in models of semantic memory.Teachable language comprehender
One of the first examples of a network model of semantic memory is the teachable language comprehender. In this model, each node is a word, representing a concept. Within each node is stored a set of properties as well as links to other nodes. A node is directly linked to those nodes of which it is either a subclass or superclass. Properties are stored at the highest category level to which they apply; for example, "is yellow" would be stored with canary, "has wings" would be stored with bird, and "can move" would be stored with animal. Nodes may also store negations of the properties of their superordinate nodes.Processing in TLC is a form of spreading activation. When a node becomes active, that activation spreads to other nodes via the links between them. In that case, the time to answer the question "Is a chicken a bird?" is a function of how far the activation between the nodes for chicken and bird must spread, or the number of links between those nodes.
The original version of TLC did not put weights on the links between nodes. This version performed comparably to humans in many tasks, but failed to predict that people would respond faster to questions regarding more typical category instances than those involving less typical instances. Allan Collins and Quillian later updated TLC to include weighted connections to account for this effect, which allowed it to explain both the familiarity effect and the typicality effect. Its biggest advantage is that it clearly explains priming: information from memory is more likely to be retrieved if related information has been presented a short time before. There are still a number of memory phenomena for which TLC has no account, including why people are able to respond quickly to obviously false questions when the relevant nodes are very far apart in the network.