Semantic memory
Encyclopedia
Semantic memory refers to the memory
of meanings, understanding
s, and other concept-based knowledge
unrelated to specific experiences. The conscious recollection of factual information and general knowledge about the world is generally thought to be independent of context and personal relevance. Semantic and episodic memory
together make up the category of declarative memory
, which is one of the two major divisions in memory. The counterpart to declarative, or explicit memory, is procedural memory
, or implicit memory
.
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.
, of the University of Toronto, 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 titled remembrances and the other memoria. The remembrance concept dealt with memories that contained the experiences of an autobiographic index, whereas the memoria’ concept dealt with those memories without the experiences of an autobiographic index. Semantic memory was to reflect our knowledge of the world around us. It holds generic information that is more than likely acquired across various contexts and is able to be used across different situations. According to Madigan in his book titled Memory, semantic memory is the sum of all knowledge you have obtained- whether it be your vocabulary, understanding of math, and all the facts you know. The use of semantic memory is quite different from that of episodic memory. Semantic memory refers to general facts and meanings we share with others whereas episodic memory refers to unique and concrete personal experiences.
Tulving's proposal of this distinction between semantic and episodic memory was widely accepted mainly because it allowed the separate conceptualization of knowledge of the world. Tulving discusses these separate systems of conceptualization of episodic and semantic memory in his book titled Elements of Episodic Memory. He states that both episodic and semantic memory differ in regards to several factors including:
Before this proposal by Tulving this area of human memory had been neglected by experimental psychologists. A number of experimenters have conducted tests to determine the validity of Tulving’s hypothesized distinction of episodic and semantic memory.
, were hypnotized. While under hypnosis they learned a list of 16 common words using a multi-trial free recall method. Once the subjects were able to perfectly recall the list twice in succession they were told that after awakening they would not remember having learned any of the words while under hypnosis. However, given the signal of the experimenter not only will they remember having learned the words but they will also remember the words from the list.
During stage one of the experiment (after subjects were awakened) the number of words recalled by the subjects were used as a measure of performance for the episodic task of free recall. Most subjects remembered learning the list of words.
During the second stage the measure of semantic memory performance was assessed. Each subject was given a semantic free association test (where stimulus words were given to elicit the learned words).
As mentioned previously, the subjects represented various levels of hypnotic susceptibility as determined by their scores on the Stanford Hypnotic Susceptibility Scale. They were grouped according to their score.
The semantic free association probabilities were relatively the same across various hypnotized groups. However, the episodic free recall probabilities were significantly different across the groups. The percentage increased as the hypnotizability of subjects decreased. The subjects in the very high susceptibility group recalled almost nothing, whereas the medium and low groups recalled 86% of the learned words.
Because the free association test was not related to the hypnotic susceptibility of the subjects shows that amnesia presented after hypnosis determined the memory for the word-events that occurred in the study phase.
This study provides evidence that supports the episodic/semantic distinction hypothesized by Tulving.
Part one
Subjects were presented with 60 words (one at a time) and were asked different questions.
Part Two
In the second phase of the experiment 60 “old words”- seen in stage one and “20 new words” not shown in stage one were presented to the subjects one at a time.
The subjects were given one of two tasks:
Results:
Conclusion:
It displays a strong distinction of performance of episodic and semantic tasks, thus supporting Tulving’s hypothesis.
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, examples of which are found below.
Processing in TLC is a form of spreading activation
. That is, 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, i.e., the number of links between the nodes "Chicken" and "Bird".
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. Collins
and Quillian later updated TLC to include weighted connections to account for this effect. This updated TLC is capable of explaining both the familiarity effect and the typicality effect. Its biggest advantage is that it clearly explains priming
: you are more likely to retrieve information from memory if related information (the "prime") 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 (like "is a chicken a meteor?"), when the relevant nodes are very far apart in the network.
. The defining feature of a semantic network is that its links are almost always directed (that is, they only point in one direction, from a base to a target) and the links come in many different types, each one standing for a particular relationship that can hold between any two nodes. Processing in a semantic network often takes the form of spreading activation (see above).
Semantic networks see the most use in models of discourse
and logic
al comprehension
, as well as in Artificial Intelligence
. In these models, the nodes correspond to words or word stems and the links represent syntactic relations between them. For an example of a computational implementation of semantic networks in knowledge representation, see Cravo and Martins (1993).
, proposed by Smith, Shoben, and Rips (1974), describes memory as being composed of feature lists for different concepts. According to this view, the relations between categories would not be directly retrieved, they would be indirectly computed. For example, subjects might verify a sentence by comparing the feature sets that represent its subject and predicate concepts. Such computational feature-comparison models include the ones proposed by Meyer (1970), Rips (1975), Smith, et al. (1974).
Early work in perceptual and conceptual categorization assumed that categories had critical features and that category membership could be determined by logical rules for the combination of features. More recent theories have accepted that categories may have an ill-defined or "fuzzy" structure and have proposed probabilistic or global similarity models for the verification of category membership.
"—a relationship between two pieces of information—is a fundamental concept in psychology, and associations at various levels of mental representation are essential to models of memory and cognition in general. The set of associations among a collection of items in memory is equivalent to the links between nodes in a network, where each node corresponds to a unique item in memory. Indeed, neural networks and semantic networks may be characterized as associative models of cognition. However, associations are often more clearly represented as an N×N matrix, where N is the number of items in memory. Thus, each cell of the matrix corresponds to the strength of the association between the row item and the column item.
Learning of associations is generally believed to be a Hebbian process; that is, whenever two items in memory are simultaneously active, the association between them grows stronger, and the more likely either item is to activate the other. See below for specific operationalizations of associative models.
In an updated version of SAM, pre-existing semantic associations are accounted for using a semantic matrix. During the experiment, semantic associations remain fixed showing the assumption that semantic associations are not significantly impacted by the episodic experience of one experiment. The two measures used to measure semantic relatedness in this model are the Latent semantic analysis (LSA) and the Word association spaces (WAS).The LSA method states that similarity between words is reflected through their co-occurrence in a local context. WAS was developed by analyzing a database of free association norms. In WAS, “words that have similar associative structures are placed in similar regions of space.”
(Adaptive Control of Thought-Rational)) theory of cognition represents declarative memory
(of which semantic memory is a part) with "chunks", which consist of a label, a set of defined relationships to other chunks (i.e., "this is a _", or "this has a _"), and any number of chunk-specific properties. Chunks, then, can be mapped as a semantic network, given that each node is a chunk with its unique properties, and each link is the chunk’s relationship to another chunk. In ACT, a chunk’s activation decreases as a function of the time since the chunk was created and increases with the number of times the chunk has been retrieved from memory. Chunks can also receive activation from Gaussian
noise
, and from their similarity to other chunks. For example, if "chicken" is used as a retrieval cue, "canary" will receive activation by virtue of its similarity to the cue (i.e., both are birds, etc.). When retrieving items from memory, ACT looks at the most active chunk in memory; if it is above threshold, it is retrieved, otherwise an "error of omission" has occurred, i.e., the item has been forgotten. There is, additionally, a retrieval latency, which varies inversely with the amount by which the activation of the retrieved chunk exceeds the retrieval threshold. This latency is used in measuring the response time of the ACT model, to compare it to human performance.
While ACT is a model of cognition in general, and not memory in particular, it nonetheless posits certain features of the structure of memory, as described above. In particular, ACT models memory as a set of related symbolic chunks which may be accessed by retrieval cues. While the model of memory employed in ACT is similar in some ways to a semantic network, the processing involved is more akin to an associative model.
from a set of discrete experiences, distributed across a number of "contexts
". Though these models differ in specifics, they generally employ an (Item × Context) matrix
where each cell represents the number of times an item in memory has occurred in a given context. Semantic information is gleaned by performing a statistical analysis of this matrix.
Many of these models bear similarity to the algorithms used in search engines (for example, see Griffiths, et al., 2007 and Anderson, 1990), though it is not yet clear whether they really use the same computational mechanisms.
(LSA). In LSA, a T × D matrix is constructed from a text corpus where T is the number of terms in the corpus and D is the number of documents (here "context" is interpreted as "document" and only words—or word phrases—are considered as items in memory). Each cell in the matrix is then transformed according to the equation:
where
is the probability that context 
is active, given that item 
has occurred (this is obtained simply by dividing the raw frequency, 
by the total of the item vector, 
). This transformation—applying the logarithm
, then dividing by the entropy
of the item over all contexts—provides for greater differentiation between items and effectively weights items by their ability to predict context, and vice versa (that is, items that appear across many contexts, like "the" or "and", will be weighted less, reflecting their lack of semantic information). A Singular Value Decomposition
(SVD) is then performed on the matrix
, which allows the number of dimensions in the matrix to be reduced, thus clustering LSA's semantic representations and providing for indirect association between items. For example, "cat" and "dog" may never appear together in the same context, so their close semantic relationship may not be well-captured by LSA's original matrix 
. However, by performing the SVD and reducing the number of dimensions in the matrix, the context vectors of "cat" and "dog"—which would be very similar—would migrate toward one another and perhaps merge, thus allowing "cat" and "dog" to act as retrieval cues for each other, even though they may never have co-occurred. The degree of semantic relatedness of items in memory is given by the cosine of the angle between the items' context vectors (ranging from 1 for perfect synonyms to 0 for no relationship). Essentially, then, two words are closely semantically related if they appear in similar types of documents.
, where 
is the distance between the two words in the frame). As in LSA
(see above), the semantic similarity between two words is given by the cosine of the angle between their vectors (dimension reduction may be performed on this matrix, as well). In HAL, then, two words are semantically related if they tend to appear with the same words. Note that this may hold true even when the words being compared never actually co-occur (i.e., "chicken" and "canary").
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.
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
. Recent evidence supports the idea that the temporal pole bilaterally is the convergence zone for unimodal semantic represenations into a multimodal representation. These regions particularly vulnerable to damage in semantic dementia
, which is characterised by a global semantic deficit.
(PFC) and the left posterior temporal
areas are other areas involved in semantic memory use. Temporal lobe
damage affecting the lateral and medial cortexes have been related to semantic impairments. Damage to different areas of the brain affect semantic memory differently.
Neuroimaging evidence suggests that left hippocampal areas show an increase in activity during semantic memory tasks. During semantic retrieval, two regions in the right middle frontal gyrus
and the area of the right inferior temporal gyrus similarly show an increase in activity. Damage to areas involved in semantic memory result in various deficits, depending on the area and type of damage. For instance, Lambon Ralph, Lowe, & Rogers (2007) found that category-specific impairments can occur where patients have different knowledge deficits for one semantic category over another, depending on location and type of damage. Category-specific impairments might indicate that knowledge may rely differentially upon sensory and motor properties encoded in separate areas (Farah and McClelland, 1991).
Category-specific impairments can involve cortical regions where living and nonliving things are represented and where feature and conceptual relationships are represented. Depending on the damage to the semantic system, one type might be favored over the other. In many cases, there is a point where one domain is better than the other (i.e. - representation of living and nonliving things over feature and conceptual relationships or vice versa)
Different diseases and disorders can affect the biological workings of semantic memory. A variety of studies have been done in an attempt to determine the effects on varying aspects of semantic memory. For example, Lambon, Lowe, & Rogers (2007) studied the different effects semantic dementia
and herpes simplex virus encephalitis have on semantic memory. They found that semantic dementia has a more generalized semantic impairment. Additionally, deficits in semantic memory as a result of herpes simplex virus encephalitis tend to have more category-specific impairments. Other disorders that affect semantic memory - such as Alzheimer's disease
- has been observed clinically as errors in naming, recognizing, or describing objects. Whereas researchers have attributed such impairment to degradation of semantic knowledge (Koenig et al. 2007).
Various neural imaging and research points to semantic memory and episodic memory
resulting from distinct areas in the brain. Still other research suggests that both semantic memory and episodic memory
are part of a singular declarative memory
system, yet represent different sectors and parts within the greater whole. Different areas within the brain are activated depending on whether semantic or episodic memory
is accessed. Certain experts are still arguing whether or not the two types of memory are from distinct systems or whether the neural imaging makes it appear that way as a result of the activation of different mental processes during retrieval.
is a semantic memory disorder that results in errors describing and naming objects, though not necessarily category-specific.Semantic dementia is another disorder associated with semantic memory. Semantic dementia
is a language
disorder characterized by a deterioration in understanding and recognizing words. Impairments include difficulty in generating familiar words, difficulty naming objects and difficulties with visual recognition. Research suggests that the temporal lobe might be responsible for category specific impairments of semantic memory disorders. In addition to category specific impairments, modality specific impairments are included in disorders of semantic memory (Cohen et al. 2002).
. Different components represent information from different sensorimotor channels. Modality specific impairments are divided into separate subsystems on the basis of input modality. Examples of different input modalities include visual, auditory and tactile input. Modality specific impairments are also divided into subsystems based on the type of information. Visual vs. verbal and perceptual vs. functional information are examples of information types. Modality specificity can account for category specific impairments in semantic memory disorders. Damage to visual semantics primarily impairs knowledge of living things, and damage to functional semantics primarily impairs knowledge of nonliving things.
methods such as positron emission tomography
(PET) and functional magnetic resonance imaging
(fMRI), which have been used to address some of the central questions about our understanding of semantic memory.
Rather than any one brain region playing a dedicated and privileged role in the representation or retrieval of all sorts of semantic knowledge, semantic memory is a collection of functionally and anatomically distinct systems, where each attribute-specific system is tied to a sensorimotor
modality
(i.e. vision) and even more specifically to a property within that modality (i.e. color
). Neuroimaging
studies also suggest a distinction between semantic processing and sensorimotor processing.
A new idea that is still at the early stages of development is that semantic memory, like perception, can be subdivided into types of visual information – color, size, form, and motion. Thompson-Schill (2003) found that the left or bilateral ventral temporal cortex appears to be involved in retrieval of knowledge of color and form, the left lateral temporal cortex in knowledge of motion, and the parietal cortex in knowledge of size.
Neuroimaging studies suggest a large, distributed network of semantic representations that are organized minimally by attribute, and perhaps additionally by category. These networks include "extensive regions of ventral (form and color knowledge) and lateral (motion knowledge) temporal cortex, parietal cortex (size knowledge), and premotor cortex
(manipulation knowledge). Other areas, such as more anterior regions of temporal cortex, may be involved in the representation of nonperceptual (e.g. verbal) conceptual knowledge, perhaps in some categorically-organized fashion."
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
of meanings, understanding
Understanding
Understanding is a psychological process related to an abstract or physical object, such as a person, situation, or message whereby one is able to think about it and use concepts to deal adequately with that object....
s, and other concept-based knowledge
Knowledge
Knowledge is a familiarity with someone or something unknown, which can include information, facts, descriptions, or skills acquired through experience or education. It can refer to the theoretical or practical understanding of a subject...
unrelated to specific experiences. The conscious recollection of factual information and general knowledge about the world is generally thought to be independent of context and personal relevance. Semantic and episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
together make up the category of declarative memory
Declarative memory
Declarative memory is one of two types of long term human memory. It refers to memories which can be consciously recalled such as facts and knowledge. Its counterpart is known as non-declarative or Procedural memory, which refers to unconscious memories such as skills...
, which is one of the two major divisions in memory. The counterpart to declarative, or explicit memory, is procedural memory
Procedural memory
Procedural memory is memory for how to do things. Procedural memory guides the processes we perform and most frequently resides below the level of conscious awareness. When needed, procedural memories are automatically retrieved and utilized for the execution of the integrated procedures involved...
, or implicit memory
Implicit memory
Implicit memory is a type of memory in which previous experiences aid in the performance of a task without conscious awareness of these previous experiences. Evidence for implicit memory arises in priming, a process whereby subjects show improved performance on tasks for which they have been...
.
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.
History
The notion of semantic memory was first introduced following a conference in 1972 between Endel TulvingEndel Tulving
Endel Tulving is an experimental psychologist and cognitive neuroscientist whose research on human memory has influenced generations of psychological scientists, neuroscientists, and clinicians...
, of the University of Toronto, 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 titled remembrances and the other memoria. The remembrance concept dealt with memories that contained the experiences of an autobiographic index, whereas the memoria’ concept dealt with those memories without the experiences of an autobiographic index. Semantic memory was to reflect our knowledge of the world around us. It holds generic information that is more than likely acquired across various contexts and is able to be used across different situations. According to Madigan in his book titled Memory, semantic memory is the sum of all knowledge you have obtained- whether it be your vocabulary, understanding of math, and all the facts you know. The use of semantic memory is quite different from that of episodic memory. Semantic memory refers to general facts and meanings we share with others whereas episodic memory refers to unique and concrete personal experiences.
Tulving's proposal of this distinction between semantic and episodic memory was widely accepted mainly because it allowed the separate conceptualization of knowledge of the world. Tulving discusses these separate systems of conceptualization of episodic and semantic memory in his book titled Elements of Episodic Memory. He states that both episodic and semantic memory differ in regards to several factors including:
- the characteristics of their operations,
- the kind of information they process, and
- their application to the real world as well as the memory laboratory.
Before this proposal by Tulving this area of human memory had been neglected by experimental psychologists. A number of experimenters have conducted tests to determine the validity of Tulving’s hypothesized distinction of episodic and semantic memory.
Kihlstrom (1980)- Experiment 1
In this study four groups of University students, varying in their levels of hypnotic susceptibilityHypnotic susceptibility
Hypnotic susceptibility measures how easily a person can be hypnotized. Several types of scales are used; however, the most common are the Harvard Group Scale of Hypnotic Susceptibility and the Stanford Hypnotic Susceptibility Scales....
, were hypnotized. While under hypnosis they learned a list of 16 common words using a multi-trial free recall method. Once the subjects were able to perfectly recall the list twice in succession they were told that after awakening they would not remember having learned any of the words while under hypnosis. However, given the signal of the experimenter not only will they remember having learned the words but they will also remember the words from the list.
During stage one of the experiment (after subjects were awakened) the number of words recalled by the subjects were used as a measure of performance for the episodic task of free recall. Most subjects remembered learning the list of words.
During the second stage the measure of semantic memory performance was assessed. Each subject was given a semantic free association test (where stimulus words were given to elicit the learned words).
As mentioned previously, the subjects represented various levels of hypnotic susceptibility as determined by their scores on the Stanford Hypnotic Susceptibility Scale. They were grouped according to their score.
The semantic free association probabilities were relatively the same across various hypnotized groups. However, the episodic free recall probabilities were significantly different across the groups. The percentage increased as the hypnotizability of subjects decreased. The subjects in the very high susceptibility group recalled almost nothing, whereas the medium and low groups recalled 86% of the learned words.
Because the free association test was not related to the hypnotic susceptibility of the subjects shows that amnesia presented after hypnosis determined the memory for the word-events that occurred in the study phase.
This study provides evidence that supports the episodic/semantic distinction hypothesized by Tulving.
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.Part one
Subjects were presented with 60 words (one at a time) 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”
Part Two
In the second phase of the experiment 60 “old words”- seen in stage one and “20 new words” not shown in stage one were presented to the subjects one at a time.
The subjects were given one of two tasks:
- Perceptual Identification task (semantic): The words were flashed on a video-screen for 35ms 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.
Results:
- The percentages correct in the Semantic task (perceptual identification) did not change with the encoding conditions of appearance, sound, or meaning.
- The percentages for the episodic task increased from the appearance condition (.50), to the sound condition (.63), to the meaning condition (.86). - The effect was also greater for the “yes” encoding words than the “no” encoding words. (see stage one)
Conclusion:
It displays a strong distinction of performance of episodic and semantic tasks, thus supporting Tulving’s hypothesis.
Models
The essence of semantic memory is that its 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 which may be said to delineate categorical and functional relationships between such objects. Thus, a complete theory of semantic memory must account not only for the representational structure of such "gists", but also for how they can be extracted from experience. Numerous models of semantic memory have been proposed; they are summarized below.Network models
NetworksNeural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
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, examples of which are found below.
Teachable Language Comprehender (TLC)
One of the first examples of a network model of semantic memory is the Teachable Language Comprehender (TLC). In this model, each node is a word, representing a concept (like "Bird"). With each node is stored a set of properties (like "can fly" or "has wings") as well as pointers (i.e., links) to other nodes (like "Chicken"). A node is directly linked to those nodes of which it is either a subclass or superclass (i.e., "Bird" would be connected to both "Chicken" and "Animal"). Thus, TLC is a hierarchical knowledge representation in that high-level nodes representing large categories are connected (directly or indirectly, via the nodes of subclasses) to many instances of those categories, whereas nodes representing specific instances are at a lower level, connected only to their superclasses. Furthermore, 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" (one level up), and "can move" would be stored with "Animal" (another level up). Nodes may also store negations of the properties of their superordinate nodes (i.e., "NOT-can fly" would be stored with "penguin"). This provides an economy of representation in that properties are only stored at the category level at which they become essential, that is, at which point they become critical features (see below).Processing in TLC is a form of spreading activation
Spreading activation
Spreading activation is a method for searching associative networks, neural networks, or semantic networks. The search process is initiated by labeling a set of source nodes with weights or "activation" and then iteratively propagating or "spreading" that activation out to other nodes linked to...
. That is, 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, i.e., the number of links between the nodes "Chicken" and "Bird".
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. Collins
Allan M. Collins
Allan M. Collins is an American cognitive scientist and Professor Emeritus of Learning Sciences at Northwestern University's School of Education and Social Policy...
and Quillian later updated TLC to include weighted connections to account for this effect. This updated TLC is capable of explaining both the familiarity effect and the typicality effect. Its biggest advantage is that it clearly explains priming
Priming (psychology)
Priming is an implicit memory effect in which exposure to a stimulus influences a response to a later stimulus. It can occur following perceptual, semantic, or conceptual stimulus repetition...
: you are more likely to retrieve information from memory if related information (the "prime") 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 (like "is a chicken a meteor?"), when the relevant nodes are very far apart in the network.
Semantic networks
TLC is an instance of a more general class of models known as semantic networks. In a semantic network, each node is to be interpreted as representing a specific concept, word, or feature. That is, each node is a symbol. Semantic networks generally do not employ distributed representations for concepts, as may be found in a neural networkNeural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
. The defining feature of a semantic network is that its links are almost always directed (that is, they only point in one direction, from a base to a target) and the links come in many different types, each one standing for a particular relationship that can hold between any two nodes. Processing in a semantic network often takes the form of spreading activation (see above).
Semantic networks see the most use in models of discourse
Discourse analysis
Discourse analysis , or discourse studies, is a general term for a number of approaches to analyzing written, spoken, signed language use or any significant semiotic event....
and logic
Logic
In philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...
al comprehension
Comprehension (logic)
In logic, the comprehension of an object is the totality of intensions, that is, attributes, characters, marks, properties, or qualities, that the object possesses, or else the totality of intensions that are pertinent to the context of a given discussion...
, as well as in Artificial Intelligence
Artificial intelligence
Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...
. In these models, the nodes correspond to words or word stems and the links represent syntactic relations between them. For an example of a computational implementation of semantic networks in knowledge representation, see Cravo and Martins (1993).
Feature models
Feature models view semantic categories as being composed of relatively unstructured sets of features. The semantic feature-comparison modelSemantic feature-comparison model
The feature-comparison model is a psychological theory of semantic memory hypothesized by Smith, Shoben and Rips . The feature-Comparison model was purposed in contrast to the semantic network approach that was purposed by Collins and Quillian...
, proposed by Smith, Shoben, and Rips (1974), describes memory as being composed of feature lists for different concepts. According to this view, the relations between categories would not be directly retrieved, they would be indirectly computed. For example, subjects might verify a sentence by comparing the feature sets that represent its subject and predicate concepts. Such computational feature-comparison models include the ones proposed by Meyer (1970), Rips (1975), Smith, et al. (1974).
Early work in perceptual and conceptual categorization assumed that categories had critical features and that category membership could be determined by logical rules for the combination of features. More recent theories have accepted that categories may have an ill-defined or "fuzzy" structure and have proposed probabilistic or global similarity models for the verification of category membership.
Associative models
The "associationAssociation (psychology)
In psychology and marketing, two concepts or stimuli are associated when the experience of one leads to the effects of another, due to repeated pairing. This is sometimes called Pavlovian association for Ivan Pavlov's pioneering of classical conditioning....
"—a relationship between two pieces of information—is a fundamental concept in psychology, and associations at various levels of mental representation are essential to models of memory and cognition in general. The set of associations among a collection of items in memory is equivalent to the links between nodes in a network, where each node corresponds to a unique item in memory. Indeed, neural networks and semantic networks may be characterized as associative models of cognition. However, associations are often more clearly represented as an N×N matrix, where N is the number of items in memory. Thus, each cell of the matrix corresponds to the strength of the association between the row item and the column item.
Learning of associations is generally believed to be a Hebbian process; that is, whenever two items in memory are simultaneously active, the association between them grows stronger, and the more likely either item is to activate the other. See below for specific operationalizations of associative models.
Search of Associative Memory (SAM)
A standard model of memory that employs association in this manner is the Search of Associative Memory (SAM) model. Though SAM was originally designed to model episodic memory, its mechanisms are sufficient to support some semantic memory representations, as well. The SAM model contains a short- term store (STS) and long a term store (LTS), where STS is a briefly activated subset of the information in the LTS. The STS has limited capacity and affects the retrieval process by limiting the amount of information that can be sampled and limiting the time the sampled subset is in an active mode. The retrieval process in LTS is cue dependent and probabilistic, meaning that a cue initiates the retrieval process and the selected information from memory is random. The probability of being sampled is dependent on the strength of association between the cue and the item being retrieved, with stronger associations being sampled and finally one is chosen. The buffer size is defined as r, and not a fixed number, and as items are rehearsed in the buffer the associative strengths grow linearly as a function of the total time inside the buffer. In SAM, when any two items simultaneously occupy a working memory buffer, the strength of their association is incremented. Thus, items that co-occur more often are more strongly associated. Items in SAM are also associated with a specific context, where the strength of that association determined by how long each item is present in a given context. In SAM, then, memories consist of a set of associations between items in memory and between items and contexts. The presence of a set of items and/or a context is more likely to evoke, then, some subset of the items in memory. The degree to which items evoke one another—either by virtue of their shared context or their co-occurrence—is an indication of the items’ semantic relatedness.In an updated version of SAM, pre-existing semantic associations are accounted for using a semantic matrix. During the experiment, semantic associations remain fixed showing the assumption that semantic associations are not significantly impacted by the episodic experience of one experiment. The two measures used to measure semantic relatedness in this model are the Latent semantic analysis (LSA) and the Word association spaces (WAS).The LSA method states that similarity between words is reflected through their co-occurrence in a local context. WAS was developed by analyzing a database of free association norms. In WAS, “words that have similar associative structures are placed in similar regions of space.”
ACT-R: a production system model
The ACT (Adaptive Control of Thought) (and later ACT-RACT-R
ACT-R is a cognitive architecture mainly developed by John Robert Anderson at Carnegie Mellon University. Like any cognitive architecture, ACT-R aims to define the basic and irreducible cognitive and perceptual operations that enable the human mind....
(Adaptive Control of Thought-Rational)) theory of cognition represents declarative memory
Declarative memory
Declarative memory is one of two types of long term human memory. It refers to memories which can be consciously recalled such as facts and knowledge. Its counterpart is known as non-declarative or Procedural memory, which refers to unconscious memories such as skills...
(of which semantic memory is a part) with "chunks", which consist of a label, a set of defined relationships to other chunks (i.e., "this is a _", or "this has a _"), and any number of chunk-specific properties. Chunks, then, can be mapped as a semantic network, given that each node is a chunk with its unique properties, and each link is the chunk’s relationship to another chunk. In ACT, a chunk’s activation decreases as a function of the time since the chunk was created and increases with the number of times the chunk has been retrieved from memory. Chunks can also receive activation from Gaussian
GAUSSIAN
Gaussian is a computational chemistry software program initially released in 1970 by John Pople and his research group at Carnegie-Mellon University as Gaussian 70. It has been continuously updated since then...
noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
, and from their similarity to other chunks. For example, if "chicken" is used as a retrieval cue, "canary" will receive activation by virtue of its similarity to the cue (i.e., both are birds, etc.). When retrieving items from memory, ACT looks at the most active chunk in memory; if it is above threshold, it is retrieved, otherwise an "error of omission" has occurred, i.e., the item has been forgotten. There is, additionally, a retrieval latency, which varies inversely with the amount by which the activation of the retrieved chunk exceeds the retrieval threshold. This latency is used in measuring the response time of the ACT model, to compare it to human performance.
While ACT is a model of cognition in general, and not memory in particular, it nonetheless posits certain features of the structure of memory, as described above. In particular, ACT models memory as a set of related symbolic chunks which may be accessed by retrieval cues. While the model of memory employed in ACT is similar in some ways to a semantic network, the processing involved is more akin to an associative model.
Statistical models
Some models characterize the acquisition of semantic information as a form of statistical inferenceStatistical inference
In statistics, statistical inference is the process of drawing conclusions from data that are subject to random variation, for example, observational errors or sampling variation...
from a set of discrete experiences, distributed across a number of "contexts
Context (language use)
Context is a notion used in the language sciences in two different ways, namely as* verbal context* social context- Verbal context :...
". Though these models differ in specifics, they generally employ an (Item × Context) matrix
Matrix (mathematics)
In mathematics, a matrix is a rectangular array of numbers, symbols, or expressions. The individual items in a matrix are called its elements or entries. An example of a matrix with six elements isMatrices of the same size can be added or subtracted element by element...
where each cell represents the number of times an item in memory has occurred in a given context. Semantic information is gleaned by performing a statistical analysis of this matrix.
Many of these models bear similarity to the algorithms used in search engines (for example, see Griffiths, et al., 2007 and Anderson, 1990), though it is not yet clear whether they really use the same computational mechanisms.
Latent Semantic Analysis (LSA)
Perhaps the most popular of these models is Latent Semantic AnalysisLatent semantic analysis
Latent semantic analysis is a technique in natural language processing, in particular in vectorial semantics, of analyzing relationships between a set of documents and the terms they contain by producing a set of concepts related to the documents and terms. LSA assumes that words that are close...
(LSA). In LSA, a T × D matrix is constructed from a text corpus where T is the number of terms in the corpus and D is the number of documents (here "context" is interpreted as "document" and only words—or word phrases—are considered as items in memory). Each cell in the matrix is then transformed according to the equation:
where
is the probability that context is active, given that item has occurred (this is obtained simply by dividing the raw frequency, by the total of the item vector, ). This transformation—applying the logarithmLogarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
, then dividing by the entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
of the item over all contexts—provides for greater differentiation between items and effectively weights items by their ability to predict context, and vice versa (that is, items that appear across many contexts, like "the" or "and", will be weighted less, reflecting their lack of semantic information). A Singular Value Decomposition
Singular value decomposition
In linear algebra, the singular value decomposition is a factorization of a real or complex matrix, with many useful applications in signal processing and statistics....
(SVD) is then performed on the matrix
, which allows the number of dimensions in the matrix to be reduced, thus clustering LSA's semantic representations and providing for indirect association between items. For example, "cat" and "dog" may never appear together in the same context, so their close semantic relationship may not be well-captured by LSA's original matrix . However, by performing the SVD and reducing the number of dimensions in the matrix, the context vectors of "cat" and "dog"—which would be very similar—would migrate toward one another and perhaps merge, thus allowing "cat" and "dog" to act as retrieval cues for each other, even though they may never have co-occurred. The degree of semantic relatedness of items in memory is given by the cosine of the angle between the items' context vectors (ranging from 1 for perfect synonyms to 0 for no relationship). Essentially, then, two words are closely semantically related if they appear in similar types of documents.Hyperspace Analogue to Language (HAL)
The Hyperspace Analogue to Language (HAL) model considers context only as the words that immediately surround a given word. HAL computes an NxN matrix, where N is the number of words in its lexicon, using a 10-word reading frame that moves incrementally through a corpus of text. Like in SAM (see above), any time two words are simultaneously in the frame, the association between them is increased, that is, the corresponding cell in the NxN matrix is incremented. The amount by which the association is incremented varies inversely with the distance between the two words in the frame (specifically,, where is the distance between the two words in the frame). As in LSALatent semantic analysis
Latent semantic analysis is a technique in natural language processing, in particular in vectorial semantics, of analyzing relationships between a set of documents and the terms they contain by producing a set of concepts related to the documents and terms. LSA assumes that words that are close...
(see above), the semantic similarity between two words is given by the cosine of the angle between their vectors (dimension reduction may be performed on this matrix, as well). In HAL, then, two words are semantically related if they tend to appear with the same words. Note that this may hold true even when the words being compared never actually co-occur (i.e., "chicken" and "canary").
Other Statistical Models of Semantic Memory
The success of LSA and HAL gave birth to a whole field of statistical models of language. A more up-to-date list of such models may be found under the topic Measures of semantic relatedness.Location of semantic memory in the brain
The cognitive neuroscienceCognitive neuroscience
Cognitive neuroscience is an academic field concerned with the scientific study of biological substrates underlying cognition, with a specific focus on the neural substrates of mental processes. It addresses the questions of how psychological/cognitive functions are produced by the brain...
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
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
systems involved in episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
. These include the medial temporal lobes (MTL) and hippocampal formation
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
. 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
Entorhinal cortex
The entorhinal cortex is located in the medial temporal lobe and functions as a hub in a widespread network for memory and navigation. The EC is the main interface between the hippocampus and neocortex...
, 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.
Other researchers believe the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
is only involved in episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
and spatial cognition
Cognition
In science, cognition refers to mental processes. These processes include attention, remembering, producing and understanding language, solving problems, and making decisions. Cognition is studied in various disciplines such as psychology, philosophy, linguistics, and computer science...
. This then raises the question where semantic memory may be located. Some believe semantic memory lives in temporal neocortex
Neocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...
. 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
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
. Recent evidence supports the idea that the temporal pole bilaterally is the convergence zone for unimodal semantic represenations into a multimodal representation. These regions particularly vulnerable to damage in semantic dementia
Semantic dementia
Semantic dementia is a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains...
, which is characterised by a global semantic deficit.
Neural correlates and biological workings
The hippocampal areas are important to semantic memory's involvement with declarative memory. The left inferior prefrontal cortexPrefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...
(PFC) and the left posterior temporal
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
areas are other areas involved in semantic memory use. Temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
damage affecting the lateral and medial cortexes have been related to semantic impairments. Damage to different areas of the brain affect semantic memory differently.
Neuroimaging evidence suggests that left hippocampal areas show an increase in activity during semantic memory tasks. During semantic retrieval, two regions in the right middle frontal gyrus
Middle frontal gyrus
The middle frontal gyrus makes up about one-third of the frontal lobe of the human brain....
and the area of the right inferior temporal gyrus similarly show an increase in activity. Damage to areas involved in semantic memory result in various deficits, depending on the area and type of damage. For instance, Lambon Ralph, Lowe, & Rogers (2007) found that category-specific impairments can occur where patients have different knowledge deficits for one semantic category over another, depending on location and type of damage. Category-specific impairments might indicate that knowledge may rely differentially upon sensory and motor properties encoded in separate areas (Farah and McClelland, 1991).
Category-specific impairments can involve cortical regions where living and nonliving things are represented and where feature and conceptual relationships are represented. Depending on the damage to the semantic system, one type might be favored over the other. In many cases, there is a point where one domain is better than the other (i.e. - representation of living and nonliving things over feature and conceptual relationships or vice versa)
Different diseases and disorders can affect the biological workings of semantic memory. A variety of studies have been done in an attempt to determine the effects on varying aspects of semantic memory. For example, Lambon, Lowe, & Rogers (2007) studied the different effects semantic dementia
Semantic dementia
Semantic dementia is a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains...
and herpes simplex virus encephalitis have on semantic memory. They found that semantic dementia has a more generalized semantic impairment. Additionally, deficits in semantic memory as a result of herpes simplex virus encephalitis tend to have more category-specific impairments. Other disorders that affect semantic memory - such as Alzheimer's disease
Alzheimer's disease
Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...
- has been observed clinically as errors in naming, recognizing, or describing objects. Whereas researchers have attributed such impairment to degradation of semantic knowledge (Koenig et al. 2007).
Various neural imaging and research points to semantic memory and episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
resulting from distinct areas in the brain. Still other research suggests that both semantic memory and episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
are part of a singular declarative memory
Declarative memory
Declarative memory is one of two types of long term human memory. It refers to memories which can be consciously recalled such as facts and knowledge. Its counterpart is known as non-declarative or Procedural memory, which refers to unconscious memories such as skills...
system, yet represent different sectors and parts within the greater whole. Different areas within the brain are activated depending on whether semantic or episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
is accessed. Certain experts are still arguing whether or not the two types of memory are from distinct systems or whether the neural imaging makes it appear that way as a result of the activation of different mental processes during retrieval.
Disorders
In order to understand semantic memory disorders, one must first understand how these disorders affect memory. Semantic memory disorders fractionate into two categories. Semantic category specific impairments and modality specific impairments are apparent in disorders of semantic memory. Understanding these types of impairments will give insight into how disorders of semantic memory function.Semantic category specific impairments
Category specific impairments can result in widespread, patchy damage or localized damage. Category specific impairments can be broken down into four categories. Perceptual and functional features, topographic organization, informativeness and intercorrelations are areas of decreased functioning in disorders of semantic memory (Warrington and Shallice, 1984). Alzheimer's diseaseAlzheimer's disease
Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...
is a semantic memory disorder that results in errors describing and naming objects, though not necessarily category-specific.Semantic dementia is another disorder associated with semantic memory. Semantic dementia
Semantic dementia
Semantic dementia is a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains...
is a language
Language
Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...
disorder characterized by a deterioration in understanding and recognizing words. Impairments include difficulty in generating familiar words, difficulty naming objects and difficulties with visual recognition. Research suggests that the temporal lobe might be responsible for category specific impairments of semantic memory disorders. In addition to category specific impairments, modality specific impairments are included in disorders of semantic memory (Cohen et al. 2002).
Modality specific impairments
Semantic memory is also discussed in reference to modalityStimulus modality
Stimulus modality also sensory modality is one aspect of a stimulus or what we percieve after a stimulus. For example the temperature modality is registered after heat or cold stimulate a receptor. There are many modalities: temperature, taste, pressure...
. Different components represent information from different sensorimotor channels. Modality specific impairments are divided into separate subsystems on the basis of input modality. Examples of different input modalities include visual, auditory and tactile input. Modality specific impairments are also divided into subsystems based on the type of information. Visual vs. verbal and perceptual vs. functional information are examples of information types. Modality specificity can account for category specific impairments in semantic memory disorders. Damage to visual semantics primarily impairs knowledge of living things, and damage to functional semantics primarily impairs knowledge of nonliving things.
Semantic refractory access and semantic storage disorders
Semantic memory disorders fall into two groups. Semantic refractory access disorders are contrasted with semantic storage disorders according to four factors. Temporal factors, response consistency, frequency and semantic relatedness are the four factors used to differentiate between semantic refractory access and semantic storage disorders. A key feature of semantic refractory access disorders is temporal distortions. Decreases in response time to certain stimuli are noted when compared to natural response times. Response consistency is the next factor. In access disorders you see inconsistencies in comprehending and responding to stimuli that have been presented many times. Temporal factors impact response consistency. In storage disorders, you do not see an inconsistent response to specific items like you do in refractory access disorders. Stimulus frequency determines performance at all stages of cognition. Extreme word frequency effects are common in semantic storage disorders while in semantic refractory access disorders word frequency effects are minimal. The comparison of 'close' and 'distant' groups tests semantic relatedness. 'Close' groupings have words that are related because they are drawn from the same category. For example, a listing of clothing types would be a 'close' grouping. 'Distant' groupings contain words with broad categorical differences. Non-related words would fall into this group. Comparing close and distant groups shows that in access disorders semantic relatedness had a negative effect. This is not observed in semantic storage disorders. Category specific and modality specific impairments are important components in access and storage disorders of semantic memory.Present and future research
Semantic memory has had a comeback in interest in the past 15 years, due in part to the development of functional neuroimagingNeuroimaging
Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...
methods such as positron emission tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
(PET) and functional magnetic resonance imaging
Functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...
(fMRI), which have been used to address some of the central questions about our understanding of semantic memory.
Rather than any one brain region playing a dedicated and privileged role in the representation or retrieval of all sorts of semantic knowledge, semantic memory is a collection of functionally and anatomically distinct systems, where each attribute-specific system is tied to a sensorimotor
Sensory-motor coupling
Sensory-motor coupling, or sensorimotor coupling, is the integration of the sensory and motor faculties of the brain.Examples of sensory-motor coupling include the feeling of being "in the groove" when listening to music and the adjustment of body posture based on visual information.- See also :*...
modality
Stimulus modality
Stimulus modality also sensory modality is one aspect of a stimulus or what we percieve after a stimulus. For example the temperature modality is registered after heat or cold stimulate a receptor. There are many modalities: temperature, taste, pressure...
(i.e. vision) and even more specifically to a property within that modality (i.e. color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
). Neuroimaging
Neuroimaging
Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...
studies also suggest a distinction between semantic processing and sensorimotor processing.
A new idea that is still at the early stages of development is that semantic memory, like perception, can be subdivided into types of visual information – color, size, form, and motion. Thompson-Schill (2003) found that the left or bilateral ventral temporal cortex appears to be involved in retrieval of knowledge of color and form, the left lateral temporal cortex in knowledge of motion, and the parietal cortex in knowledge of size.
Neuroimaging studies suggest a large, distributed network of semantic representations that are organized minimally by attribute, and perhaps additionally by category. These networks include "extensive regions of ventral (form and color knowledge) and lateral (motion knowledge) temporal cortex, parietal cortex (size knowledge), and premotor cortex
Premotor cortex
The premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for...
(manipulation knowledge). Other areas, such as more anterior regions of temporal cortex, may be involved in the representation of nonperceptual (e.g. verbal) conceptual knowledge, perhaps in some categorically-organized fashion."
External links
- http://www.mtsu.edu/~sschmidt/Cognitive/semantic/semantic.html#VI.%20Conclusions%20on%20Semantic%20Memory
- http://www.newscientist.com/article.ns?id=dn10012&feedId=brain_rss20
- http://diodor.eti.pg.gda.pl An application of computational semantic memory model. Plays 20 questions game on animals domain
- S-Space Package, an open source Java library that includes several semantic memory implementations,such as PEN and IS for generating Statistical semanticsStatistical semanticsStatistical semantics is the study of "how the statistical patterns of human word usage can be used to figure out what people mean, at least to a level sufficient for information access"...
from a text corpus