In 1972 the cognitive scientist Endel Tulving (b. 1927) argued that conscious recollection (i.e., declarative memory) is composed of two separate memory domains, each having distinct functionality, knowledge access, and neurological localization. Whereas episodic memory involves our conscious recall of past events within some specific context, semantic memory involves our recall of factual knowledge that is not context-dependent.
Psychologists have assessed semantic memory through a large array of tests. For example, in recalling a previously studied list of words, participants often recall groups of words in sequence (i.e., cluster) on the basis of thematic or taxonomic relations, even though the words were ordered randomly at presentation. Measures of the time required to make various forms of judgments also reflect semantic memory structure. Variation in the time required to name visually presented words, or to discriminate words from nonwords (lexical decision), reflects access to semantic information about words. Similarly, the time required to make category judgments, such as whether a robin is a bird, can be used to infer some properties of semantic memory structure.
Analyses of lexical access and category judgment tasks have led to the development of network models, which describe how the components of semantic information (e.g., concepts) are organized within semantic memory. In a hierarchical network model, for example, access to each lower-level item depends on the higher-order category (e.g., robin–bird). In contrast, other models propose that semantic memory is not organized hierarchically, though information is related to varying degrees. In these latter models, memory retrieval occurs in parallel, as a result of the activation of related information within the network. Recently, connectionist models have furthered this notion and have depicted semantic memory networks as highly distributed yet unitary systems whose multiple elements work largely in parallel.
The network and connectionist models are based on accounts of behavioral data and mathematical simulation. The question of how semantic and episodic memory abilities are represented in the brain is currently controversial. Studies of neurological patients suggest that damage to medial temporal regions of the brain can impair episodic memory while leaving semantic memory largely intact. Neuroimaging studies of healthy individuals also indicate some degree of differentiation in the pattern of brain activation occurring during episodic and semantic memory tasks, but there is also a substantial degree of overlap. Though details vary according to task demands, neuroimaging research suggests that left prefrontal regions, medial temporal regions, ventral visual processing areas, the hippocampal formation, and parts of parietal cortex can all play a role in semantic memory processing. Accessing semantic information, however, can also be a form of episodic encoding. That is, the best way to prepare for a later episodic test is to encode the meaning of the information to be remembered (as opposed to its physical structure). Typically, the left prefrontal region of the brain is implicated in both semantic retrieval and episodic encoding, and the right prefrontal region is implicated in episodic memory retrieval. This complex network of cortical involvement reflects the growing recognition among researchers that semantic memory comprises several component processes.
SEE ALSO Neuroeconomics
Chang, Tien Ming. 1986. Semantic Memory: Facts and Models. Psychological Bulletin 99 (2): 199–220.
Martin, Alex, and Linda L. Chao. 2001. Semantic Memory and the Brain: Structure and Processes. Current Opinion in Neurobiology 11 (2): 194–201.
Matthew C. Costello
David J. Madden
[Semantic memory is our acquired knowledge about the everyday world. The following two sections characterize our current understanding about semantic memory from the cognitive and the neurbiological perspectives. In both sections, two common themes in research on semantic memory are discussed: whether semantic memory is fundamentally distinct from memory for personal experiences (episodic memory) and how to characterize the organization of semantic knowledge in terms of its cognitive dimensions and its representation in the brain.
Semantic memory is often distinguished from episodic memory as what one "knows" independent of what one "remembers," that is, consciously recollects learning in a particular personal episode. Endel Tulving, who introduced the distinction between semantic and episodic memory, has argued that semantic memory is a distinct memory system, independent from that of episodic memory. Yet, all of our conscious learning experiences are initially a part of episodic memory, consistent with a common alternative view that semantic memory is knowledge abstracted from the information common to many episodes. One central question in research on semantic memory is whether and how episodic memory and semantic memory are mediated by different neural structures or brain systems. Several lines of evidence suggest semantic memory is mediated by cerebral cortical areas without involvement of the hippocampus, whereas episodic memory relies critically on hippocampal function.
As yet we have only a preliminary understanding of the cognitive dimensions of semantic memory.C ognitiveE ffectsoutlines research suggesting three principal cognitive dimensions: categorization of objects by their features, judgments about relationships between concepts, and the capacity to combine multiple elementary concepts into distinct and more complex concepts.
It is generally viewed that semantic memory is mediated by the cerebral cortex, as best demonstrated in cases of semantic dementia, a selective loss of semantic memory, consequent to cortical deterioration. However, there is considerable controversy over whether separate areas of the cerebral cortex process specific domains of semantic memory, and about the nature of the functional distinctions. There have now been several observations of deficits in specific domains of semantic knowledge following damage to particular areas of the cerebral cortex, as well as observations of activation of particular cortical areas during naming in specific domains of objects. It is not clear whether these selectivities reflect specificcategories of semantic knowledge or distinct types of cognitive or perceptual processing associated with particular categories of information.N eurobiologicalP erspectiveoutlines this issue.]
The part of long-term memory dealing with words, their symbols, and meanings.
Semantic memory allows humans to communicate with language. In semantic memory, the brain stores information about words, what they look like and represent, and how they are used in an organized way. It is unusual for a person to forget the meaning of the word "dictionary," or to be unable to conjure up a visual image of a refrigerator when the word is heard or read. Semantic memory contrasts with episodic memory, where memories are dependent upon a relationship in time. An example of an episodic memory is "I played in a piano recital at the end of my senior year in high school."
The "tip of the tongue" phenomenon provides some insight into the way information is stored in semantic memory. Most people have experienced this situation where they are trying to recall a person's name. As the person searches through his or her memory for the name Stern, for example, he or she will recall other similar names—Stone, Stein—but not Douglas or Zimmer. Semantic memory appears to categorize information that has similar meaning (in this case, surnames), that begins with the same letter, and has the same number of syllables.
Words and other memories that are stored in semantic memory contribute to episodic memory and the two work together to function as an effective long-term memory system.
Bolles, Edmund Blair. Remembering and Forgetting: Inquiries Into the Nature of Memory. New York: Walker and Co.,1988.