Semantic memory comprises all our knowledge and understanding about the meaning of words, objects, people, concepts, and events (for reviews, see Caramazza & Mahon, 2006;; Patterson, Nestor, & Rogers, 2007;; Rogers, Lambon Ralph, Garrard, Bozeat, McClelland, Hodges, 2004). In particular, semantic memory, together with episodic memory, constitutes declarative memory-that is, memories that we can declare or describe explicitly. However, in contrast to episodic memory, semantic memory represents knowledge of objects or events that transcend particular places, times, or contexts. Episodic memory, however, represents memories of specific events and experiences (Tulving, 1972).
Collins and Quillian (1969) proposed that semantic knowledge is underpinned by a set of nodes, each representing a specific feature or concept, which are all connected to one another. Nodes that related in some way, such as often coincident in time, are more strongly connected.
For example, in the model developed by Collins and Quillian (1969), each node represents a specific word, such as "bird". Each node is stored together with a set of properties, such as "has wings" or "can fly. Furthermore, in this model, connections link categories to exemplars, representing a hierarchical arrangement. For example, "bird" is connected to "chicken". Features, such as "can fly" are stored only at the category level, such as "bird", in which they represent key properties.
To retrieve this knowledge, some cue or stimulus activates one set of nodes, which then activate other related nodes, called spreading activation. To illustrate, in response to the question "Is a chicken a bird", the time to answer this question depends on the number of connections that intervene between the node that represents chicken and the node that represents "bird".
Collins and Loftus (1975) then refined this model. They weighted the connections to explain the typicality effect-the finding that typical instantiations of a category are recognized more rapidly. Nevertheless, this model cannot explain a finding, observed by Glass, Holyoak, and Kiger (1979), that individuals can readily respond to questions that are patently false, like "Is a chicken a meteor". In this instance, the nodes are far apart, but the responses are rapid.
Later, more sophisticated network models were developed (for an example, see Cravo & Martins, 1993). These models are similar to the propositions proposed by Collins and Loftus (1975). However, each node might represent some other element, like a concept or feature, rather than merely a word. Furthermore, the links or connections can represent a variety of relationships, not just hierarchy.
According to feature models, such as the propositions developed by Smith, Shoben, and Rips (1974), semantic memory is assumed to comprise lists of features for each concept. Hence, according to the perspective, the connections between concepts do not represent the relationships between concepts. Instead, to uncover these relationships, individuals must compare the sets of features between two concepts (see also Meyer, 1970;; Rips, 1975).
Raaijmakers and Schiffrin (1981) proposed one of the seminal examples of an associative model to characterize episodic and semantic memory, called the Search of Associative Memory model. According to this model, when two items occupy working memory concurrently, the strength of their association increases. Over time, items, such as stimuli or features, that often coincide are connected by strong links.
Some scholars assume the semantic knowledge is represented as a single system (see Caramazza, Hillis, Rapp, & Romani, 1990;; Lambon Ralph, Patterson, & Hodges, 1997). In contrast, other researchers argue that semantic knowledge might be represented in multiple systems. Each system, for example, might correspond to one modality. Alternatively, each system might correspond to one category of concepts (see Farah, Hammond, Mehta, & Ratcliff, 1989;; Humphreys & Forde, 2001;; McCarthy & Warrington, 1988).
The proposition that semantic memory might constitute multiple systems emerged from case studies, in which individuals demonstrated selective impairments in specific modalities or categories (Warrington & Shallice, 1984;; see also Lambon Ralph, Patterson, & Hodges, 1997). That is, as a consequence of lesions, often following stroke or herpes simplex encephalitis, some participants demonstrated impairments in only one modality. They might, for example, not be able to describe an object that is presented visually. Nevertheless, they might be able to describe this object if the stimulus was defined verbally. Alternatively, participants demonstrated impairments in a subset of categories. They could, for example, describe living organisms but not inanimate objects or vice versa (Lambon Ralph, Lowe, & Rogers, 2007).
Nevertheless, some scholars can reconcile these findings, and other observations, to the perspective that semantic memory corresponds to one system (Caramazza, Hillis, Rapp, & Romani, 1990;; Caramazza & Mahon, 2006). According to proponents of this perspective, semantic knowledge is represented as a single, central system, whereas perceptual knowledge is represented in stores that are specific to one modality (e.g., Riddoch, Humphreys, Coltheart, & Funnell, 1988).
Indeed, some research findings challenge the proposition that semantic memory might constitute multiple systems. In patients with Alzheimer's Disease, for example, semantic deficits in one modality tend to coincide with similar semantic deficits in other modalities (Hodges, Patterson, Graham, & Dawson, 1996).
Some scholars assume that perceptual and semantic representations of objects correspond to separate systems (e.g., Biederman, 1987;; Riddoch, Humphreys, Coltheart, & Funnel, 1988). According to these conceptualizations, one system represents only the perceptual features of objects, such as their shape, color, weight, and intensity. This system is used to identify the object-that is, to ascertain which features of a stimulus match one of the objects in this store.
When the stimulus is recognized, a separate representation of this object in semantic memory is activated. This representation comprises all the semantic features of this object, such as its utility, consequences, problems, and so forth,
Other scholars, however, argue the perceptual and semantic representations of objects are represented in the same system (e.g., Rogers, Hodges, Lambon Ralph, & Patterson, 2003). According to this conceptualization, the physical representations of objects in different modalities-such as the appearance, sound, smell, or size of these stimuli-and the interactions and associations across these features integrate to generate semantic knowledge. In other words, semantic knowledge is derived from the relationships between perceptual features.
Tulving (1972, 1985) argued that semantic memory is merely a specialized form-a subset-of procedural memory, which stores skills and sequences of actions. Furthermore, according to Tulving (1972, 1985), episodic memory represents a subset, or specialized form, of semantic memory. Consistent with this proposition, several studies, such as research conducted by Dalla Barba and Goldblum (1996) and Goldblum, Gomez, Dalla Barba, Boller, Deweer, Hahn et al. (1998) for example, imply that episodic memory depends on the integrity of semantic memory.
Nevertheless, some studies have uncovered some findings that contradict this perspective. Dudas, Clague, Thompson, Graham, and Hodges (2005), for example, conducted a study with older participants who showed mild cognitive impairment, which is often regarded as a potential precursor of Alzheimer's Disease. These participants completed a task that involved recognizing an object, representing episodic memory, and naming this object, representing semantic memory. Performance on these two facets of the task were uncorrelated with one another, challenging the perspective that deficits in semantic memory should degrade episodic memory. Furthermore, patients with semantic dementia do show deficits in semantic, but not episodic, memory-at least during the early stages of this disease (Graham, Simons, Pratt, Patterson, & Hodges, 2000;; Hodges & Patterson, 2007).
In the olfactory modality, however, semantic memory seems to be related to episodic memory. For example, the semantic encoding of odors facilitates subsequent recognition of these smells (Jehl, Royet, & Holley, 1997;; Lehrner, Gluck, & Laska, 1999).
Some studies, often conducted with patients who exhibit various forms of dementia, indicate that semantic memory is underpinned by circuits primarily located in the right, rather than left hemisphere. Giffard, Laisney, Mezenge, de la Sayette, Eustache, and Desgranges (2008), for example, conducted an FDG-PET study. They discovered that hypometabolism in the superior temporal lobe, especially in the right hemisphere, related to performance on a semantic priming task.
In contrast, some studies imply that semantic memory is primarily underpinned by circuits primarily located in the left hemisphere. Zahn, Juengling, Bubrowski, Jost and Dykierek et al. (2004), for example, also conducted an FDG-PET study. They showed that hypometabolism in the left anterior temporal, posterior inferior temporal, inferior parietal and medial occipital lobes were related to performance-in both verbal and nonverbal semantic tasks.
In addition to hemisphere, whether semantic memory is primarily represented in posterior or anterior regions is unclear. Some studies show that hypometabolism in posterior association regions, including the temporoparietal and temporo-occipital areas, is associated with deficits on semantic tasks (Meguro, LeMestric, Landeau, Desgranges, Eustache, & Baron, 2001;; Mosconi, Pupi, De Cristofaro, Fayyaz, Sorbi, & Herholz, 2004). Research on patients with semantic dementia, for example, indicate that both anterior temporal lobes, including the perirhinal cortex, are involved in semantic processing (Chan Fox, Scahill, Crum, Whitwell, Leschziner, 2001;; Davies, Graham, Xuereb, Williams, & Hodges, 2004;; Patterson, Nestor, & Rogers 2007),
According to Patterson, Nestor and Rogers (2007), the anterior temporal lobes might function as a nucleus in the distributed semantic network. That is, semantic knowledge might be distributed across cortical association areas-some of which assimilate information from multiple modalities. The anterior temporal lobes, as corroborated by their connections with all other sensory systems, might integrate this information.
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Last Update: 6/19/2016