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Semantic Networks

Lecture 13 & McClelland Reading

QuestionAnswer
semantic tasks are those that require a subject to produce/verify semantic info about an object, a depiction of an object, or a set of objects indicated verbally by a word
semantic info is info that has not previously been associated with the particular stimulus object itself & which is not available more-or-less directly from perceptual input provided by the object/object description
semantic task performance is thought to depend on a mediating process of categorization where there exists a representation in memory corresponding to each of many concepts/categories & info about the concepts is either stored in the representations or is only accessible from it
within model performance on semantic tasks depends on access to relevant category representations
categorization based approaches lie implicitly/explicitly at the base of theorizing about semantic knowledge & its use in semantic tasks
the 3 constructs that are frequently invoked in categorization based theories to explain empirical data are hierarchical structure, privilege categories, category prototypes
hierarchical structure.. directs the sharing of info across related concepts at different levels of abstraction
privileged categories contain info that is accessed directly & not by means of spreading activation in the processing hierarchy
category prototypes are the means of computing similarity between individual instances & stored category representations
the 3 constructs used in categorization based theories offer an incomplete & in some ways paradoxical basis for accounting for relevant empirical phenomena
for many natural domains there exists constraints on category membership such that exemplars of one category must also be members of another & these class inclusion constraints can be described by a taxonomic hierarchy
Quillian pointed out that a taxonomic hierarchy can provide an efficient mechanism for storying/retrieving semantic info with the key aspect being the observation that category membership at each level entails a # of properties that are shared by the members of the more specific included categories
Quillian proposed a spreading activation mechanism that permitted the activation of a category representation to spread to taxonomically superordinate concepts & this model provided a mechanism for property inheritance & generalization of new knowledge
the time it takes to verify category membership at various levels can often violate the predictions of the taxonomic hierarchy model
in neuropsychology Warrington accounts for pattern of deficits in cases of progressing fluent aphasia or semantic dementia
semantic dementia of fluent aphasia is the progressive deterioration of semantic knowledge while other cognitive faculties remain relatively spared
in semantic dementia what info is lost earlier in the progression of the disease? info about specific categories at the bottom of the taxonomy
studies by Cambridge group show that semantic dementia patients exhibit relatively preserved general knowledge in a variety of semantic tasks
the structure apparent in impair performance of semantic tasks reveals the organizational structure of concepts in memory
warrington suggested that categories located and the top of taxonomy are the first to be activated during access & the first to be acquired in development
Neil used children's responses to the judgement or propositions & their negations to construct predictability trees which describe children's ontological distinctions
concepts on predictability trees occupy the same node if they can reasonably enter into same set of propositions
the taxonomic structure is used to explain the influence of general category membership on the processing of more specific category info
info stored with more general representations can determine the feature weights for more specific categorizations but the hypothesis doesn't explain why a given feature is important for some categories & not others or how the relevant knowledge was acquired in the 1st place
illusory correlations are a way general knowledge may influence the processing of more specific info
studies show that people are not good at estimating the frequency objects & properties co-occur across a particular set of events
subjects persist in the belief that particular objects/properties have occurred together frequently even with contrasting evidence & may discount/ignore co-occurrence of object-property pairs during learning
children in Massey & German's experiment are able to categorize objects at general level of animate/inanimate & attributed typical properties to unfamiliar stimuli
2nd major influence on categorization based theories of semantic task performance stems from studies on basic level of categorization
subjects often perform best in semantic tasks requiring them to identify objects at basic level
Roach et al. demonstrated that at the basic level subjects fastest to verify membership prefer to use the basic label in pic-naming tasks fastest to discriminate objects at the basic level show a larger effect of visual priming to basic level children first learn to name objects w/ their basic-level name
basic-level advantages are observed because the cognitive system exploits representations that correspond to into-rich bundles of co-occurring attributes in the environment
basic categories tend to have special statistical properties
Murphy & Lassaline describe basic categories as maximizing informativeness & distinctiveness
more general categories are not very informative because their exemplars have little in common
more specific categories are informative but not particularly distinctive because they have few distinguishing characteristics
objects from same basis category tend to share many properties with each other& few with contrasting categories & are considered to be particularly useful
Rosch proposed that cognitive faculties develop to take advantage of the basic level structure by forming summary representations of maximally listing & informative categories of objects
what constitutes the basic level reflects shared/distinctive properties as these are known within a particular culture
the basic level has advantage for typical members but not atypical
"basic level" concept representations constitute the entry point into a taxonomic processing hierarchy
a given stimulus directly activates only its basic category representation
info stored at other levels of abstraction must be retrieved by means of spreading activation in the taxonomy
privileged access theories introduce a distinction between semantic info retrieved directly by means of categorization & indirectly through reference to stored class-inclusion links
basic-level phenomena are observed because info stored at more general/specific levels of abstraction is only accessible through the prior activation of base-level concept representations
Jolicoeur et al. stipulated that entry-level categories don't need to reside at same level of taxonomic specificity for all exemplars & might be found at a more specific level of the hierarchy for atypical instances
basic-level advantages can shift with increasing expertise in a domain which suggests that such effects are in part constrained by experience
experts prefer to name at the subordinate level in their domain & novices prefer to name at basic-level
experts in a given domain may derive somewhat different conceptions about how items in the domain are similar to one another depending upon the kind of expertise they have acquired
the processes by which we construct semantic representations are sensitive to the details of the experience we have within the categories
evidence that a level of category structure intermediate between between the most specific & most general has primacy over the most general level
Roach argued natural categories have a tendency to share family resemblances
attributes have a tendency to occur in clusters
the cognitive system exploits such discontinuities by forming summary representations of categories that correspond to clusters of correlated attributes in the environment
membership in most natural categories is graded (some objects are better examples of categories than others)
poor/atypical members of a category take longer to verify than good/typical members
Roach & Mervis: semantic system may store summary category representations as a prototypical set of descriptors that are generally, but not necessarily, true of the category's exemplars
time taken to classify a given instances is inversely proportional to the # of attributes it shares with a prototypical category members
prototype theory natural categories are represented by summary descriptions that are abstracted through exposure to instances in the environment
novel instances are compared to stored summary descriptions & are assigned to the category with the closest match under some measure of similarity
in prototypes theory the time taken to perform assignment depends upon feature overlap between novel instance, correct category, & competing categories
Mervis extended prototype theory to account for data from lexical acquisition
child-basic category prototypes include a small # of properties relative to adult category prototypes
category prototype representations may be used to explain effect of typicality on reaction times in category & property-verification tasks over-extension of familiar names to inappropriate but related objects in lexical acquisition & dementia & inappropriate restriction during early word learning
similarity-based models that attempt to do away with taxonomic processing structure will have difficulty explaining how more general category membership can influence categorization processing at more specific levels
hierarchically organized processing models will have difficulty explaining the strong influence of typicality on semantic judgements
categorization-based theories face further challenges from the theory-theory approach
basic tenet of theory-theory approach is that semantic cognition is constrained to a large extent by the naive domain knowledge ( a "theory") that people hold about casual relations that exist between entities in a domain
it is difficult to characterize just what "nice theories" are
the key function of the theory-theory approach is to explain/predict observed events in the environment & it serves this function with reference to stored knowledge about the casual properties of & relations between objects
in the TLC model a node inherits the properties of its "parents" and "grandparents"
the TLC model cannot account for reverse distance effects typicality effects basic level effects
the original TLC model is hierarchical with all links the same length while the revised TLC model is not s strict hierarchy because concepts can be associated via a direct link & connections vary in length
in the revised TLC model the length of the link corresponds to the strength of the relationship
properties in the original TLC are given by walking up the links to inherit the properties
in the revised TLC model the properties are inherited by spreading activation
being semantically primed means that a person is faster to respond to a lexical decision task if it was preceded by a related word
Created by: kzegelien2005
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