The task of explaining the organization of conceptual knowledge is both promising and difficult. It is promising because if conceptual knowledge is organized, then in principle it should be possible to form theories which would account for the nature of conceptual organization in normal brains and brains which present for cognitive deficits of one kind or another. We should think that this organization would obtain modularly; that is, we should think that conceptual knowledge should be organized according to functionally individuated cognitive mechanisms. There is reason to be wary of this promise, however. The most relevant body of possible evidence for this task spells difficulty for cognitive theorists. Subjects who present for selective deficits of semantic processing of one kind or another are notoriously difficult to explain. Cases of category-specific semantic deficits, in which individuals behave as if unable to name and/or recognize items of a particular semantic category, presently resist satisfactory explanation and therefore prevent the success of any theory of semantic architecture.
[...] It is not the case that in any given instance of category-specific deficits the error in the relevant cognitive system is either the functional semantic system or the sensory semantic system, for this presupposes both of the following: that sensory and functional information are the most essential to any category- specific deficit and that the correct identification of a given item of a given category is contingent on functional information but not sensory information, or sensory information but not functional information. [...]
[...] Given that the category ‘living things' can be damaged independently of the category ‘nonliving things', the plausible inference to suggest is that the semantic system is organized according to categorical schemas. However, though some suggested this hypothesis initially (for instance, Warrington, 1981), it was subsequently discarded; the interpretation of the data did not fit the notion that the organization of the semantic system reflected definite categorical boundaries. Warrington, who found reason to doubt the initial (Warrington, 1981) hypothesis that category-specific semantic deficits may result from damage to the categorical organization of the semantic system, proposed the Sensory-Functional hypothesis. [...]
[...] is a brain-damaged subject who has a disproportionate category-specific semantic deficit for the category of animals; she is impaired equally for both visual and functional attributes of this category, and so it is not something which the Sensory-Functional theory can coherently account for. The results of E.W.'s testing can be summed up as follows: E.W.'s category-specific semantic deficit is restricted to the category of animate objects; that is, her performance on other living things, such as fruits and vegetables, is within the normal range; the same is true of her performance on any food item. [...]
[...] Finally, while unitary-content-hypothesis represents one of the more recent efforts to explain category-specific semantic deficits, the complexity and implications inherent in both the Sensory-Functional account and the Domain- Specific hypothesis disallow the current discussion to treat this third hypothesis if this current discussion is to at all remain committed to a standard of through analysis. In all cases, we shall recourse to the experiments said to support each, and criticize methodology and interpretation where reasonable. In many instances, initial support has lead to retrospective denial of the same evidence-class. [...]
[...] Essentially, their model instantiates the notion that the onset of disproportionate category-specific semantic deficits are supervenient on, caused by, and implemented as a result of damage to the specific way in which a particular type of knowledge is processed. Thus, Caramazza and Shelton's claim that Farah and McClelland's model can be reinterpreted and reduced to the mere approximation of categorical distinctions between two semantic domains completely alters the essential motive of this PDP model, and partakes in an element of equivocation, because the meaning of the supervenience exhibited by the PDP cannot be transmitted to Caramazza's and Shelton's thesis of domain-specificity, seeing as how the notion of ‘domain-specificity' presupposes a fundamental role in the organization of semantic knowledge. [...]
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