Principales actividades del chaku

problems with Warrington and McCarthy's revised version of the functional/sensory hypothesis. Firstly, they have argued that it is not obvious that the categories of living things which have been shown to dissociate from one another (i.e. animals and fruits and vegetables) do in fact differ in terms of the importance of shape and colour to the differentiation of their members. Indeed, although the pair of fruits (blackberry/raspberry) chosen as an example by Warrington and McCarthy are primarily differentiated from one another in terms of their colour, this is not the case for other possible pairs of fruits, such as banana/lemon, peas/green beans or apple/pear. Caramazza et al. also attempt to make the complementary argument about the category of flowers - i.e. that shape information is not necessarily more important for their differentiation than information about colour. However, although they do provide an example (daisy/black-eyed susan) that illustrates that it is possible for this to be the case, this would appear to be the exception rather than the rule. Few flowers are so similar to other flowers in shape that colour becomes a crucial factor, and few have a colour that is so distinctive as to render information about shape superfluous. It is perhaps more accurate to say that, while both colour and shape information may be extremely important in the semantic representations of fruits and vegetables, colour information may be less important than shape for flowers. Damage to stored colour information would therefore result in a semantic deficit specific to the category of fruits and vegetables.

However, Caramazza et al. (1994) have argued that an explanation of MD's inability to name fruits and vegetables in terms of an impairment to information about colour is undermined by his ability to name colours, and by his inability to name fruits and vegetables from tactile stimulation (Hart, Bemdt and Caramazza 1985). However, this is not necessarily the case. Firstly, the ability to name colours has been demonstrated to be dissociable from the ability to identify the correct colours of objects see, for example, Geschwind and Fusillo 1966; Farah, Levine and Calvanio 1988). This suggests that the knowledge that is necessary for naming colours is stored separately from knowledge about the typical colours of objects. It is the latter type of information that would be crucial in the semantic representations of fruits and vegetables. Secondly, the fact that MD's impairment was also apparent with tactile input is not necessarily problematic for Warrington and

McCarthy's theory. Caramazza et al. (1994) have taken the view that the visual/verbal and fimctional/sensory distinctions made by Warrington and McCarthy are synonymous with one another. This is demonstrated by their statement that "On this hypothesis, a category- specific deficit for living things would be the result of selective damage to a visual semantic subsystem and a category-specific deficit for non-living things would be the result of selective damage to a fimctional semantic (verbal) subsystem" (p. 74). However, this is not an accurate characterization of Warrington and McCarthy's theory. The latter authors have argued that semantic information is organised by both input modality and information type. Thus they have argued that the organization of semantic memory into functional and sensory attributes occurs within semantic systems that are accessed by different modalities of input. Hence, damage to colour information could occur within systems accessed by visual, verbal or tactile input. Even within Caramazza et al.'s characterization of the multiple semantics hypothesis (which corresponds to the Modality-Specific Content hypothesis) it would not necessarily be the case that an impairment to the visual/sensory system would only affect tasks involving visual stimuli. The identification of fruits and vegetables from tactile input might also be affected if information stored in the visual/sensory system is necessary for the identification of members of this category.

Nevertheless, Caramazza et al. maintain that there are a number of observed dissociations which are difficult to explain in terms of the revised fimctional/sensory hypothesis. They argue that it is difficult, for example, to identify specific functional or sensory channels which, if damaged, would explain impaired processing of animals and vegetables, but not modes of transport (patient JBR [Warrington and Shallice 1984]), or impaired processing of vegetables but not animals and modes of transport (patient JJ, late in his recovery [Hillis and Caramazza 1991]). Caramazza et al. conclude that the proposal that category-specific deficits reflect damage to specialized subregions of functional and sensory channels cannot, therefore, account for the reported data. However, although such fine-grained fractionations of semantic categories are indeed, at first sight, difficult to incorporate within the revised fimctional/sensory hypothesis, they do not necessarily represent insurmountable problems for it. The dissociations between the categories of animals, fhiits and vegetables and forms of transport may be explicable in terms of different weightings on information about shape.

colour, function and motility in their representations. For example, the selective impairment of knowledge about the broad category of living things (observed in JBR) may be the result of damage to sensory information in general, while the selective preservation of knowledge about animals and modes of transport (observed in JJ) may be the result of a selective preservation of knowledge about motility.

The criticisms put forward by Caramazza et al. do not, therefore, constitute a strong case against the revised form of the fimctional/sensory hypothesis favoured by Warrington and McCarthy. However, a number of alternative accounts of category-specific deficits of semantic memory have been advanced.