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Neural Representation of Semantic Knowledge

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  1. In order to demonstrate the insight of their hypothesis we must first assess the preceding theories and why these are unsatisfactory.
  2. As such, greater cortical mass and connectivity would be required to correctly perform these tasks.
  3. PET scanning has shown that particular regions of the cortex respond to tasks in which subjects are asked to identify either animals or tools.
  4. But we come back to the question of why should the cortex have separate areas for animals, faces, and tools?
  5. A counter argument to Damasio's hypothesis is that it is not a parsimonious view of the brain.

The human ability to look at and label objects, though taken for granted even in very young children, has long puzzled and fascinated scientists investigating the organization of knowledge systems in the brain. A central question in this investigation is the way knowledge of objects are categorized, encoded, and stored within the neural network. This study of the ?semantic organization? of the cortex has relied in large part on patients who, through the damaging effects of stroke, disease, or physical injury, have cerebral injuries which impair their faculties of recognition and object identification. After extensive testing with these patients, researchers have arrived at varying conclusions. Some hold that the deficits point to a category-specific semantic organization, others suggest a modality-specific structure, and still others claim the deficits are simply generalized cognitive impairment. Yet, many of these are at best, partial explanations and at worst, misinterpretations of observations.

[...] Yet, this scheme of the brain's semantic structure offers no explicit reason why this should be so. These logical problems are why category-specificity is not the only neural framework offered by scientists. In contrast to the idea of category specificity in the brain's semantic organization is the theory of modality-specificity. This theory posits that the agnosia of these patients is purely a function of the visual properties of the stimulus and not the categorical or semantic properties. For example, in an experiment where both auditory and visual identification were observed, ?there was an insignificant degree of consistency for comparison between modalities In contrast, there was a significant degree of response consistency for all responses within a modality (Warrington & Shallice, 1984). [...]

[...] As important as this faculty is to an organism's survival, it is surprising that the brain does not recruit even more cortex for the task. The more cortex devoted to the lexical nodes and the systems they subserve, the more function is preserved in the case of a lesion. Finding a parsimonious, economical model for the brain's semantic organization seems almost paradoxical in this sense. In contrast to the category-specific, modality-specific, and visual- difficulty hypotheses, the ?neural nodes? theory of semantic organization as stated by Damasio and others reconciles many problems quite well. [...]

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