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Five patients who had undergone radiation therapy for cerebral tumors and whose conditions were deteriorating were examined by means of positron emission tomography (PET) with [18F] fluorodeoxyglucose. All five cases had similar clinical and computed tomographic findings. Using the PET technique the two cases of radiation necrosis were distinguished from the three recurrent tumors. In the two cases of radiation necrosis the rate of glucose utilization in the lesion was markedly reduced compared with the normal brain parenchyma. In the recurrent gliomas, however, the glucose metabolic rate was elevated. All five diagnoses were confirmed by biopsy or autopsy.

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The structure of people's conceptual knowledge of concrete nouns has traditionally been viewed as hierarchical (Collins & Quillian, 1969). For example, superordinate concepts (vegetable) are assumed to reside at a higher level than basic-level concepts (carrot). A feature-based attractor network with a single layer of semantic features developed representations of both basic-level and superordinate concepts. No hierarchical structure was built into the network. In Experiment and Simulation 1, the graded structure of categories (typicality ratings) is accounted for by the flat attractor network. Experiment and Simulation 2 show that, as with basic-level concepts, such a network predicts feature verification latencies for superordinate concepts (vegetable ). In Experiment and Simulation 3, counterintuitive results regarding the temporal dynamics of similarity in semantic priming are explained by the model. By treating both types of concepts the same in terms of representation, learning, and computations, the model provides new insights into semantic memory.