SEMANTIC MEMORY

[Semantic memory is our acquired knowledge about the everyday world. The following two sections characterize our current understanding about semantic memory from the cognitive and the neurbiological perspectives. In both sections, two common themes in research on semantic memory are discussed: whether semantic memory is fundamentally distinct from memory for personal experiences (episodic memory) and how to characterize the organization of semantic knowledge in terms of its cognitive dimensions and its representation in the brain.

Semantic memory is often distinguished from episodic memory as what one "knows" independent of what one "remembers," that is, consciously recollects learning in a particular personal episode. Endel Tulving, who introduced the distinction between semantic and episodic memory, has argued that semantic memory is a distinct memory system, independent from that of episodic memory. Yet, all of our conscious learning experiences are initially a part of episodic memory, consistent with a common alternative view that semantic memory is knowledge abstracted from the information common to many episodes. One central question in research on semantic memory is whether and how episodic memory and semantic memory are mediated by different neural structures or brain systems. Several lines of evidence suggest semantic memory is mediated by cerebral cortical areas without involvement of the hippocampus, whereas episodic memory relies critically on hippocampal function.

As yet we have only a preliminary understanding of the cognitive dimensions of semantic memory.C ognitiveE ffectsoutlines research suggesting three principal cognitive dimensions: categorization of objects by their features, judgments about relationships between concepts, and the capacity to combine multiple elementary concepts into distinct and more complex concepts.

It is generally viewed that semantic memory is mediated by the cerebral cortex, as best demonstrated in cases of semantic dementia, a selective loss of semantic memory, consequent to cortical deterioration. However, there is considerable controversy over whether separate areas of the cerebral cortex process specific domains of semantic memory, and about the nature of the functional distinctions. There have now been several observations of deficits in specific domains of semantic knowledge following damage to particular areas of the cerebral cortex, as well as observations of activation of particular cortical areas during naming in specific domains of objects. It is not clear whether these selectivities reflect specificcategories of semantic knowledge or distinct types of cognitive or perceptual processing associated with particular categories of information.N eurobiologicalP erspectiveoutlines this issue.]