Semantic memory, distinguished from episodic memory by Endel Tulving (1972), stores our general knowledge about the world: facts (Paris is the capital of France), word meanings (a "bachelor" is an unmarried man), conceptual knowledge (birds can fly), and the categorical and associative structure that organizes this knowledge. Unlike episodic memory, semantic memory is not tied to any specific personal experience — you know that 2 + 2 = 4 without remembering when or where you learned it.
Organization of Semantic Memory
How is the vast store of human knowledge organized? Several models have been proposed. Collins and Quillian's hierarchical network model (1969) represented concepts as nodes in a tree-like hierarchy (canary → bird → animal) with properties stored at the most general level (has wings → bird). Collins and Loftus' spreading activation model (1975) replaced the strict hierarchy with a more flexible network where related concepts are connected by weighted links, and activating one concept spreads activation to related concepts.
Semantic priming: "doctor" speeds recognition of "nurse" by spreading activation through associative links.
Eleanor Rosch's prototype theory proposed that categories are organized around prototypes — the most typical members. A robin is a more prototypical bird than a penguin, and category membership is graded rather than all-or-nothing. Typicality effects — faster verification and more frequent listing for typical members — are among the most robust findings in semantic memory research.
Semantic dementia — a variant of frontotemporal dementia involving progressive deterioration of the anterior temporal lobes — produces a selective, progressive loss of semantic knowledge while episodic memory, working memory, and perception remain relatively intact. Patients gradually lose knowledge of word meanings, object identities, and conceptual categories. The disorder provides powerful evidence that semantic memory is a distinct system with a specific neural substrate, and the pattern of breakdown (more specific knowledge lost before more general knowledge) reveals the hierarchical organization of semantic representations.
Neural Substrates
The anterior temporal lobes serve as a "semantic hub" that integrates information from modality-specific cortical regions to create amodal conceptual representations. But semantic knowledge is also distributed across sensory and motor cortices: knowledge about how objects look activates visual areas, knowledge about actions activates motor areas, and knowledge about sounds activates auditory areas. This "hub-and-spoke" model, proposed by Matthew Lambon Ralph and colleagues, reconciles the evidence for both localized and distributed semantic representations.
Relationship to Episodic Memory
Semantic and episodic memory are not entirely independent. New semantic knowledge is often initially acquired through episodic experiences and gradually becomes decontextualized. The hippocampus may be involved in the initial acquisition of new semantic facts, though established semantic knowledge can be accessed independently of the hippocampus. The interaction between these systems is a continuing area of active research.