In 1972 Tulving(2) produced an influential paper distinguishing two types of memory: semantic and episodic. Semantic memory is memory for our knowledge about the world, for example remembering that Dublin is the capital of Eire, or that a fox has a bushy tail. Semantic memory is also concerned with our knowledge of social customs, the meanings of words, the colours and textures of objects, and how things smell. Most memory-impaired patients do not forget this kind of information, although they may have difficulty adding to their store of semantic knowledge. Amnesic patients are often unable, for example, to learn new words that enter the vocabulary after their neurological insult. Thus CW, an ex-patient of mine, cannot understand the terms 'AIDS' or 'e-mail', or 'mad cow disease', all of which came into widespread usage after 1985 when CW developed herpes simplex encephalitis.
Episodic memory, on the other hand, represents what most of us would think of as memory, in that it refers to a specific episode that has been experienced and can be recalled. Thus remembering what you ate for dinner last night, when you last phoned your mother, or what you read a few minutes ago are all examples of episodic memory. This system is frequently damaged in people with organic memory impairment and episodic memory deficits are perhaps the most noticeable characteristic of the amnesic syndrome.
A third system that operates differently from either semantic or episodic memory is procedural memory, the system used for learning skills such as riding a bicycle or learning to type. People get better with practice and can demonstrate the skill even though they may not remember how they learned to ride a bicycle or type. JC, a young amnesic patient of mine, successfully learned to type even though he had no conscious recollection of learning. In his words, 'Practical skills developed without me being aware of how this came about. I could do things without being able to explain how'. Procedural memory is typically normal or nearly normal in amnesic patients.
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