Disorders of semantic memory.

Memory is a complex and multifaceted aspect of human cognition, as reflected by the broad range of neuropsychological deficits which present with complaints of "poor memory". We will review disorders of semantic memory, the component of long term memory which represents our knowledge of objects, facts, and concepts, as well as words and their meaning. The distinction between episodic and semantic memory was first drawn by Tulving1 on psychological grounds: episodic memory corresponds to the recollection of personally experienced episodes or events which are time and place specific. By contrast, semantic memory is culturally shared, usually overlearned, and not temporally specific. According to this dichotomy, remembering the details of a meal in a Parisian bistro on holiday last year depends on episodic memory systems, whereas knowing the meaning of the word "bistro" and that Paris is the capital of France, draws on semantic memory. There is clearly a degree of interdependence between episodic and semantic memory: knowledge must first be acquired and is initially temporally specific, whereas all experiences must be understood at some level. Some theorists maintain, therefore, that they represent no more than the extremes of a spectrum of information acquisition.23 Yet examples of patients with profoundly impaired episodic memory and preserved general knowledge, such as Korsakoff amnestics, together with patients in whom semantic memory is selectively disrupted (see below), argue that these two systems are, at least partially, separable on neuropsychological grounds. From an anatomical perspective, the medial temporal structures, in particular