[Amnesic syndromes].

Amnesic syndromes are rare, but they constitute very useful models to study the neurobiological substrates of human memory. The causes and semiological formulae of these syndromes are varied, but they are all characterized by anterograde amnesia combined with a retrograde amnesic disorder and by elective damage of episodic and declarative memories, the semantic and procedural memories, as well as intelligence, being usually spared. Amnesic syndromes are divided into two main types: the simplest one, type 1, is thought to be a defective storage, whilst the more complex type 2 is regarded as defective encoding associated with disturbances in retrieval dependent on more diffuse meso-diencephalic, septal, hypothalamic and sometimes frontal lesions. The lesional substrate of amnesic syndromes comprises the hippocampo-mammilo-thalamo-cingulum axial complex and its connections with the mesencephalon and the basal forebrain.