Involvement of the amygdala in learning and memory: a critical review, with emphasis on anatomical relations

The amygdala has been attributed with a considerable number of very diverse functions. Its involvement in learning and memory, though, has found increased attention. Following a short description of the connections of the amygdala and its critical neurotransmitters, studies are reviewed here in which the amygdala's activity was manipulated or observed by different methods (lesions, electrical brain stimulation, neurochemical intra-amygdaloid injections, single-unit recordings). Some of the major conclusions resulting from this data analysis indicate an advantage in performing subtotal amygdaloid lesions over an amygdalectomy, a point of view that is especially supported by the heterogeneous anatomical connections of different amygdaloid nuclei. Thereafter, an evaluation is made of different tasks with respect to their discriminative sensitivity to amygdaloid manipulations. Some species-specific differences in performing certain tasks after amygdaloid injuries are discussed in relation to the different expansion of amygdalo-cortical connections in higher and less highly encephalized species. Finally, some general assumptions are made on the specific role of each of the amygdaloid nuclei during the mnemonic processes attributed to the amygdala. It is concluded that emotionally significant information is encoded and can be retrieved on the basis of the structures and connections of the basolateral limbic circuit.