HIERARCHICAL NEURONAL MODELING OF COGNITIVE FUNCTIONS - FROM SYNAPTICTRANSMISSION TO THE TOWER-OF-LONDON

Recent progress in the molecular biology of synaptic transmission, in particular of neurotransmitter receptors, offers novel information rel evant to 'realistic' modeling of neural processes at the single cell a nd network level. Sophisticated computer analyses of 2D crystals by hi gh resolution electron microscopy yield images of single neurotransmit ter receptor molecules with tentative identifications of ligand bindin g sites and of conformational transitions. The dynamics of conformatio nal changes can be accounted for by a 'multistate allosteric network' model. Allosteric receptors also possess the structural and functional properties required to serve as coincidence detectors between pre-and post-synaptic signals and, therefore, can be used as building blocks for a chemical Hebb synapse. These properties were introduced into net works of formal neurons capable of producing and detecting temporal se quences. In more elaborate models of pre-frontal cortex functions, all osteric receptors control the selection of transient 'pre-representati ons' and their stabilization by external or internal reward signals. W e apply this scheme to Shallice's Tower of London test, and we show ho w a hierarchical neuronal architecture can implement executive or plan ning functions associated with frontal areas. ((C) Academie des scienc es/Elsevier, Paris.)