Hierarchical neuronal modeling of cognitive functions: from synaptic transmission to the Tower of London

Recent progress in the molecular biology of synaptic transmission, in parti cular of neurotransmitter receptors, offers novel information relevant to ' realistic' modeling of neural processes at the single cell and network leve l. Sophisticated computer analyses of two-dimensional crystals by high reso lution electron microscopy yield images of single neurotransmitter receptor molecules with tentative identifications of ligand binding sites and of co nformational transitions. The dynamics of conformational changes can be acc ounted 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, therefor e, can be used as building blocks for a chemical Hebb synapse. These proper ties were introduced into networks of formal neurons capable of producing a nd detecting temporal sequences. In more elaborate models of pre-frontal co rtex functions, allosteric receptors control the selection of transient 'pr e-representations' and their stabilization by external or internal reward s ignals. We apply this scheme to Shallice's Tower of London test, and we sho w how a hierarchical neuronal architecture can implement executive or plann ing functions associated with frontal areas. (C) 2000 Elsevier Science B.V. All rights reserved.