Oscillatory Event-Related Brain Dynamics, Edited

According to one brain-theoretical view, high~!:.brainful1ctionsarebased on processing_!!l1j.ts called cell as~~mbJiys. <::ell assemblies are large groups of cortical--._....__··---~-_.o =..~'-t):~u,(:i· " '7.",' ". ' _.. """'......-pyramidal neurons with strong and reclproc-al internalconnections. C~lla,ssemblies develop in a randomIy-connected neuronal network when sets of neurons are fre-quently attive-simillfaneolisly so that their connectionsEstrengthen (Hebb's law, Hebb,f949~Gustafsso·neCa1.,1987, Bonhoffer et aI., 1989, Ahissar et aI., 1992). Neurons making up one assembly do not need to be located in a small cortical area. They may be distributed widely over various cortical regions. Such transcortical as-semblies are likely to be held together through long axonsof pyramidal cells which are well-known to connect distant cortical areas (Pandya and Yeterian, 1985, Braitenberg and Schiiz, 1991, Deacon, 1992a, Deacon, 1992b). Due to strong intra-assembly connections, excitation of some neurons of an assembly leads to spreading activation in the network and, finally, to an "ign'ilioiiiJ>&tthe whole _ assembly. Overshooting activity will occur in the cortex when too many,assemblies ig1\:rt~at the same time. Therefore, a regulation mechanism is required which keeps the level of cortical excitation close to a target value (Braitenberg, 1978). This regulatiop.ll1ech-anism guarantees that only one or a limited number of assemblies will igrilit ~ at a time. What is the purpose of transcortical assemblies? One obvious answer is the fol-lowing:tianscortical assembhesi11ake it possible to "bind " information represented in different parts of the corte~. Such binding may occur between neurons in the vi-sual ~iSteJ!l:stihiulated by features of a perceived object. If assemblies include vari