Coherent Assembly Dynamics in the Cortex: Multi-Neuron Recordings, Network Simulations and Anatomical Considerations

The anatomical structure of the neo-cortex and, particularly, its massive connectivity strongly suggest that the functional organization of this part of the brain is based upon interactions within and among groups of cells (Braitenberg and Schuz, 1991). This observation has prompted neurobiologists as early as Sherrington (1941) and Hebb (1949) (see also James, 1890) to speculate that cortical neurons do not act in isolation, but rather that they organize into cell assemblies for various computational tasks (see Gerstein et al., 1989, for a review of different definitions of the concept of cell assembly). One operational definition for the cell assembly has been particularly influential: near-simultaneity or some other specific timing relation in the firing of the participating neurons. As, for instance, elaborated in the concept of the ‘synfire chain’ (Abeles, 1982, 1991), the synaptic influence of multiple neurons converging onto others in the cortical network is much stronger if they fire in (near-) coincidence. Thus, temporal coherence or synchronous firing, postulated as a mechanism for perceptual integration (Hebb, 1949), would in fact be directly available to the brain as a potential neural code (Perkel and Bullock, 1968; Gerstein and Michalski, 1981; Johannesma et al., 1986).