DYNAMICAL CELL ASSEMBLY HYPOTHESIS - THEORETICAL POSSIBILITY OF SPATIOTEMPORAL CODING IN THE CORTEX

This paper is an attempt to understand how knowledge and events are re presented and processed in the brain. An important point is the questi on of what carries information in the brain - the mean firing rate or the timing of spikes? The idea we want to pursue is that, contrary to the traditional view, the brain might use higher order statistics, whi ch means in essence that timing of spikes plays a critical role in enc oding, representing, and processing knowledge and events in the brain. A recently revealed salient nature of cortical pyramidal cells, i.e., the high variability of inter-spike intervals suggests that a cortica l neuron may function effectively as a coincidence detector. At the sa me time, non-classical experimental phenomena of task-related, short t ime-scaled dynamical modulations of temporal correlations between neur ons suggest a non-classical view on the dynamics working in the brain. In response to contexts or external events, a group of neurons, a dyn amical cell assembly, spontaneously organizes, linked temporarily by c oincident timing of incident spikes, showing correlated firing with ea ch other. This is an emergent property of neuronal populaton in the co rtex. We make a theoretical exploration on issues as (1) the descripti on of such emergent dynamics of dynamical cell assemblies based on the working hypothesis that a cortica neuron functions effectively as a c oincidence detector, and (2) the principle of spatio-temporal coding b ased on the hypothetical emergent dynamics. Note that the conventional rate coding hypothesis does not give satisfactory answers to fundamen tal questions on the representation and processing of knowledge or eve nts in the brain, e.g., the questions of cross-modular integration of information or the binding problem, and representation of hierarchical knowledge etc. The first goal is to give a non-encyclopedic review on (1) the temporal structure of spike sequences,focusing on the questio n of the basic code in the brain; (2) the paradigms on representation of knowledge and events proposed from a theoretical or experimental ba sis. The classical paradigms of Hebb and Barlow with their experimenta l and theoretical critiques, and more recently proposed experiment-bas ed paradigms are reviewed. Also a review is given on (3) the experimen tally observed spatio-temporal structure of spike dynamics. The second goal is to give a description of the dynamical cell assembly - the ce ntral concept in this paper. Aside from the question of physiological basis, we make a theoretical study, under a working hypothesis that a cortical neuron functions effectively as a coincidence detector, on th e emergent dynamics of cell assemblies, and also examine howl the obse rved experimental data could be explained within this theoretical sett ing. We also try to give the principle of spatio-temporal coding based on the dynamical cell assembly framework. A key concept is the intern al mechanism of ''dialogue'' among neuronal pools in the brain. This p rovides a dynamical foundation of bi-directional interactions for the linkage of distant modules to create integrated information. We presen t a simple model in order to illustrate the working principle of coinc idence detector systems. Relations with other temporal coding paradigm s me also discussed. Copyright (C) 1996 Elsevier Science Ltd.