Dynamical Cell Assembly Hypothesis - Theoretical Possibility of Spatio-temporal Coding in the Cortex.

This paper is an attempt to understand how knowledge and events are represented and processed in the brain. An important point is the question 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, which means in essence that timing of spikes plays a critical role in encoding, 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 cortical neuron may function effectively as a coincidence detector. At the same time, non-classical experimental phenomena of task-related, short time-scaled dynamical modulations of temporal correlations between neurons suggest a non-classical view on the dynamics working in the brain. In response to contexts or external events, a group of neurons, a dynamical cell assembly, spontaneously organizes, linked temporarily by coincident timing of incident spikes, showing correlated firing with each other. This is an emergent property of neuronal populations in the cortex.We make a theoretical exploration on issues as (1) the description of such emergent dynamics of dynamical cell assemblies based on the working hypothesis that a cortica neuron functions effectively as a coincidence detector, and (2) the principle of spatio-temporal coding based on the hypothetical emergent dynamics. Note that the conventional rate coding hypothesis does not give satisfactory answers to fundamental questions on the representation and processing of knowledge or events 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 question of the basic code in the brain; (2) the paradigms on representation of knowledge and events proposed from a theoretical or experimental basis. The classical paradigms of Hebb and Barlow with their experimental and theoretical critiques, and more recently proposed experiment-based paradigms are reviewed. Also a review is given on (3) the experimentally observed spatio-temporal structure of spike dynamics.The second goal is to give a description of the dynamical cell assembly - the central 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 the emergent dynamics of cell assemblies, and also examine how the observed experimental data could be explained within this theoretical setting.We also try to give the principle of spatio-temporal coding based on the dynamical cell assembly framework. A key concept is the internal mechanism of "dialogue" among neuronal pools in the brain. This provides a dynamical foundation of bi-directional interactions for the linkage of distant modules to create integrated information. We present a simple model in order to illustrate the working principle of coincidence detector systems. Relations with other temporal coding paradigms are also discussed. Copyright 1996 Elsevier Science Ltd.