Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Variable Discharge of Cortical Neurons


The Journal of Neuroscience, May 15, 1998, 18(10):3870-3896

The Variable Discharge of Cortical Neurons: Implications for Connectivity, Computation, and Information Coding

Michael N. Shadlen1 and William T. Newsome2

1 Department of Physiology and Biophysics and Regional Primate Research Center, University of Washington, Seattle, Washington 98195-7290, and 2 Howard Hughes Medical Institute and Department of Neurobiology, Stanford University School of Medicine, Stanford, California 94305


Cortical neurons exhibit tremendous variability in the number and temporal distribution of spikes in their discharge patterns. Furthermore, this variability appears to be conserved over large regions of the cerebral cortex, suggesting that it is neither reduced nor expanded from stage to stage within a processing pathway. To investigate the principles underlying such statistical homogeneity, we have analyzed a model of synaptic integration incorporating a highly simplified integrate and fire mechanism with decay. We analyzed a "high-input regime" in which neurons receive hundreds of excitatory synaptic inputs during each interspike interval. To produce a graded response in this regime, the neuron must balance excitation with inhibition. We find that a simple integrate and fire mechanism with balanced excitation and inhibition produces a highly variable interspike interval, consistent with experimental data. Detailed information about the temporal pattern of synaptic inputs cannot be recovered from the pattern of output spikes, and we infer that cortical neurons are unlikely to transmit information in the temporal pattern of spike discharge. Rather, we suggest that quantities are represented as rate codes in ensembles of 50-100 neurons. These column-like ensembles tolerate large fractions of common synaptic input and yet covary only weakly in their spike discharge. We find that an ensemble of 100 neurons provides a reliable estimate of rate in just one interspike interval (10-50 msec). Finally, we derived an expression for the variance of the neural spike count that leads to a stable propagation of signal and noise in networks of neurons that is, conditions that do not impose an accumulation or diminution of noise. The solution implies that single neurons perform simple algebra resembling averaging, and that more sophisticated computations arise by virtue of the anatomical convergence of novel combinations of inputs to the cortical column from external sources.

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The reliability of spike transmission increases steeply for approximately 20 to 40 synchronous thalamic inputs in a time window of 5 milliseconds, when the reliability per spike is most energetically efficient. The optimal range of synchronous inputs is influenced by the balance of background excitation and inhibition in the cortex, which can gate the flow of information into the cortex. Ensuring reliable transmission by spike synchrony in small populations of neurons may be a general principle of cortical function. (Spike Synchrony in Small Populations of Neurons)



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