Softky, William R. and Koch, Christof (1993) The highly irregular firing of cortical cells is inconsistent with temporal integration of random EPSPs. Journal of Neuroscience, 13 (1). pp. 334-350. ISSN 0270-6474. http://resolver.caltech.edu/CaltechAUTHORS:20130816-103228752
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How random is the discharge pattern of cortical neurons? We examined recordings from primary visual cortex (V1; Knierim and Van Essen, 1992) and extrastriate cortex (MT; Newsome et al., 1989a) of awake, behaving macaque monkey and compared them to analytical predictions. For nonbursting cells firing at sustained rates up to 300 Hz, we evaluated two indices of firing variability: the ratio of the variance to the mean for the number of action potentials evoked by a constant stimulus, and the rate-normalized coefficient of variation (Cv) of the interspike interval distribution. Firing in virtually all V1 and MT neurons was nearly consistent with a completely random process (e.g., Cv approximately 1). We tried to model this high variability by small, independent, and random EPSPs converging onto a leaky integrate-and- fire neuron (Knight, 1972). Both this and related models predicted very low firing variability (Cv << 1) for realistic EPSP depolarizations and membrane time constants. We also simulated a biophysically very detailed compartmental model of an anatomically reconstructed and physiologically characterized layer V cat pyramidal cell (Douglas et al., 1991) with passive dendrites and active soma. If independent, excitatory synaptic input fired the model cell at the high rates observed in monkey, the Cv and the variability in the number of spikes were both very low, in agreement with the integrate-and-fire models but in strong disagreement with the majority of our monkey data. The simulated cell only produced highly variable firing when Hodgkin-Huxley- like currents (INa and very strong IDR) were placed on distal dendrites. Now the simulated neuron acted more as a millisecond- resolution detector of dendritic spike coincidences than as a temporal integrator. We argue that neurons that act as temporal integrators over many synaptic inputs must fire very regularly. Only in the presence of either fast and strong dendritic nonlinearities or strong synchronization among individual synaptic events will the degree of predicted variability approach that of real cortical neurons.
|Additional Information:||© 1993 by Society for Neuroscience. Received Apr. 30, 1992; revised July 16, 1992; accepted July 23, 1992. We are very grateful to J. Knierim, D. Van Essen, and W. Newsome for their provision of hard-won data for this analysis, to W. Bair for his formatting of it, and to 6. Bernander for making his simulated neuron available to us. Special thanks are due to R. Douglas, F. Crick, and B. Kehoe for many productive discussions and suggestions. We also thank D. H. Perkel, who pioneered these investigations and introduced the parents of one of us (W.R.S.). This research was funded by an NSF Presidential Young Investigator Award, by the Office of Naval Research, by the HFSPO Program in Strasbourg, and by the James S. McDonnell Foundation.|
|Group:||Koch Laboratory, KLAB|
|Subject Keywords:||awake monkey, randomness, coincidence detection, variability, temporal integration, dendritic spikes, integrate-and-fire neuron, visual cortex, biophysical modeling|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||KLAB Import|
|Deposited On:||11 Mar 2010 05:57|
|Last Modified:||03 Apr 2014 20:56|
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