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A Brief History of Time (Constants)

Koch, Christof and Rapp, Moshe and Segev, Idan (1996) A Brief History of Time (Constants). Cerebral Cortex, 6 (2). pp. 93-101. ISSN 1047-3211. doi:10.1093/cercor/6.2.93.

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That the cerebral cortex processes information at prodigious speeds cannot be doubted. Yet the passive time constant, τ_m of neurons, often thought of as a measure of the neuron's “response time” to synaptic input, is relatively long. In the 1950s, τ_m was estimated to be only a few milliseconds for mammalian central neurons; with improvement in recording techniques, its estimated value grew over the years and it now stands near 20–100 msec. However, as we will argue here, the functional meaning of τ_m is ambiguous. On the basis of a newly introduced definition of local delay, we show that the time window for synaptic integration in passive dendritic trees can be much smaller than the time constant. We argue that the voltage response to very brief synaptic inputs is essentially independent of τ_m. We discuss how τ_m can change dynamically with the global activity of the network, as well as the difficulties of defining a time constant in structures with voltage-dependent elements. We conclude that the classically defined τ_m only provides a very rough estimate, typically an overestimate, of the response time of neurons and that alternative measures are required to capture the dependency of the time course of the membrane potential on ligand-gated and/or voltage-dependent membrane conductances.

Item Type:Article
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Koch, Christof0000-0001-6482-8067
Additional Information:© 1996 Oxford University Press. We thank Ōjvind Bernander and Hagai Agmon-Snir for their detailed comments and their help in preparing the manuscript. The research reported here was funded by grants to C.K. from ONR and the NIMH Center for Neuroscience and by grants to I.S. from ONR and BSE I.S. acknowledges the hospitality of the Mathematical Research Branch (NIDDK) at the NTH in Bethesda, and C.K., the hospitality of the Santa Fe Institute in Santa Fe, New Mexico. Early versions of the manuscript were written while these two authors spent part of their summer at these institutions.
Group:Koch Laboratory (KLAB)
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Office of Naval Research (ONR)UNSPECIFIED
National Institute of Mental Health (NIMH)UNSPECIFIED
Issue or Number:2
Record Number:CaltechAUTHORS:20120208-103527408
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Official Citation:Christof Koch, Moshe Rapp, and Idan Segev A Brief History of Time (Constants) Cereb. Cortex (1996) 6(2): 93-101 doi:10.1093/cercor/6.2.93
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:29195
Deposited By: Ruth Sustaita
Deposited On:08 Feb 2012 18:55
Last Modified:09 Nov 2021 17:04

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