A BRIEF-HISTORY OF TIME (CONSTANTS)

That the cerebral cortex processes information at prodigious speeds ca nnot be doubted. Yet the passive time constant, tau(m) of neurons, oft en thought of as a measure of the neuron's ''response time'' to synapt ic input, is relatively long. In the 1950s, tau(m) was estimated to be only a few milliseconds for mammalian central neurons; with improveme nt in recording techniques, its estimated value grew over the years an d it now stands near 20-100 msec. However, as we will argue here, the functional meaning of tau(m) is ambiguous. On the basis of a newly int roduced definition of local delay, we show that the time window for sy naptic integration in passive dendritic trees can be much smaller than the time constant. We argue that the voltage response to very brief s ynaptic inputs is essentially independent of tau(m). We discuss how ta u(m) can change dynamically with the global activity of the network, a s well as the difficulties of defining a time constant in structures w ith voltage-dependent elements, We conclude that the classically defin ed tau(m) only provides a very rough estimate, typically an overestima te, 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 conductanc es.