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Neural Computation

July 1, 1997, Vol. 9, No. 5, Pages 1001-1013
(doi: 10.1162/neco.1997.9.5.1001)
© 1997 Massachusetts Institute of Technology
Shunting Inhibition Does Not Have a Divisive Effect on Firing Rates*
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Shunting inhibition, a conductance increase with a reversal potential close to the resting potential of the cell, has been shown to have a divisive effect on subthreshold excitatory postsynaptic potential amplitudes. It has therefore been assumed to have the same divisive effect on firing rates. We show that shunting inhibition actually has a subtractive effecton the firing rate in most circumstances. Averaged over several interspike intervals, the spiking mechanism effectively clamps the somatic membrane potential to a value significantly above the resting potential, so that the current through the shunting conductance is approximately independent of the firing rate. This leads to a subtractive rather than a divisive effect. In addition, at distal synapses, shunting inhibition will also have an approximately subtractive effect if the excitatory conductance is not small compared to the inhibitory conductance. Therefore regulating a cell's passive membrane conductance—for instance, via massive feedback—is not an adequate mechanism for normalizing or scaling its output.