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Input synchrony and the irregular firing of cortical neurons

Abstract

Cortical neurons in the waking brain fire highly irregular, seemingly random, spike trains in response to constant sensory stimulation, whereas in vitro they fire regularly in response to constant current injection. To test whether, as has been suggested, this high in vivo variability could be due to the postsynaptic currents generated by independent synaptic inputs, we injected synthetic synaptic current into neocortical neurons in brain slices. We report that independent inputs cannot account for this high variability, but this variability can be explained by a simple alternative model of the synaptic drive in which inputs arrive synchronously. Our results suggest that synchrony may be important in the neural code by providing a means for encoding signals with high temporal fidelity over a population of neurons.

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Figure 1: Spontaneous synaptic event recorded in a layer 2/3 cortical neuron.
Figure 2: Fluctuating currents affect the fine structure of the spike train but not the mean rate.
Figure 3: Variability in response to mixed excitatory and inhibitory input is less than in vivo.
Figure 4: Responses elicited by hypertonic solution evoked increases in the rate of miniature EPSC release.
Figure 5: Input synchrony yields in vivo variability.

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Acknowledgements

We thank G. Buracas for the MT data and L. Dobrunz, E. Huang, K. Miller, P. Latham, T. Troyer and K. Zhang for comments. This work was supported by the Howard Hughes Medical Institute (C.F.S.), National Institutes of Health grant NS 12961 (C.F.S.) and the Sloan Center for Theoretical Neurobiology (A.M.Z.).

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Correspondence to Anthony M. Zador.

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Stevens, C., Zador, A. Input synchrony and the irregular firing of cortical neurons. Nat Neurosci 1, 210–217 (1998). https://doi.org/10.1038/659

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