INPUT SYNCHRONY AND THE IRREGULAR FIRING OF CORTICAL-NEURONS

Cortical neurons in the waking brain fire highly irregular, seemingly random, spike trains in response to constant sensory stimulation, wher eas in vitro they fire regularly in response to constant current injec tion. To test whether, as has been suggested, this high in vivo variab ility could be due to the postsynaptic currents generated by independe nt synaptic inputs, we injected synthetic synaptic current into neocor tical neurons in brain slices. We report that independent inputs canno t account for this high variability, but this variability can be expla ined by a simple alternative model of the synaptic drive in which inpu ts arrive synchronously. Our results suggest that synchrony may be imp ortant in the neural code by providing a means for encoding signals wi th high temporal fidelity over a population of neurons.