Characteristics of the UP-DOWN states and the protocol for injection of artificial EPSPs. A, Simultaneous extracellular and intracellular recording of the UP and DOWN states of activity in layer 5 of the prefrontal cortex. The UP state is associated with the arrival of a barrage of EPSPs that depolarize the membrane potential, increase the membrane conductance, and increase the membrane potential variance. MU, Multiple unit. B, Artificial EPSPs of random amplitude conductance (limited by steps of 2 nS; see Materials and Methods) were injected at a rate of 10 Hz and then separated into those that occurred during the UP and DOWN states. C, Expansion of the transition from DOWN to UP for detail. D, Overlay of responses of the cell to the 30 nS EPSP. Three measures were derived: probability that the EPSP will generate a spike, latency to onset of the first action potential, and finally the spike jitter, measured as the SD of the spike latency.