Spontaneously occurring UP states result in an increase in responsiveness to mimicked synaptic conductances as well as a change in slope of the input-output relation. A, Examination of the probability of different-amplitude mimicked synaptic conductances to evoke an action potential during the DOWN and UP states. The UP state is associated with an increase in responsiveness to inputs and a decrease in slope of the input-output relation. B, Normalized data from seven cells illustrating the reproducibility of these effects. The synaptic conductance for each cell was normalized such that the conductance that resulted in a 0.5 probability of action potential discharge during the DOWN state was given a value of 1. The amplitude of each synaptic conductance was randomly chosen from a distribution of 2 to 60 nS in steps of 2 nS (see Materials and Methods). Extrapolation of the graphs to 0 conductance yields an estimate of the probability of spontaneous activity generating a response (denoted by an X).