current steps and voltage noise from transient synaptic conductances contributes to the frequency of spike output. If the opening of a tonic inhibitory conductance occurs in combination with an increase in the variability of driving excitatory input (Mitchell and Silver, 2003) or if a noisy barrage of mixed excitatory and inhibitory synaptic conductances (an increase in background synaptic activity) is added to the driving input (Chance et al., 2002), the slope of the input-output relationship of individual neurons can be changed.
