A, The firing rate of an example YFP-16 neuron during 1 s steps of depolarizing somatic current injection of varying amplitudes (inset). Neuronal firing rates, averaged over the 1 s step, were linearly related to injected current. B, Synaptic stimulation for 1 s drives increases in firing rate in the same neuron shown in A. Somatic current injection was used to maintain a stable baseline firing rate of ~5 Hz. C, The postsynaptic firing rate in this neuron at the two synaptic stimulation rates shown (20 and 100 Hz). D, Linear input to firing rate relationship for the neuron shown in C. The slope is less than 1, indicating the linear synaptic summation of input currents over time. E, Summary of goodness of linear fits in response to stimuli that ranged from 20 to 150 Hz rates in the population of YFP-16 and GIN neurons. F, Summary of firing response gains across the same population. Of the 8 GIN and 10 YFP-16 neurons tested, only one fired action potentials with every presynaptic stimulus (gain = 1).