The brain operates through a coordinated interplay of numerous neurons, yet little is known about the collective behavior of individual neurons embedded in a huge network. We used large-scale optical recordings to address synaptic integration in hundreds of neurons. In hippocampal slice cultures bolus-loaded with Ca2+ fluorophores, we stimulated the Schaffer collaterals and monitored the aggregate presynaptic activity from the stratum radiatum and individual postsynaptic spikes from the CA1 stratum pyramidale. Single neurons responded to varying synaptic inputs with unreliable spikes, but at the population level, the networks stably output a linear sum of synaptic inputs. Nonetheless, the network activity, even though given constant stimuli, varied from trial to trial. This variation emerged through time-varying recruitment of different neuron subsets, which were shaped by correlated background noise. We also mapped the input-frequency preference in spiking activity and found that the majority of CA1 neurons fired in response to a limited range of presynaptic firing rates (20~40 Hz), acting like a band-pass filter, although a few neurons had high pass-like or low pass-like characteristics. This frequency selectivity depended on phasic inhibitory transmission. Thus, our imaging approach enables linking single-cell behaviors to their communal dynamics, and we discovered that, even in a relatively simple CA1 circuit, neurons could be engaged in a concordant information processing.