The hippocampal CA3 slice culture causes a power-law distributed neuronal activity spontaneously. In computer simulations, we reproduce three stochastic properties (synchrony size, spike-count rate and inter-spike interval) of the spontaneous activity observed from the CA3 slice cultures. When the background activity of a neuron, that is not the activity driven by synaptic inputs, is uniform in every neuron, the stochastic properties are not reproduced at the same time. However, when the background activity of a neuron differs in every neuron, the stochastic properties are simultaneously reproduced. These results suggest that the diverse background activities hide in the spontaneous activity of hippocampal CA3 slice culture. It has been suggested that a neuronal network causing power-law distributed neuronal activity optimizes input/output processing. The hippocampal CA3 slice culture causing power-law spontaneous activity may optimize input/output processing on the diverse background activities.