Irregular firing pattern in excitatory–inhibitory networks driven by background input current which is heterogeneous and fluctuating. Neurons numbered from 1 to 800 are excitatory, while neurons numbered from 801 to 1,200 are inhibitory. mEb = 1.7 μA/cm2, mEs = 0 μA/cm2, αEb = 0.5, βEb = 0.1, mIb = 0.9 μA/cm2, mIs = 0 μA/cm2, αIb = 0.5, βIb = 0.1