Noise had different effects on f–I relationships in layer 2/3 pyramidal neurons and FS interneurons. A, Voltage traces showing steady-state firing in a layer 2/3 pyramidal cell at mean currents of 0.4 nA (top) and 1.0 nA (bottom), without noise (left) and with noise (σ = 4; right). Noise clearly increased the firing rate at both current levels. B, f–I relationships of the same pyramidal neuron at multiple noise levels (σ = 0, 2, 4, and 6), showing that gain increased progressively with higher noise. C, Voltage traces showing firing in a layer 5 FS interneuron at a mean current slightly below threshold (top) and a current slightly above threshold (bottom), without noise (left) and with noise (σ = 4; right). Noise caused rapid firing at the just-subthreshold current but had little effect on the firing rate with suprathreshold current. D, f–I relationships of the same FS interneuron at multiple noise levels (σ = 0, 2, 4, and 6), showing an abrupt rate increase at threshold without noise and progressive gain reduction by noise. Inset, Average maximal (Max) gain in FS interneurons (n = 8) at three noise levels.