Lack of effect of optical activation in animals treated with control virus. Optical stimulation (5 mW, 10 Hz) in the IL of animals injected with a control virus that lacked the ChR2 sequence (rAAV5/CaMKIIa-EYFP) had no effect on background (A) and evoked (B) activity of IL pyramidal cells (n = 5 neurons). (A) Left, Peristimulus time histograms (PSTHs, bin width, 1 s) show number of action potentials (spikes/s) before, during and after laser stimulation in an individual neuron. Right, Bar histograms show summary data (means ± SE) for the sample of neurons tested (n = 5). (B) Left, PSTHs show responses of an individual neuron to innocuous (Innoc.) and noxious (Nox.) stimuli (horizontal bars) before, during and after laser stimulation. Right, Bar histograms show summary data (means ± SE) for the sample of neurons tested (n = 5). Background activity preceding each stimulus has been subtracted from the total activity during stimulation to obtain the net evoked response.