(A) Confocal images showing NpHR-YFP expression (green) was confined to SOM INs (red) in layer 4 barrel cortex of an adult SOM-Cre mouse. (B) Yellow light induced a large amplitude hyperpolarization in a layer 4 NpHR-YFP-expressing SOM interneuron. (C) Summary plot of the membrane potential of layer 4 NpHR-YFP-expressing SOM INs measured in the absence or presence of yellow light. (D) Yellow light reduced the spiking of a layer 4 NpHR-YFP-expressing SOM interneuron during a thalamus-triggered UP state due to the hyperpolarization induced by photostimulation. (E) Population results showing that yellow light nearly completely suppressed spiking activity of layer 4 NpHR-YFP-expressing SOM INs during UP states. (F) Yellow light increased spiking of a layer 4 FS interneuron during a thalamus-triggered UP state. (G) Population results showing that yellow light significantly increased spiking of FS INs during UP states. (H) Yellow light decreased spikes of a layer 4 PN during a thalamus-triggered UP state. (I) Population results showing that yellow light significantly decreased spiking of PNs during UP states. In B, D, F, and H, yellow bars represent photostimulation (591 nm, 1 s, 5 mW) and arrows represent thalamic stimulation. In C, E, G and I, data from the same cell are connected by a line. Open symbols represent the mean values for individual cells, and filled symbols represent mean values across cells. Error bars indicate s.e.m. ** p < 0.01, *** p < 0.001, paired t test.