In some experiments, optical responses were evoked by electrical stimulation via a bipolar concentric electrode (outer diameter , inner diameter ; FHC, Bowdoin, Maine). Current pulses (0.5 ms duration) were generated by a Grass S88 stimulator (Grass Products). In other experiments, optical responses were evoked by photostimulating pyramidal neurons in slices from line 18 Thy1-ChR2 mice.24 When characterizing light-evoked responses in multiple cell types with whole-cell patch clamp recordings (Fig. 2), slices were excited with wide-field illumination (465 to 495 nm, 1.05 mW) from a mercury arc lamp under a objective. In these experiments, a potassium gluconate intracellular solution was used, containing (in mM); 130 K-Gluconate, 10 KOH, 10 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid, 4 , 0.4 , 5 EGTA, 5 Disodium Phosphocreatine, 2.5 (290 to 295 mOsm, pH 7.25). Patch clamp recordings were made with pipettes (5 to 6 MΩ) pulled with a vertical puller (Narishige, Japan).