Animals were paralyzed with pancuronium bromide (1.5 mg/kg induction, 0.2 mg/kg/hr maintenance) and artificially ventilated through a tracheal cannula to maintain end tidal CO2 at 3.6%–4.0%). The thoracic vertebrae were suspended and a bilateral pneumothoracotomy was performed for recording stability. Core temperature was maintained at 37.8°C. Anesthetic

depth was assessed by EEG and heart rate, and the anesthetic infusion rate was adjusted accordingly. All procedures were approved by the Northwestern University Animal Care and Use Committee. The nictitating membranes were retracted with 2.5% phenylephrine hydrochloride and pupils dilated with 1% atropine. Contact lenses and external corrective lenses focused the retina on a computer

monitor (ViewSonic, A-1210477 in vitro Walnut, CA) ∼48 cm distant (refresh rate 100 Hz; mean luminance 20 cd/m2). Visual stimuli were generated with the Psychophysics Toolbox (Brainard, 1997 and Pelli, 1997) for Matlab (Mathworks, Natick, MA). Sparse noise stimuli for receptive field mapping consisted of 0.5° × 0.5° pixels over an extent of 5° × 5°. Drifting grating stimuli (2 or 4 cycles/s) were presented at the optimal spatial frequency (0.3–1.2 cyc/deg), 13 orientations and 5 contrasts (2%–32%). For LGN VX 809 recordings, grating size and position were set to overlap the receptive fields of all LGN neurons under study. For V1 recordings, high contrast (64%) gratings at optimal spatial frequency and size were used to determine preferred orientation and receptive field location. Flashed gratings at 5 phases were used to determined optimal phase at 6 different orientations (see Figure S1A). We presented flashed gratings at 6 orientations and 4 contrasts (4%– 32%)

for 23 cells, and a shorter stimulus set (2 orientations 4 contrasts) for 12 cells. Whole-cell current clamp recordings were obtained with glass-electrodes (Sutter Instrument, Novato, CA) filled with standard K-gluconate mafosfamide solution with blind-patch techniques. Electrode impedance ranged from 7–12 MΩ. The pipette was positioned such that its tip, after ∼600 μm travel through the cortex, was within 1 mm of the metal electrode used for cortical inactivation. Warm agarose (3%) was poured over the craniotomy to dampen cortical pulsations. Signals were low-pass filtered and digitized at 4,096 samples/s. For a cell to be included in the data set, we required that its resting potential at break-in be more hyperpolarized than −50 mV, and that the resting potential be stable over the course of the recording (Figure S1B). Electrical stimuli (300–400 μA, electrode negative; 200 μs) were delivered to the cortex with low impedance (<2 MΩ) epoxy-coated tungsten electrodes (A-M Systems, Carlsborg, WA) placed at a distance of less than 1 mm from the patch pipette and ∼400 μm below the cortical surface. Such stimulation creates a short window (∼50–100 ms) during which cortical spiking activity is silenced (Chung and Ferster, 1998), but LGN activity is spared.