3 kHz from the output JQ1 of the Multiclamp 700B amplifier. Calculations used look-up tables for the voltage dependence of τ(Vm) and n∞(Vm) and were completed in 40 μs. I(t) was then updated with 8 pA resolution, low-pass filtered at 10 kHz and injected into the cell via the Multiclamp 700B amplifier. Improper bridge balance (e.g., >20 MΩ or changed by  >∼2MΩ) caused strong oscillations that in some cases even triggered spikes. Only recordings without such oscillations were analyzed. Outside-out patches were pulled from identified OFF Alpha ganglion

cells in order to study voltage-gated currents. After establishing a seal of >5 GΩ on the soma and correcting for the pipette capacitance, the cell membrane was disrupted to establish a whole-cell configuration with Vhold = −60mV. The pipette was slowly removed from the cell using the manipulator’s piezo drives, while

constantly checking Rs, Rin, and capacitance. After reaching >100 MΩ of Rs (from originally 10–20 MΩ), the pipette was quickly pulled away from the cell by several hundred micrometers. Initial membrane capacitance and Rin were recorded from the membrane patch. Cases where Vm was positive VE-822 mw to −30 mV or when the ratio of Rin to Rs was <10 were not studied further. Voltage-clamp recordings were performed without Rs compensation at 10 kHz with a 4 kHz Bessel filter. Capacitance artifacts and leak currents were measured during the voltage-clamp recordings with 5 mV steps from Vholds and used to record changes in membrane parameters. Recordings with roughly constant leak current were used for analysis. Capacitance artifacts were fitted with Thymidine kinase a double exponential

function and together with the leak current subtracted from the current traces. Because of imperfect fits of the first two recorded points in the capacitance artifact, the first 0.2 ms after a voltage step were omitted. We thank Mania Kupershtok for technical assistance and Dr. Josh Singer for comments on the manuscript. Supported by a Research to Prevent Blindness Career Development award, an Alfred P. Sloan Foundation fellowship and the National Institutes of Health (EY14454; EY14454-S1; core grant EY07003). “
“The activity of even a single thalamic axon can generate robust, widespread inhibition in somatosensory cortex (Swadlow and Gusev, 2000 and Swadlow and Gusev, 2002). This is not because thalamic afferents are inhibitory—they release the excitatory transmitter glutamate (Kharazia and Weinberg, 1994)—but because they can efficiently fire cortical inhibitory neurons through one of the cortex’s most powerful synapses (Cruikshank et al., 2007, Gabernet et al., 2005, Hull et al., 2009, Porter et al., 2001, Swadlow and Gusev, 2000 and Swadlow and Gusev, 2002). These GABAergic interneurons in turn synapse onto local excitatory neurons, creating a robust feedforward inhibitory circuit (Gabernet et al., 2005, Inoue and Imoto, 2006 and Sun et al.