Figure 4.

A broadening of inhibitory tuning primarily contributes to the contrast-dependent sharpening of OS. A, Average normalized excitatory and inhibitory tuning curves at the two different contrasts (25% and near 100%). Bar, SE; n = 14. B, Top, Simulated excitatory and inhibitory synaptic conductance waveforms (1 cycle). Bottom, Membrane potential response generated by the integrate-and-fire neuron model when the evoked synaptic conductances shown on top were integrated. C, Tuning profiles of excitation and inhibition at 25 and 100% contrast applied in our model. Ge, excitation. Gi, inhibition. D, Tuning curves of PSP generated by our model. Black, 25% contrast. Red, 100% contrast. Dash, 100% contrast but with the inhibitory tuning profile prevented from broadening (i.e., it is the same as the one at 25% contrast). The applied strength of synaptic conductance at different contrasts was according to the tuning curve shown in C. Right, Normalized PSP tuning curves. E, Left, Average subthreshold V_m responses of an example cell recorded under the current-clamp mode. Spikes were removed by a 15 ms median filter before averaging. Each trace is 1.5 s. Right, Polar graphs of evoke spike rate and peak depolarization voltage. F, gOSI of peak V_m response at 25% and near 100% contrast. Data points for the same cell are connected with a line. **p < 0.01, paired t test; n = 8. G, Left, Average peak V_m responses at the preferred and orthogonal orientations from our current-clamp recording results. n = 8. Right, V_m responses derived by feeding the experimentally determined excitatory and inhibitory conductances into the neuron model. White column represents the V_m response generated when the inhibitory tuning was prevented from broadening (i.e., the recorded inhibitory responses at near 100% contrast are scaled according to the average inhibitory tuning curve at 25% contrast). *p < 0.05; **p < 0.01, paired t test. n = 14.