Fig. 2.

Power law behavior of the voltage-rate relation causes the rate tuning to be contrast invariant. The rate tuning curves are shown for different levels of noise: A, ς = 1 mV; B, ς = 3 mV; C, ς = 6 mV. Each panel shows the rate response to a Gaussian mean voltage, with half-width at half-maximal of 30°, with different maximum voltage: 5 mV (solid line), 7 mV (long dashed line), 10 mV (short dashed line), and 15 mV (dotted line). Theinsets show the corresponding normalized rate tuning curves. Note that in A the response to 5 and 7 mV is so small as to be indistinguishable from 0 Hz. Other parameters are as in Figure 1. Only for a voltage noise ς = 3 mV are the tuning curves for the firing rate contrast invariant. For ς = 1 mV, the firing rate exceeds the region in which the power law holds, causing broadening of the tuning curve for larger contrast. For large voltage fluctuations, ς = 6 mV, the rate does not go to 0 Hz at the null orientation, resulting in contrast dependence at the null orientation for the normalized tuning curves.