Fig. 12.

Effect of changes in membrane potential on contrast–response function. A, Average contrast–response function of action potential discharges in cortical neurons (n = 12) at two different membrane potentials. Hyperpolarization of the membrane potential by an average of 7.9 mV (n = 12) results in a decrease in the response of the neuron at each level of contrast. Fitting the two sets of data points with a modified Hill equation revealed that the hyperpolarization resulted in a decrease in the measure of maximal response (R_max), a decrease in slope (s) of the contrast response function, and an increase in the contrast that gives 50% of the maximal spike response (C₅₀). Two sequences of increasing contrast were used: from 2.5 to 40% and from 5 to 80%. We combined the data of these two different cell groups and plotted contrast in octaves. B, Plot of R_max in control versus after hyperpolarization for all cells tested. Note the shift in the R_max to the bottom right, indicating consistent shift to lower levels with hyperpolarization. C, Plot ofC₅₀ in control versus with hyperpolarization. With hyperpolarization this is shifted to thetop left, indicating a consistent increase in the contrast value that gave a half-maximal response. D,Plot of the slope of the contrast–response function before and after hyperpolarization reveals a slight but nonsignificant change in the slope of the contrast–response function with hyperpolarization.