Figure 4. Univariance of cone inputs to midget and parasol cells.

A, For each RGC, four cone stimulus regions were selected (colored outlines, top panels), and responses were recorded to a series of contrast steps. Bottom panels show the average number of spikes recorded during stimulation as a function of stimulus contrast (colored points). This function was scaled separately for each cone along the contrast axis so as to align the four functions (see Experimental Procedures). Black point represents baseline response with no stimulus. Response time courses corresponding to groups of points with similar response amplitudes are shown as colored traces. Vertical scale bars for the time course traces correspond to 10 Hz for the ON midget RGC and 40 Hz for the other RGCs. Scale bars for receptive field maps, 15 μm. B, For each of 47 RGCs tested (13 OFF Parasols, 22 OFF Midgets, 12 ON Midgets), the residual deviation (see Experimental Procedures) after scaling of the individual cone contrast response functions is plotted as a percentage of the maximum response amplitude. Filled symbols are the RGCs shown in (A).