Responses of S+, S−, and P-cells to stimulus modulation in a three-dimensional color space. A, S+ cell. B, S− cell. C, P-cell. Same cells as in Figure 1. The top panels show the amplitude of response (the Fourier harmonic of the discharge at the temporal frequency of stimulation) as a function of angle (azimuth) in the isoluminant plane (elevation 0°). The middle panels show responses as a function of angle (elevation) in the plane formed by the L–M axis (azimuth 0°) and the achromatic axis (elevation 90°). Bottom panels show responses as a function of angle (elevation) in the plane formed by the S-cone axis (azimuth 90°) and the achromatic axis. The sign of the response is determined by its phase. SEMs are all smaller than the data points. The smooth lines show for each cell the best-fitting prediction of a linear model of cone summation described in Materials and Methods. Insets in each panel show the average responses to one cycle of modulation for stimuli modulated along 0° (left) and 90° (right) in the relevant chromatic plane (calibration in A, 75 spikes/s; B, 160 spikes/s; C, 115 spikes/s). The stimulus was a spatially uniform field (diameter of 8°) modulated sinusoidally in time at 5 Hz.