Contrast responses of S+ and S− cells. The average amplitude of the modulated response of S+ cells (n = 34; gray circles) and S− cells (n = 18; black circles) to S-cone modulation. The stimulus was a drifting grating of preferred spatial frequency, or modulation of a spatially uniform field, at ∼5 Hz. Contrast is specified relative to the maximum attainable on the monitor; 1.0 corresponds to ∼0.85 S-cone contrast. Error bars (sometimes smaller than the data point) report SEMs.