Figure 1. Overview and experimental design. a, Postreceptoral mechanisms arising from additive and opponent combination of cone signals. b, Spectral sensitivities of the L, M, and S cones and melanopsin. c, Left, Our stimuli all used a common, neutral background with the spectral power distribution shown. Right, Modulation spectra were added to the background. The L + M + S modulation is illustrated. Modulation spectra varied sinusoidally between an extreme positive (black line) and negative (red line) modulation. d, The maximal positive and negative modulations for each set of targeted photoreceptor(s) studied. Maximal contrast values are given in square brackets. Wavelength axis is as in c. e, Schematic of experimental apparatus and design. The subject viewed sinusoidal spectral modulations produced by the digital spectral integrator through an artificial pupil with a pharmacologically dilated eye. During fMRI scanning, different stimulus frequencies targeting a given class of photoreceptor(s) were presented in 12 s blocks. The sinusoidal flicker was ramped at the start and end of each block by a 3 s half-cosine. The stimulus was occasionally interrupted by an attention event.