The relationship between spectral sensitivity and spatial sensitivity for the primate r-g X-channel.

Which visual channel detects high spatial frequencies during careful fixation? Color vision models based on psychophysical data contradict electrophysiological results. According to electrophysiology, the channel which mediates foveal acuity originates in the small, tonic color-opponent r-g units of the X-cell pathway. However, psychophysical models assign acuity to the V lambda channel because when acuity is used as a criterion for equating luminosity it is additive. In all opponent-color models the r-g channel is subadditive and hence is excluded from mediating acuity. We show that the r-g channel adds cone signals for high spatial frequencies and subtracts them for low, and conclude that the major achromatic channel for human foveal vision originates within the r-g color-opponent channel. Quantitative analysis makes explicit the interaction between the spatial and spectral variables for the simple-opponent cells which predominate in primate foveal vision.