To put it another way, one does not see reddish greens or yellowish blues. We do see yellowish greens, bluish reds, yellowish reds etc.
Hering also observed that there was a distinct pattern to the color of the after images we see. For example if one looks at a unique red patch for about a minute and then switches the gaze to a homogeneous white area they will see a greenish patch in the white area.
Hering hypothesized that trichromatic signals from the cones fed into subsequent neural stages and exhibited two major opponent classes of processing. 1. Spectrally opponent processes which were red vs. green and yellow vs. blue. 2. Spectrally non-opponent processes which was black vs. white. This opponent process model lay relatively dormant for many years until a pair of visual scientists working at Eastman Kodak at the time, conceived of a method for quantitatively measuring the opponent processes responses. Leo Hurvich and Dorothea Jameson invented the hue cancellation method to psychophysically evaluate the opponent processing nature of color vision.
Due in large measure to the efforts of Hurvich and Jameson the opponent processes theory attained a central position shared with the the trichromatic theory. One very fortuitous scientific event to that also took place in the 1950s was the discovery of electrophysiological responses that emulated opponent processing. Consequently, with the quantitative data provided by the psychophysics and direct neurophysiological responses provided by electrophysiology opponent processing is no longer questioned.
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