### Hue Cancellation Method

Hurvich and Jameson reasoned that when red and green a mixed together the produce yellow, not reddish green. Further when yellow and blue are mixed together they produce white and not yellowish blue. Red and green cancel each other as do yellow and blue. They further reasoned that if one started with a color such as bluish green it should be possible to mix this color with a unique yellow to cancel out the blue content leaving only green.
This is the basic idea behind the hue cancellation technique.
The first step was to identify for each observer their unique red, green, yellow and blue. Then monochromatic lights were presented one at a time and the observer's task was to cancel out one of the perceived hues if the wavelength was not already a unique hue. So at the shortest wavelengths the observer would add a certain amount of green to cancel out the redness. Then in the middle wavelengths unique red was used to cancel green, unique blue was used to cancel yellow and in the longer wavelengths unique green was used to cancel red.

The amount of the cancellation color used was taken as an indicator of the strength of the cancelled hue. These data were then converted to produce the opponent processing curves. Although it is mentioned in the figure, it is worth repeating that the sign (plus or minus) attributed to these functions is arbitrary and chosen to represent the opposite nature of the opponent processes.

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