Perceptual Coherence : Hearing and Seeing

The Perception of Quality: Visual Color 323
test and surround will also remain the same. Thus, the constancy of relative
ratios should lead to color constancy.
Kraft and Brainard (1999) equalized the local surround by using different
combinations of illumination and surface reflectance. The logic of the
experiments is that if we illuminate a gray paper with two different illuminants
but somehow balance the excitation due to illumination × reflectance,
observers will not be able to achieve color constancy because they cannot
untangle the illumination from the surface reflectance. In the first combination,
the background wall was gray cardboard, and the illumination in the
entire chamber was a neutral light producing a uniform reflectance spectrum
(figure 7.8, local surround A). In the second combination, the background
wall was blue and the illumination in the entire chamber was
orange-red, which also produced a neutral uniform reflectance spectrum
from the back wall (local surround B). Thus, the reflection from the back
wall surrounding the test patch was identical in the two conditions, although
the light reflected from the test patch and all of the other surfaces in
the chamber differed. (Note that the difference between the local surround
condition and the minimizing control condition described in the above
paragraph is the inclusion of the colored surfaces and objects in the local
surround condition. In both conditions, the local surround is matched.) If
the local surround is the only cue for constancy, then equalizing the local
surround should eliminate any constancy. But if the local surround is but
one of several cues and if observers can use the more orange appearance of
other objects in the chamber to infer the illuminant, then constancy should
simply decrease. In fact, constancy did decrease to roughly 0.50, although
this is still relatively good.
The second part of the experiment investigated the role of the global surround.
The notion parallels that for the local surround, but here the three
cone responses to the test patch are compared to the three cone responses
averaged across the entire scene. On this basis, Kraft and Brainard (1999)
compared neutral illumination of the chamber (spatial mean A) to pale red
illumination that equated the average cone responses across the entire
scene (spatial mean B). In this condition, if subjects are judging the test
patch in terms of the entire scene, then constancy should be zero, but if they
are making use of other cues, then constancy should simply decrease. The
constancy did decrease to 0.40, demonstrating that a difference in the
global surround is not the sole mechanism for constancy.
The third part of the experiment investigated the role of absorption from
the most intense region of the scene, the white areas described previously.
In one condition, the yellow frame was illuminated by a neutral light
(maximum flux A), while in the second condition a magenta frame was
illuminated by a yellow light (maximum flux B). For both conditions, the
background was the identical dark gray cardboard, and all the other surfaces