It is widely accepted that human color vision is based
on two types of cone-opponent mechanism, one differencing
L and M cone types (loosely termed “red–green”),
and the other differencing S with the L and M cones (loosely
termed “blue–yellow”). The traditional
view of the early processing of human color vision suggests
that each of these cone-opponent mechanisms respond in
a bipolar fashion to signal two opponent colors (red vs.
green, blue vs. yellow). An alternative possibility is
that each cone-opponent response, as well as the luminance
response, is rectified, so producing separable signals
for each pole (red, green, blue, yellow, light, and dark).
In this study, we use psychophysical noise masking to determine
whether the rectified model applies to detection by the
postreceptoral mechanisms. We measured the contrast-detection
thresholds of six test stimuli (red, green, blue, yellow,
light, and dark), corresponding to the two poles of each
of the three postreceptoral mechanisms. For each test,
we determined whether noise presented to the cross pole
had the same masking effect as noise presented to the same
pole (e.g. comparing masking of luminance increments by
luminance decrement noise (cross pole) and luminance increment
noise (same pole)). To avoid stimulus cancellation, the
test and mask were presented asynchronously in a “sandwich”
arrangement (mask-test-mask). For the six test stimuli,
we observed that noise masks presented to the cross pole
did not raise the detection thresholds of the test, whereas
noise presented to the same pole produced a substantial
masking. This result suggests that each color signal (red,
green, blue, and yellow) and luminance signal (light and
dark) is subserved by a separable mechanism. We suggest
that the cone-opponent and luminance mechanisms have similar
physiological bases, since a functional separation of the
processing of cone increments and cone decrements could
underlie both the separation of the luminance system into
ON and OFF pathways as well as the splitting of the cone-opponent
mechanisms into separable color poles.
