The PC pathway conveys both chromatic and achromatic information, with
PC neurons being more responsive to chromatic (L−M) than to
achromatic (L+M) stimuli. In considering the evolution of color vision, it
has been suggested that the dynamic range of chromatic PC-pathway
processing is tuned to the chromatic content of the natural environment.
Anomalous trichromats, with reduced separation of their L- and M-cone
spectral sensitivities, have diminished chromatic input to PC-pathway
cells. Dichromats, with absent L or M cones, should have no chromatic
input to PC-pathway cells. Therefore, the PC-pathway dynamic range of
color defectives should be released from any constraint imposed by the
chromatic environment. Here we ask whether this results in compensatory
enhancement of achromatic PC-pathway processing in color defectives. This
study employed a psychophysical method designed to isolate PC-pathway
processing using achromatic stimuli. In a pulsed-pedestal condition, a
four-square stimulus array appeared within a uniform surround. During a
trial, one of the test squares differed from the other three, and the
observer's task was to choose the square that was different. A
four-alternative, forced-choice method was used to determine thresholds as
a function of the contrast of the four-square array to the surround. Seven
color defective and four normal observers participated. Results showed no
systematic differences between normals and color defectives. There was no
enhancement of achromatic processing as compensation for reduced chromatic
processing in the PC-pathway system in color defectives. From
physiological recordings, PC-pathway achromatic contrast gains of
dichromatic and trichromatic New World primates and trichromatic Old World
macaques have also been shown to be similar to each other. Our study and
the animal studies imply that PC-pathway contrast gain parameters were
regulated by factors other than the environmental chromaticity gamut, and
may have arisen in a nontrichromatic common ancestor to both Old and New
World primates.