Bipolar or rectified chromatic detection mechanisms?

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 "r ed-green"), and the other differencing S with the L and M cones (loosely te rmed "blue-yellow"). The traditional view of the early processing of human color vision suggests that each of these cone-opponent mechanisms respond i n 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 signa ls for each pole (red, green, blue, yellow, light, and dark). In this study , we use psychophysical noise masking to determine whether the rectified mo del applies to detection by the postreceptoral mechanisms. We measured the contrast-detection thresholds of six lest stimuli (red, green, blue, yellow , light, and dark), corresponding to the two poles of each of the three pos treceptoral mechanisms. For each rest, we determined whether noise presente d to the cross pole had the same masking effect as noise presented to the s ame pole (e.g. comparing masking of luminance increments by luminance decre ment noise (cross pole) and luminance increment noise (same pole)). To avoi d stimulus cancellation, the test and mask were presented asynchronously in a "sandwich" arrangement (mask-test-mask). For the six rest stimuli, we ob served that noise masks presented to the cross pole did not raise the detec tion thresholds of the test, whereas noise presented to the same pole produ ced a substantial masking. This result suggests that each color signal (red , green, blue, and yellow) and luminance signal (light and dark) is subserv ed by a separable mechanism. We suggest that the cone-opponent and luminanc e mechanisms have similar physiological bases, since a functional separatio n of the processing of cone increments and cone decrements could underlie b oth the separation of the luminance system into ON and OFF pathways as well ;is the splitting of the cone-opponent mechanisms into separable color pole s.