Ratio model for suprathreshold hue-increment detection

We use psychophysical techniques to investigate the neural mechanisms subse rving suprathreshold chromatic discrimination in human vision. We address t wo questions: (1) How are the postreceptoreal detection mechanism responses combined to form suprathreshold chromatic discriminators? and (2) How do t hese discriminators contribute to color perception? We use a pedestal parad igm in which the subject is required to distinguish between a pedestal stim ulus and the same pedestal added to a chromatic increment (the test). Our s timuli are represented in a cardinal space, in which the axes express the r esponses of the three postreceptoral detection mechanisms normalized relati ve to their respective detection thresholds. In the main experiment the tes t (a hue increment) was fixed in the direction orthogonal to the pedestal i n our cardinal space. We found that, for high pedestal contrasts, the test threshold Varied proportionally with the pedestal contrast. This result sug gests the presence of a hue-increment detector dependent on the ratio of th e outputs from the red-green and blue-yellow postreceptoral detection mecha nisms. The exception to this was for pedestals and tests fixed along the ca rdinal axes. In that case detection was enhanced by direct input from the p ostreceptoral mechanism capable of detecting the test in isolation. Our res ults also indicate that discrimination in the red-green/luminance and blue- yellow/luminance planes exhibits a behavior similar to discrimination withi n the isoluminant plane. In the final experiment we observed that threshold s for hue-increment identification (e.g., selecting the bluer of two stimul i) are also governed by a ratio relationship. This finding suggests that ou r ratio-based mechanisms play an important role in color-difference percept ion. (C) 1999 Optical Society of America [S0740-3232(99)01711-1].