Achromatic parvocellular contrast gain in normal and color defective observers: Implications for the evolution of color vision

Abstract

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.

Achromatic luminance contrast sensitivity in X-linked color-deficient observers: An addition to the debate

“…The results of our study support earlier studies that found enhancement in various visual functions in X-linked color-defective individuals (Jägle et al, 2006; Sharpe et al, 2006) and also contradict the only known study that addressed almost exactly the same issue (Lutze et al, 2006). In this case, achromatic CS was found to be enhanced in dichromatic observers, especially at higher spatial frequencies.…”

Color Compensation in Anomalous Trichromats Assessed with fMRI

“…Although there are increasing signs of short-term plasticity, specifically contrast adaptation, at these early stages, [54][55][56] contrast adaptation is known to be substantially stronger in the visual cortex, 36 especially within the parvocellular pathway, along which the L versus M signals are carried. 54,57 The adjustments we measured could potentially represent a very long-term form of visual plasticity, though we cannot rule out a process that acts more rapidly. Color vision adjusts to changes in the environment or observer over widely varying timescales, 58 and compensation for the color losses could similarly involve multiple timescales and processes.…”

Color compensation in anomalous trichromats assessed with fMRI

“…The emergence of compensation in the cortex would mean that the retina and LGN remain in a weakened, unadjusted state for processing the L vs M signals. While there are increasing signs of short-term plasticity, specifically contrast adaptation, at these early stages [54][55][56], contrast adaptation is known to be substantially stronger in the visual cortex [36], especially within the parvocellular pathway along which the L vs M signals are carried [54,57].…”