Changes in induced hues at low luminance and following dark adaptation suggest rod-cone interactions may differ for luminance increments and decrements

Abstract

Color contrast describes the influence of one color on the perception of colors in neighboring areas. This study addressed two issues: (1) the accurate representation of the color changes; (ii) the underlying visual mechanisms. Observers matched the hue that was induced in a neutral square when it was set in one of four standard colored surrounds: "red" (+L(-M) relative to neutral), "green" (-L(+M)), "purple" (+S), and "yellow" (-S). The standard and matching displays were viewed haploscopically. The standard neutral square was either a luminance increment, or decrement, both of which appeared the complementary color to the surrounds in which they were inset. In Experiment 1, the surround luminance in each eye's display was either equal, at 18 cd x m(-2), or the match surround luminance was reduced to 2.5 cd x m(-2). The matches with equal surround luminances could be represented as vector shifts in a logarithmic MacLeod-Boynton (r, b) chromaticity diagram, as described previously (Shepherd, 1997, 1999). The low luminance matches were, however, displaced further from neutral, as if larger chromatic differences were needed. The precise direction of the displacements differed for luminance increments and decrements: the red, green and yellow decrement matches were also displaced vertically downwards in the MacLeod-Boynton diagram. In Experiment 2, dark-adapting before setting repeat color matches displaced the decrement matches vertically, but did not affect the increment matches. Thus, rod intrusion in S-cone pathways may have boosted the S-cone signal for the lowest luminance decrement matches in Experiment 1 and account for the vertical shift in MacLeod-Boynton co-ordinates. The distinct pattern of displacements for low luminance increments and decrements may be explained if the match is set at a cone-opponent, rather than a cone contrast, site and if rod signals have an input only to S-cone decrement, perhaps S-OFF, pathways.

Vision under mesopic and scotopic illumination

“…In addition to affecting the perceived hue, brightness, and saturation, rod activity can alter cone-mediated chromatic discrimination ( Stabell and Stabell, 1977 ; Nagy and Doyal, 1993 ; Knight et al, 1998 , 2001 ; Cao et al, 2008b ; Shepherd and Wyatt, 2008 ; Volbrecht et al, 2011 ). In comparison to measurements under photopic illumination, chromatic sensitivity measured under mesopic illuminations is differentially altered in the areas of the protan, deutan, and tritan confusion lines, with the greatest sensitivity loss near the tritan axis, but in general, the magnitude of the rod intrusion is small when measured with luminance contrast masking techniques ( Walkey et al, 2001 ).…”

Links between global and local shape perception, coloured backgrounds, colour discrimination, and non-verbal IQ

“…The background colours that were chosen for the present study were selected to differentially activate the cone opponent pathways early in the visual system (cardinal colour directions). Care was taken to ensure that the colours selected for the experiment were equally saturated (after Shepherd, 1997b;Shepherd and Wyatt;2008) and of equal measured luminance. The saturation was limited only by the colour gamut available on a CRT monitor at a reasonable luminance (20 cdm -2 ), which was five times that used by Michimata et al (1999).…”

“…The CRT monitor was the only source of light in an otherwise dark room. Rod intrusion cannot be ruled out with these screen luminances, however, Shepherd (1994) and Shepherd and Wyatt (2008) have previously used similar screen colours with luminances ranging from 58 cdm -2 to 2.5 cdm -2 and found, in a colour matching task, rod intrusion to become apparent only below 4.5 to 7.0 cd m -2 . This is returned to in the Discussion.…”

“…The violet was also the most saturated that could be displayed at 20 cdm -2 that lay on a tritan line running through the neutral in the MB diagram (a vertical line); the mustard was selected to be equal distance from the neutral in the MB diagram when the b axis is logarithmically transformed (log10). These colours were selected so that they would appear equally saturated for the pink and cyan, and violet and mustard, colours (see the vector model for colour contrast reported in Shepherd, 1997b;Shepherd and Wyatt, 2008 for further details). The black elements of the mask had a luminance of 2 cdm -2 , the holes in the mask had the same luminance and chromaticity as the coloured backgrounds.…”

Rod hue biases produced on CRT displays

“…Studies of rod hue biases have not revealed additional neural pathways for rod influence on blue/yellow hue appearance. However, some studies of chromatic discrimination have suggested there may be differences in rod influence on S-on and S-off pathways [40,41].…”