AUTHOR=Cormenzana Méndez Iñaki , Martín Andrés , Charmichael Teaire L. , Jacob Mellina M. , Lacerda Eliza M. C. B. , Gomes Bruno D. , Fitzgerald Malinda E. C. , Ventura Dora F. , Silveira Luiz C. L. , O'Donell Beatriz M. , Souza Givago S. 

TITLE=Color Discrimination Is Affected by Modulation of Luminance Noise in Pseudoisochromatic Stimuli

JOURNAL=Frontiers in Psychology

VOLUME=Volume 7 - 2016

YEAR=2016

URL=https://www.frontiersin.org/journals/psychology/articles/10.3389/fpsyg.2016.01006

DOI=10.3389/fpsyg.2016.01006

ISSN=1664-1078

ABSTRACT=Pseudoisochromatic stimuli have been widely used to evaluate colour discrimination and to identify colour vision deficits. Luminance noise is one of the stimulus parameters used to ensure that subject’s response is due to their ability to discriminate target stimulus from the background based solely on the hue between the colours that compose such stimuli. We studied the influence of contrast modulation of the stimulus luminance noise on threshold and reaction time colour discrimination. We evaluated colour discrimination thresholds using the Cambridge Colour Test (CCT) at six different stimulus mean luminances. Each mean luminance condition was tested using two protocols: constant absolute difference between maximum and minimum luminance of the luminance noise (constant delta protocol, CDP), and constant contrast modulation of the luminance noise (constant contrast protocol, CCP). MacAdam ellipses were fitted to the colour discrimination thresholds in the CIE 1976 colour space to quantify the colour discrimination ellipses at threshold level. The same CDP and CCP protocols were applied in the experiment measuring RTs at three levels of stimulus mean luminance. The colour threshold measurements show that for the CDP, ellipse areas decreased as a function of the mean luminance and they were significantly larger at the two lowest mean luminances, 10 cd/m2 and 13 cd/m2, compared to the highest one, 25 cd/m2. For the CCP, the ellipses areas also decreased as a function of the mean luminance, but there was no significant difference between ellipses areas estimated at six stimulus mean luminances. The exponent of the decrease of ellipse areas as a function of stimulus mean luminance was steeper in the CDP than CCP. Further, reaction time increased linearly with the reciprocal of the length of the chromatic vectors varying along the four chromatic half-axes. It decreased as a function of stimulus mean luminance in the CDP but not in the CCP. The findings indicated that visual performance using pseudoisochromatic stimuli was dependent on the Weber’s contrast of the luminance noise. Low Weber's contrast in the luminance noise is suggested to have a reduced effect on chromatic information and, hence, facilitate desegregation of the hue-defined target from the background.