Color Discrimination Is Affected by Modulation of Luminance Noise in Pseudoisochromatic Stimuli
Iñaki Cormenzana MéndezAndrés MartínTeaire L. CharmichaelMellina Monteiro JacobEliza Maria da Costa Brito LacerdaBruno GomèsMalinda E.C. FitzgeraldDora Fix VenturaLuiz Carlos L. SilveiraBeatriz M. O’DonellGivago da Silva Souza
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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.Keywords:
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Response characteristics of two types of cells in the lateral geniculate nucleus of the squirrel monkey have been examined in experiments in which the luminance of a stimulus is shifted in stepped increments and decrements from an adaptation luminance. These two types of cells are found to show opposite changes in discharge frequency in response to shifts in stimulus luminance: some respond with an increase in firing rate to increases in luminance and show a decrease in response rate when luminance is decreased; others behave in the opposite fashion. The magnitude of change of the response is graded according to the amount of change of the stimulus. For any cell, the direction of change of the response for a given change of stimulus is dependent on the adaptation luminance. The total range of change of luminance over which any cell shows a good differentiation is usually not more than ±1 log unit around the adaptation luminance. An analysis of the discriminatory ability of these units as a function of adaptation luminance and the amount of change of the stimulus is presented.
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The responses to red stimuli presenting different distributions of time luminance are compared. The height of the x wave seems to be independent of time distribution of the light stimulus while the height of the b wave is greater, the greater the time of variation of the luminance. The latencies to peak of both waves are greater with a slowly rising stimulus than with a steep stimulus.
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The hue of induced colour was studied as a function of surround/test field luminance ratio using a chromatic surround and an achromatic central test field. The hue of the test field was determined by means of colour naming methods. Three inducing colours were used: blue (Wr No. 47), green (Wr No. 58), and red (Wr No. 25). The number of subjects was 9–11 in the two experiments. The luminance ratio (ranging from 0.07 to 17.1) was varied by varying the luminance of the test field (Experiment 1) or of the surround (Experiment 2). For the blue surround the results show a hue shift in accordance with the Bezold‐Brücke phenomenon. For the inducing colours green and red the induced colours are weak, and the hue shifts are more or less unsystematic though there are individual subjects showing a trend in the Bezold‐Brücke direction. It is concluded that the hue shifts depend on the luminance relations rather than on the test field luminance.
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Mouse hue and wavelength-specific luminance contrast sensitivity are non-uniform across visual space
Abstract Mammalian visual behaviors, as well as responses in the neural systems thought to underlie these behaviors, are driven by luminance and hue contrast. With tools for measuring activity in cell-type specific populations in the mouse during visual behavior gaining traction, it is important to define the extent of luminance and hue information that is behaviorally-accessible to the mouse. A non-uniform distribution of cone opsins in the mouse potentially complicates both luminance and hue sensitivity: opposing gradients of short (UV-shifted) and middle (blue/green) cone opsins suggest that hue discrimination and wavelength-specific luminance contrast sensitivity may differ depending on retinotopic location. Here we ask if, and how well, mice can discriminate color and wavelength-specific luminance across visuotopic space. We found that mice were able to discriminate hue, and were able to do so more broadly across visuotopic space than expected from the cone-opsin distribution. We also found wavelength-band specific differences in luminance sensitivity.
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Four data sets are analyzed to quantify three effects of luminance of samples on chromaticity discrimination: on ellipse area, axis dimensions (a and b), and a/b ratio. Ellipses for aperture, surface, and simulated surface colors in CIE 1931 and 1964 x, y, Y color spaces are shown to reduce axis dimensions with higher luminance by different functions for the major and minor axes. Reduction is greater for major than minor axes, thus improving ellipse circularity. The functions plot straight lines in log-log scale as power law equations, except luminances below 3 cd/m2. We give formulae to predict a and b axes, a/b ratio, and ellipse area for almost any luminance in x, y, Y spaces. Effect of luminance is remarkable on ellipse area, which on average halves with every 3.5 times higher luminance. To illustrate the substantial effects of luminance, RIT-DuPont ellipses are predicted for three levels of equal luminance at 42, 212, and 2120 cd/m2. In the latter, ellipses are much smaller and are nearer circular than in the former. Higher luminance is known to improve color discrimination, so reduced ellipse area is to be expected but does not occur in CIELAB and DIN99 spaces because of lack of luminance-level dependency. We discuss our results' implications on uniform color space. Weber fraction ΔY/Y indicates brightness discrimination decreases with increasing luminance and is thus independent of chromaticity discrimination. © 2005 Wiley Periodicals, Inc. Col Res Appl, 30, 186–197, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20107
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