Response times were measured for a visual search task in which the observer was required to find a target that differed from distracting stimuli only in color. In the first experiment the search time was measured as a function of display density for both small and large color differences. With small color differences response time increased with display density, indicating a serial search, but with large color differences response time was constant, indicating a parallel search. In the second experiment the color difference required for parallel search was measured in eight different directions from the distracter chromaticity. These color differences were much larger than threshold color differences and were not well represented by the ellipse used to describe the threshold contour around a point in color space.
Color-discrimination ellipses derived from the variability of color-matching data of six observers are analyzed in a normalized constant-luminance cone-excitation space. The analysis shows that the ellipses do not vary significantly in shape with chromaticity, observer, or experimental conditions. The discrimination contours are predictable from the thresholds on the two cardinal axes of this space; these are used to normalize the data at each chromaticity for each observer. Thresholds on these two axes vary with chromaticities, individuals, and experimental conditions in accordance with simple and familiar laws.
Visual search times were measured as a function of chromaticity and luminance differences between a target and distractor stimuli. Results showed that mean search time increased linearly with the number of distractors if the luminance difference between target and distractors was small but was roughly constant if the luminance difference was large. Similar results were previously found for chromaticity differences. With the number of distractor stimuli held constant, the mean search time decreased with increases in the difference between target and distractors, up to some critical difference. Further increases in target-distractor difference had little effect. Results were similar for targets defined by luminance and chromaticity. There was some advantage to combining luminance differences with chromaticity differences when the target was dimmer than the distractors. Generally there was no advantage for combining a chromaticity difference with a luminance difference when the target was brighter than the distractors.
Many previous studies have shown that background color affects the discriminability and appearance of color stimuli. However, research on visual search has not typically considered the role that the background may play. Rosenholtz (2001a) has suggested that color search asymmetries result from the relationship between the stimuli and the background. Here we test the hypothesis that background color should have an effect on asymmetries in visual search based on color, using searches for color stimuli on different colored backgrounds. Observers searched for a single known target stimulus among homogeneous distractor stimuli. The target stimulus differed from the distractors only in chromaticity, but targets and distractors both differed from the backgrounds in luminance so that they were easily visible regardless of chromaticity. Target/distractor pairs differed primarily in saturation (Experiments 1, 2, & 3) or in hue (Experiment 4). Each member of each pair of colors served as target and distractor color on both achromatic and red backgrounds. When the stimuli were presented on an achromatic background, response times were shorter when the more saturated member of each pair of colors served as the target color. When the same stimuli were presented on a red background, the asymmetry was either reversed or abolished. When target and distractors differed in hue, there was little asymmetry on the achromatic background but a sizable asymmetry for some color pairs on the red background. On both backgrounds, the magnitude of the asymmetry varied with the difference between the stimulus colors and the background color. Results confirm that asymmetries in color search are dependent on the relationship between the stimulus colors and the background color. Two candidate models are suggested that show promise in predicting these experimental results: Rosenholtz' saliency model (1999, 2001a) and a modification to signal detection theory models in which the observation noise is proportional to the difference between target/distractor color and background color.
We conducted two studies that investigated display characteristics related to color (hue, saturation, brightness, and transparency) and contrast with a background for displaying information qualifiers (termed meta-information) such as uncertainty, age, and source quality. Level of detail (or granularity) of the meta-information and task demands were also manipulated. Participants were asked to rank and rate colored regions overlaid on different map backgrounds based on the level of meta-information the regions displayed. Results from Study 1 indicated that participants could appropriately rank and rate levels of meta-information across saturation, brightness, and transparency conditions, and results from Study 1 and Study 2 showed that the natural direction of ordering is complex and dependent on the relevance of different information to the task and the contrast of the overlay region with the background.
Observers were required to search for and find a target stimulus that differed from distractor stimuli only in chromaticity. Pairs of target and distractor colors were chosen so that in some conditions the two members of a pair differed in hue, in saturation, or in both hue and saturation. For each type of condition, a number of pairs of colors representing varying degrees of perceptual difference were chosen. Each member of each pair of colors served as both the target and distractor color while the other member of the pair served in the remaining role. The largest asymmetries in search times occurred when the target and distractors differed in saturation. Somewhat smaller asymmetries occurred with differences in both saturation and hue, and no asymmetries occurred with hue differences. Results suggest that the asymmetries are related to the time required to encode and transmit the chromatic information centrally rather than the properties of the search process itself.
A color-naming method was used to examine the large-field red/green discriminations of dichromats screened with standard tests. The stimulus was a 12 degrees annular field with the central 4 degrees removed, flashed for 300 ms. Four wavelengths were equated in brightness for each observer at two retinal illuminance levels, approximately 10 and 100 trolands. The stimuli were then presented in random order and the observer was asked to name each, using one of four color terms. The entire experiment was done with the observers dark-adapted and also with the rods bleached. For all four deuteranopes and two of four protanopes, color names were very significantly related to both illuminance and wavelength in both adaptation conditions. The relationship between name and wavelength was similar to that of a normal trichromat, but the performance of a dichromat was very poor by comparison. Performance was generally somewhat better in the dark than with the rods bleached. However, the result in the bleached condition is consistent with recent evidence that at least some observers who are classified as dichromats with standard small-field screening procedures actually have a weak residual third cone mechanism.
Experiments were designed to investigate the effects of set size and variation in the chromaticity of distractor stimuli on thresholds for detecting a target stimulus that differed from distractors only in chromaticity. Distractor chromaticities were selected from a line in the isoluminant color plane and targets were selected from lines approximately orthogonal to the distractor line. With uniform distractors thresholds increased with set size as predicted by a signal detection model. When targets and distractors were selected from lines parallel to the Cardinal directions in color space, thresholds were lower with variable distractors than with uniform distractors and variations in the location of the target along the distractor line had no effect on threshold. Results with diagonally oriented distractor lines were similar. Results suggest that many pairs of orthogonal directions in the isoluminant color plane represent independent color coding mechanisms that mediate search. Results also show that information in independent color coding mechanisms tuned to orthogonal directions in the isoluminant plane can be combined to facilitate detection of the target.
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