A comparison between colour and luminance contrast in a spatial linking task

Contour Integration in Dynamic Scenes: Impaired Detection Performance in Extended Presentations

Since scenes in nature are highly dynamic, perception requires an on-going and robust integration of local information into global representations. In vision, contour integration (CI) is one of these tasks, and it is performed by our brain in a seemingly effortless manner. Following the rule of good continuation, oriented line segments are linked into contour percepts, thus supporting important visual computations such as the detection of object boundaries. This process has been studied almost exclusively using static stimuli, raising the question of whether the observed robustness and "pop-out" quality of CI carries over to dynamic scenes. We investigate contour detection in dynamic stimuli where targets appear at random times by Gabor elements aligning themselves to form contours. In briefly presented displays (230 ms), a situation comparable to classical paradigms in CI, performance is about 87%. Surprisingly, we find that detection performance decreases to 67% in extended presentations (about 1.9-3.8 s) for the same target stimuli. In order to observe the same reduction with briefly presented stimuli, presentation time has to be drastically decreased to intervals as short as 50 ms. Cueing a specific contour position or shape helps in partially compensating this deterioration, and only in extended presentations combining a location and a shape cue was more efficient than providing a single cue. Our findings challenge the notion of CI as a mainly stimulus-driven process leading to pop-out percepts, indicating that top-down processes play a much larger role in supporting fundamental integration processes in dynamic scenes than previously thought.

The physiology and psychophysics of the color-form relationship: a review

The relationship between color and form has been a long standing issue in visual science. A picture of functional segregation and topographic clustering emerges from anatomical and electrophysiological studies in animals, as well as by brain imaging studies in human. However, one of the many roles of chromatic information is to support form perception, and in some cases it can do so in a way superior to achromatic (luminance) information. This occurs both at an early, contour-detection stage, as well as in late, higher stages involving spatial integration and the perception of global shapes. Pure chromatic contrast can also support several visual illusions related to form-perception. On the other hand, form seems a necessary prerequisite for the computation and assignment of color across space, and there are several respects in which the color of an object can be influenced by its form. Evidently, color and form are mutually dependent. Electrophysiological studies have revealed neurons in the visual brain able to signal contours determined by pure chromatic contrast, the spatial tuning of which is similar to that of neurons carrying luminance information. It seems that, especially at an early stage, form is processed by several, independent systems that interact with each other, each one having different tuning characteristics in color space. At later processing stages, mechanisms able to combine information coming from different sources emerge. A clear interaction between color and form is manifested by the fact that color-form contingencies can be observed in various perceptual phenomena such as adaptation aftereffects and illusions. Such an interaction suggests a possible early binding between these two attributes, something that has been verified by both electrophysiological and fMRI studies.

Perceived duration of chromatic and achromatic light

Luminance and color information are considered to be processed in parallel systems. The integration of information from these two separate systems is crucial for the visual system to produce a coherent percept. To investigate how luminance and color lights are perceived in time, we measured the perceived duration of light stimuli with and without colors in a paradigm involving simultaneous perception with presentation of two successive stimulus frames. Luminance contrast and color contrast of the stimuli were set with a chromatic substitution technique. In Experiment 1, the perceived duration of both chromatic stimuli and achromatic stimuli increased as the luminance contrast decreased. Experiment 2 tested if the duration of the percept was influenced by color contrast which was defined by colorimetric purity of the stimuli, when luminance contrast was set as low as practically possible. The result showed that the duration of the percept decreased with increasing color contrast of the stimuli. Moreover, Experiment 3 demonstrated that the trend of perceived duration was consistent with the four primary colors, provided that the effective color contrast of stimulus was corrected based on the contrast sensitivity to the color. These experiments indicate that, with a high luminance contrast level, perceived duration of a stimulus is predominantly defined by luminance contrast, whereas in low luminance contrast conditions, the duration depends on the color contrast. The perceived duration of color stimuli showed an "inverse color contrast effect", similar to the well-known "inverse intensity effect" for luminance stimuli. The similarities and the differences between the two systems, as well as their priorities in processing temporal information of visual stimuli are further discussed.

Chromatic tuning of contour-shape mechanisms revealed through the shape-frequency and shape-amplitude after-effects

We investigated whether contour-shape processing mechanisms are selective for color direction using the shape-frequency and shape-amplitude after-effects, or SFAE and SAAE [Gheorghiu, E. & Kingdom, F. A. A. (2006). Luminance-contrast properties of contour-shape processing revealed through the shape-frequency after-effect. Vision Research, 46(21), 3603-3615. Gheorghiu, E. & Kingdom, F. A. A. (2007). The spatial feature underlying the shape-frequency and shape-amplitude after-effects. Vision Research, 47(6), 834-844]. All contours were defined along the 'red-green', 'blue-yellow' and 'luminance' axes of cardinal color space. Adapting and test contours were defined along the same or along opposite polarities within a cardinal axis, and along the same or along different cardinal axes. We found (i) little transfer of the after-effects across different within-axis polarities, for all cardinal axes and for both even-symmetric and odd-symmetric contours; (ii) little transfer between the red-green and blue-yellow cardinal axes; (iii) little transfer between the chromatic and luminance cardinal directions for the SAAE; (iv) large transfer between the chromatic and luminance cardinal directions for the SFAE. We conclude that contour-shape mechanisms are selective for within-cardinal axis polarity and for the chromatic axes within the isoluminant plane. However for certain types of contour-shape processing they are poorly selective along the chromatic versus luminance dimension. Overall our results suggest that contour-shape encoding mechanisms are selective for color direction and that color is important for contour-shape processing.

Luminance and chromatic cues in a spatial integration task

These experiments explore the way in which cues provided by luminance and chromatic contrast interact in the spatial integration of elements. The stimuli were composed of bidimensional and isotropic Gauss functions. The elements were placed so that when experimentally manipulating the separations between the lines, subjects could generate an oriented percept from the elements sharing luminance or chromaticity. Results showed that, in most cases, grouping elements that share chromatic content is possible, in spite of variations in luminance content. Grouping elements as a function of luminance is more difficult when chromaticity alternates from one element to another. Lastly, if competing groupings are generated, the stimulus is structured as a function of chromatic content and not of luminance content.

Importance of color in the segmentation of variegated surfaces

We examined how variations in color and brightness are used by the visual system in distinguishing textured surfaces that differed in their first- or second-order statistics. Observers viewed a 32 x 32 array containing two types of square elements differing in chromaticity or luminance or both. The spatial distributions of the two kinds of elements were varied within the array until observers could distinguish two juxtaposed regions. At low but not at high contrast, observers are better able to distinguish regions when the elements differ only in chromaticity than when they differ only in luminance. The advantage of color at low contrasts results from the greater visibility of the arrays defined by color variation. An observer's capacity to distinguish textures defined by variations in first-order chromatic statistics is little affected by the addition of achromatic noise but is more affected by the addition of chromatic noise. The relative robustness of chromatic cues in the face of achromatic noise leaves the visual system well equipped to exploit color variations in segmenting complex scenes, even in the presence of variations in brightness. This capacity seems to depend on mechanisms that sum over large regions: When surfaces differ in their second-order statistics and cannot be distinguished by mechanisms that sum over large regions, the advantage of color is much diminished.

Contour integration in color vision: a common process for the blue-yellow, red-green and luminance mechanisms?

We compare the performance of the red-green, blue-yellow and luminance postreceptoral mechanisms on a contour integration task requiring the linking of oriented Gabor elements across space to extract a winding 'path' or contour. We first establish that for all three mechanisms curvature and contrast are independent; losses in performance due to one cannot be compensated by changes in the other. We then compare contour integration by the three mechanisms using a method that controls for their differences in cone contrast thresholds. Our results show that despite the poor orientation discrimination thresholds and poor spatial sampling found for the blue-yellow mechanism, all three mechanisms perform similarly on contour integration over a wide range of curvatures. Furthermore, all three mechanisms have the same dependence on path curvature. We also investigate the effects of adding external orientation noise. Our results imply that the internal orientation noise for extracting 'aligned' path elements is similar in the three mechanisms and for all path curvatures, and the relative efficiencies are also similar for the three mechanisms. To account for our results, we propose that the three postreceptoral mechanisms use a common contour integration process. This linking process, however, cannot be color-blind; our last experiment shows that linking between different chromatic mechanisms or between opposite spatial phases disrupts contour integration. We thus propose that the common integration process remains sensitive to the color contrast and phase of its inputs.

Attention-based texture segregation

Luminance- or color-defined +/- 45 degrees-oriented bars were arranged to yield single-feature or double-conjunction texture pairs. In the former, the global edge between two regions is formed by differences in one attribute (orientation, or color, or luminance). In the color/orientation double-conjunction pair, one region has +45 degrees red and -45 degrees green textels, the other -45 degrees red and +45 degrees green textels (the luminance/orientation double-conjunction pair is similar); such a pair contains a single-feature orientation edge in the subset of red (or green) textels, and a color edge in the subset of +45 degrees (or -45 degrees) textels. We studied whether edge detection improved when observers were instructed to attend to such subsets. Two groups of observers participated: in the test group, the stimulus construction was explained to observers, and they were cued to attend to one subset. The control group ran through the same total number of sessions without explanations/cues. The effect of cuing was week but statistically significant. Feature cuing was more effective for color/orientation than for luminance/orientation conjunctions. Within each stimulus category, performance was nearly the same no matter which subset was attended to. On average, a global performance improvement occurred over time even without cuing, but some observers did not improve with either cuing or practice. We discuss these results in the context of one-versus two-stage segregation theories, as well as by reference to signal enhancement versus noise suppression. We conclude that texture segregation can be improved by attentional strategies aimed to isolate specific stimulus features.

Contour integration with colour and luminance contrast

In this study, we consider how colour contrast can be used to integrate form and how it interacts with luminance contrast in the task. The performance of form integration was assessed by measuring the detection of a winding "contour" of aligned gabor elements embedded in a background of randomly oriented gabors, using both luminance and isoluminant (red/green) contrast. Performance on the task improves with gabor element contrast, and identical performance for colour and luminance contour detection is achieved at high screen contrasts, showing that colour is able to support a complex form integration task. In a second experiment, we investigate whether colour and luminance contrast can be combined in contour integration by measuring the detection of a path with alternating isoluminant colour and luminance elements. We find that contour detection uses both colour and luminance information cooperatively, but performance is much poorer than would be expected from a single common contour integration process which fails to distinguish the two types of contrast. This suggests that there are specific contour integration processes for colour and luminance. In a third experiment, we measure the effects of variations in colour and luminance contrast on contour detection using elements that combine colour and luminance contrast. We find that varying the colour contrast of elements tends to worsen the detection of a luminance contour, as do luminance contrast variations for colour contour detection. These results suggest no special role for colour in integrating contours, and are discussed with regard to their ecological significance.

The effect of nearby luminance contrast polarity on color boundary localization

The study measured the shift in apparent position of a color-defined target boundary as a function of the distance, luminance polarity and amount of contrast of a nearby luminance-defined flanking boundary. In general, the position of the target boundary shifted towards the flank with the attraction being somewhat greater for negative than positive polarity flanks, and for high compared to low contrast flanks. High contrast, negative polarity flanks resulted in greater attraction at 3.69 min arc separation. For low contrast flanks, the apparent shift in position of the target boundary depended on the polarity and position of the flank relative to the target. For example, for small separations ( < 3 min arc) flank polarity had little influence, while for larger separations ( > or = 3.69 min arc), negative polarity flanks exhibited attraction while positive polarity flanks began to show repulsion. The results support the notion that luminance and color processing may share a common representation for the localization of boundaries. Position judgments based on this representation appear to be influenced by the amount of luminance contrast in a nearby boundary.