A unified approach to the perception of motion, stereo, and static-flow patterns

Saliency interactions between the 'L-M' and 'S' cardinal colour directions

Two sub-systems characterize the early stages of human colour vision, the 'L-M' system that differences L and M cone signals and the 'S' system that differences S cone signals from the sum of L and M cone signals. How do they interact at suprathreshold contrast levels? To address this question we employed the method used by Kingdom et al. (2010) to study suprathreshold interactions between luminance and colour contrast. The stimulus employed in one condition was similar to that used by Regan and Mollon (1997) for studying the relative 'organizing power' of the two sub-systems, and consisted of obliquely-oriented red-cyan (to isolate the L-M sub-system) and violet-chartreuse (to isolate the S sub-system) stripes within a lattice of circles. In our experiment there were two conditions, (1) the Separated condition, in which the L-M and S modulations were of opposite orientation and presented separately as a forced-choice pair, and (2) the Combined condition, in which the L-M and S modulations were added. In the Separated condition the task was to indicate the stimulus with the more salient orientation structure, whereas in the Combined condition the task was to indicate the orientation that was more salient. Psychometric functions were used to estimate the ratio of L-M to S contrast at the 'balance-point' i.e. point-of-subjective-equality (PSE) in both conditions. We found that across 20 subjects an average of 8% more S than L-M contrast was needed to achieve a PSE in the Combined compared to Separated condition. We consider possible reasons for this PSE difference and conclude that it is either due to an early-stage interaction between the S and L-M sub-systems, or to a later stage in which new colours that arise from their combination are selectively grouped.

Attention and visual memory in visualization and computer graphics

A fundamental goal of visualization is to produce images of data that support visual analysis, exploration, and discovery of novel insights. An important consideration during visualization design is the role of human visual perception. How we "see" details in an image can directly impact a viewer's efficiency and effectiveness. This paper surveys research on attention and visual perception, with a specific focus on results that have direct relevance to visualization and visual analytics. We discuss theories of low-level visual perception, then show how these findings form a foundation for more recent work on visual memory and visual attention. We conclude with a brief overview of how knowledge of visual attention and visual memory is being applied in visualization and graphics. We also discuss how challenges in visualization are motivating research in psychophysics.

The chromatic parameters of isoluminant chromatic motion examined with dissimilarity judgements

A moving edge or contrast between equiluminant colours produces a weaker motion percept than that defined by luminance contrast. However, the specific contrasts that determine chromatic motion have not been explored systematically. Here, rivalrous chromatic motion displays were produced by superimposing two gratings, one drifting from left to right and the other in the opposite direction. Each grating oscillated between equiluminant endpoints chosen from a pool of 16 colours. The rivalrous ambiguity resolved spontaneously for each display as the more-dissimilar colour pair dominated the less-dissimilar pair, and produced a percept of motion in the corresponding direction. These dissimilarity judgements were analysed with multidimensional scaling to represent 'motion salience' as distances in a colour map, to gauge whether chromatic motion is enhanced or weakened if the oscillation aligns with particular directions in the colour plane. Judgements were compared with other control judgements involving standard subjective dissimilarities between the same stimuli. Notably, chromatic motion was strongest when grating endpoints were separated along an orange-blue direction. This does not coincide with either cardinal axis of cone space, (L-M) or S(0) , but rather is a direction that would arise if motion is computed from a combination of the (L-M) and S(0) signals.

Escher in color space: individual-differences multidimensional scaling of color dissimilarities collected with a gestalt formation task

The structure of color perception can be examined by collecting judgments about color dissimilarities. In the procedure used here, stimuli are presented three at a time on a computer monitor and the spontaneous grouping of most-similar stimuli into gestalts provides the dissimilarity comparisons. Analysis with multidimensional scaling allows such judgments to be pooled from a number of observers without obscuring the variations among them. The anomalous perceptions of color-deficient observers produce comparisons that are represented well by a geometric model of compressed individual color spaces, with different forms of deficiency distinguished by different directions of compression. The geometrical model is also capable of accommodating the normal spectrum of variation, so that there is greater variation in compression parameters between tests on normal subjects than in those between repeated tests on individual subjects. The method is sufficiently sensitive and the variations sufficiently large that they are not obscured by the use of a range of monitors, even under somewhat loosely controlled conditions.

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.