Rumi Tokunaga

Material and lighting dimensions of object colour

The dimensionality of the object colour manifold was studied using a multidimensional scaling technique, which allows for the representation of a set of coloured papers as a configuration in a Euclidean space where the distance between papers corresponds to the perceptual dissimilarities between them. When the papers are evenly illuminated they can be arranged as a three-dimensional configuration. This is in line with the generally accepted view that the object colour space is three-dimensional. Yet, we show that under variegated illumination another three dimensions emerge. We call them lighting dimensions of object colour in order to distinguish from the traditional three referred to as material dimensions of object colour.

Lightness constancy and illumination discounting

Contrary to the implication of the term “lightness constancy”, asymmetric lightness matching has never been found to be perfect unless the scene is highly articulated (i.e., contains a number of different reflectances). Also, lightness constancy has been found to vary for different observers, and an effect of instruction (lightness vs. brightness) has been reported. The elusiveness of lightness constancy presents a great challenge to visual science; we revisit these issues in the following experiment, which involved 44 observers in total. The stimuli consisted of a large sheet of black paper with a rectangular spotlight projected onto the lower half and 40 squares of various shades of grey printed on the upper half. The luminance ratio at the edge of the spotlight was 25, while that of the squares varied from 2 to 16. Three different instructions were given to observers: They were asked to find a square in the upper half that (i) looked as if it was made of the same paper as that on which the spotlight fell (lightness match), (ii) had the same luminance contrast as the spotlight edge (contrast match), or (iii) had the same brightness as the spotlight (brightness match). Observers made 10 matches of each of the three types. Great interindividual variability was found for all three types of matches. In particular, the individual Brunswik ratios were found to vary over a broad range (from .47 to .85). That is, lightness matches were found to be far from veridical. Contrast matches were also found to be inaccurate, being on average, underestimated by a factor of 3.4. Articulation was found to essentially affect not only lightness, but contrast and brightness matches as well. No difference was found between the lightness and luminance contrast matches. While the brightness matches significantly differed from the other matches, the difference was small. Furthermore, the brightness matches were found to be subject to the same interindividual variability and the same effect of articulation. This leads to the conclusion that inexperienced observers are unable to estimate both the brightness and the luminance contrast of the light reflected from real objects lit by real lights. None of our observers perceived illumination edges purely as illumination edges: A partial Gelb effect (“partial illumination discounting”) always took place. The lightness inconstancy in our experiment resulted from this partial illumination discounting. We propose an account of our results based on the two-dimensionality of achromatic colour. We argue that large interindividual variations and the effect of articulation are caused by the large ambiguity of luminance ratios in the stimulus displays used in laboratory conditions.

Material and lighting hues of object colour

Observers can easily differentiate between a pigmented stain and the white surface that it lies on. The same applies for a colour shadow cast upon the same surface. Although the difference between these two kinds of colour appearance (referred to as material and lighting hues) is self‐evident even for inexperienced observers, it is not one that has been captured by any colour appearance model thus far. We report here on an experiment supplying evidence for the dissociation of these two types of hue in the perceptual space. The stimulus display consisted of two identical sets of Munsell papers illuminated independently by yellow, neutral, and blue lights. Dissimilarities between all the paper/light pairs were ranked by five trichromatic observers, and then analysed by using non‐metric multidimensional scaling (MDS). In the MDS output configuration, the Munsell papers lit by the same light made a closed configuration retaining the same order as in the Munsell book. The paper configurations for the yellow and blue lights were displaced transversally and in parallel to each other, with that of the neutral light located in between. The direction of the shift is interpreted as the yellow‐blue lighting dimension. We show that the yellow‐blue lighting dimension cannot be reduced to that of the reflected light.

Dissimilarity of yellow-blue surfaces under neutral light sources differing in intensity: Separate contributions of light intensity and chroma

Observers viewed two side-by-side arrays each of which contained three yellow Munsell papers, three blue, and one neutral Munsell. Each array was illuminated uniformly and independently of the other. The neutral light source intensities were 1380, 125, or 20 lux. All six possible combinations of light intensities were set as illumination conditions. On each trial, observers were asked to rate the dissimilarity between each chip in one array and each chip in the other by using a 30-point scale. Each pair of surfaces in each illumination condition was judged five times. We analyzed this data using non-metric multi-dimensional scaling to determine how light intensity and surface chroma contributed to dissimilarity and how they interacted. Dissimilarities were captured by a three-dimensional configuration in which one dimension corresponded to differences in light intensity.

Chromatic induction from surrounding stimuli under perceptual suppression

The appearance of colors can be affected by their spatiotemporal context. The shift in color appearance according to the surrounding colors is called color induction or chromatic induction; in particular, the shift in opponent color of the surround is called chromatic contrast. To investigate whether chromatic induction occurs even when the chromatic surround is imperceptible, we measured chromatic induction during interocular suppression. A multicolor or uniform color field was presented as the surround stimulus, and a colored continuous flash suppression (CFS) stimulus was presented to the dominant eye of each subject. The subjects were asked to report the appearance of the test field only when the stationary surround stimulus is invisible by interocular suppression with CFS. The resulting shifts in color appearance due to chromatic induction were significant even under the conditions of interocular suppression for all surround stimuli. The magnitude of chromatic induction differed with the surround conditions, and this difference was preserved regardless of the viewing conditions. The chromatic induction effect was reduced by CFS, in proportion to the magnitude of chromatic induction under natural (i.e., no-CFS) viewing conditions. According to an analysis with linear model fitting, we revealed the presence of at least two kinds of subprocesses for chromatic induction that reside at higher and lower levels than the site of interocular suppression. One mechanism yields different degrees of chromatic induction based on the complexity of the surround, which is unaffected by interocular suppression, while the other mechanism changes its output with interocular suppression acting as a gain control. Our results imply that the total chromatic induction effect is achieved via a linear summation of outputs from mechanisms that reside at different levels of visual processing.

P1-26: Influence of Depth from Luminance Contrast on Vergence Eye Movements

A vergence eye movement is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision. It has been shown that a vergence movement is induced not only by binocular depth but also by the changing size of the stimuli, which produces perception of motion in depth. That is, a monocular depth cue influences the direction of the eye movement, even when the eye movement contradicts depth from the disparity cue. Despite that a number of monocular depth cues are known, the influence on the vergence movement is known only with changing size. In this study, we focused on luminance contrast as a monocular depth cue and examined whether it influences the vergence movement. The stimuli were a Gabor patch with contrast changing sinusoidally in time at a given temporal frequency. When the observer looks at the stimuli, apparent depth changes with the contrast change. Eye movement measurements showed vergence movements synchronizing with luminance changes. Change in perceive...