The peculiar nature of simultaneous colour contrast in uniform surrounds

We present evidence from asymmetric colour matching experiments which strongly suggests that uniform surrounds evoke induction effects of a very peculiar nature, not representative of colour induction effects in variegated surrounds. Given the widespread use of uniform surrounds in studies of colour vision, this finding is of interest in relation to a number of current research issues, such as contrast coding of colour, functionally equivalent surrounds and colour constancy. A framework that systematises the seemingly complex colour appearance changes induced by uniform surrounds is presented and its implications are discussed.     

Color appearance: The limited role of chromatic surround variance in the "gamut expansion effect"

R. O. Brown and D. I. MacLeod (1997) observed that chromatic patches appear much more saturated against an equiluminant, uniform gray surround than against a chromatically variegated surround with the same space-average color. Using asymmetric color matching, we investigated what stimulus conditions are critical for the occurrence of this "gamut expansion effect." We found (a) that the effect diminishes rapidly with increasing color contrast between target and surround, (b) that the amount and the spatial distribution of color variance in the surround plays but a very limited role, (c) that the effect is mainly local, and (d) that basically the same effect can be obtained by comparing two uniform surrounds. These findings, particularly the latter, argue strongly against an explanation solely in terms of contrast adaptation. We suggest that the main features of our findings can be explained in terms of color scission.     

Classification of apparent motion percepts based on temporal factors

As pointed out by M. Wertheimer (1912), a number of qualitatively different motion impressions, such as "optimal motion," "part motion," and "pure phi," may be evoked by manipulating the temporal parameters of two-element apparent motion sequences. We investigated how the transitions between the different percepts depend on temporal variables over a large range of interstimulus intervals and stimulus onset asynchronies. On the basis of these data, we present a hierarchical classification scheme describing the critical temporal conditions for alternative percepts. A particularly interesting finding is that the distinction between pure phi, on the one hand, and part and optimal motion, on the other, depends on the temporal duty cycle of the stimulus elements. It is suggested that this temporal variable may be used as a cue to resolve occlusion-related ambiguities in classical motion stimuli.     

New laws of simultaneous contrast?

Drawing on many seemingly disparate and unrelated lines of evidence, we argue that the direction of the simultaneous contrast effect in three-dimensional colour space is given by the difference vector between target and surround ('direction hypothesis'). This challenges the traditional idea according to which the direction of the simultaneous contrast effect is complementary to the colour of the surround ('complementarity law'). We also argue that the size of the simultaneous contrast effect is either constant or decreases with the difference between target and surround in three-dimensional colour space. The latter proposal challenges Kirschmann's fourth law. Within our theoretical framework, the universally presumed validity of the complementarity law and Kirschmann's fourth law can be understood as resulting from the failure to take various confounding factors into account when interpreting empirical data, the most prominent of which is the influence of temporal von Kries adaptation.     

Recovery of spatial memory and persistence of spatial orientation deficits after traumatic brain injury during childhood

The present study set out to examine the recovery of spatial learning and cognitive mapping skills after severe TBI in childhood. A prospective investigation was carried out with repeated measures. Children with TBI (n = 18) and healthy matched controls (n = 18) were investigated while the children with TBI stayed in a rehabilitation facility (t₀) and 4 years later (t₁). Children were assessed with the Kiel Locomotor Maze, where they had to remember defined locations in an experimental chamber with completely controlled intra- and extra-maze cues until the learning criterion was reached. During probe trials, cognitive mapping strategies were assessed. Results showed (i) that spatial learning is functionally restituted 4 years post-trauma and (ii) that cognitive mapping skills are still impaired 4 years post-trauma. It was concluded that cognitive performance of children who survived a severe TBI may be overestimated, having far reaching consequences for the children.     

The natural center of chromaticity space is not always achromatic: a new look at color induction

Although current theories of color vision differ in many respects, they all assume the existence of a uniquely defined neutral point in chromaticity space. It generally is assumed that this point satisfies several criteria simultaneously. One of these criteria is that it is perceived as achromatic. A further criterion shared by most theories is the structural assumption that lines in chromaticity space of constant hue converge on the neutral point. The basic assumption that these two criteria coincide is clearly true for isolated spots of light presented in darkness, and it usually is taken for granted that this coincidence generalizes to more complex visual stimuli. Here, we show that this is not the case. Our experiments with infields in chromatic surrounds revealed that the point in chromaticity space that appears gray is clearly different from the point on which lines of constant hue converge. A plausible interpretation of this apparently paradoxical finding in terms of color scission is proposed.     

The strengths of simultaneous colour contrast and the gamut expansion effect correlate across observers: evidence for a common mechanism

We present the results of an experiment aiming to clarify the relation between simultaneous colour contrast and Brown and MacLeod’s (1997) gamut expansion effect. These two context effects are often thought to be due to two different mechanisms, but this assumption has not previously been subjected to empirical test. Here we used inter-individual variability in the susceptibility to these effects to test this assumption. The individual variability was found to be quite substantial for both context effects. As would be expected if a common underlying mechanism contributes to both effects, a significant correlation across observers was found. It is suggested that this putatively common mechanism of ‘crispening’ accounts completely for the gamut expansion effect, and partially for the simultaneous colour contrast effect, which seems to depend on von Kries adaptation also.     

Disparity, motion, and color information improve gloss constancy performance

S. Nishida and M. Shinya (1998) found that observers have only a limited ability to recover surface-reflectance properties under changes in surface shape. Our aim in the present study was to investigate how the degree of surface-reflectance constancy depends on the availability of information that may help to infer the reflectance and shape properties of surfaces. To this end, we manipulated the availability of (i) motion-induced information (static vs. dynamic presentation), (ii) disparity information (with the levels "monocular," "surface disparity," and "surface + highlight disparity"), and (iii) color information (grayscale stimuli vs. hue differences between diffuse and specular reflections). The task of the subjects was to match the perceived lightness and glossiness between two surfaces with different spatial frequency and amplitude by manipulating the diffuse component and the exponent of the Phong lighting model in one of the surfaces. Our results indicate that all three types of information improve the constancy of glossiness matches--both in isolation and in combination. The lightness matching data only revealed an influence of motion and color information. Our results indicate, somewhat counterintuitively, that motion information has a detrimental effect on lightness constancy.