Effects of surround/test field luminance ratio on induced colour

Abstract.— According to Kirschmann's third law the induced colour is at its maximum when the inducing and induced fields are of equal luminance. Later studies (Kinney, 1962) show the induced colour to be most pronounced at a luminance ratio (inducing/induced) of about 4/1. In the present study the amount of colour induced into an achromatic test field was determined for one inducing colour, red, by letting observers judge the colour strength of the induced field. The test (or induced) field luminance was varied to give luminance ratios between 0.5/1 and 2/1. The results show that both colour strength and blackness increase as the luminance ratio is increased. The fact that the test field was judged even to have maximum chromatic colour strength and maximum blackness at the same time is discussed in relation to the method used and in relation to earlier studies on the "mode of appearance" of colours and the bidimensionality of achromatic colours.     

Effects of algebraic and absolute luminance differences on achromatic surface grouping

The algebraic luminance difference corresponding to a uniform achromatic surface on a uniform achromatic background determines the dark or light quality and the extent of this quality in the achromatic colour of the surface, while the absolute value of this difference determines the amount of perceived contrast between the achromatic colours of the surface and of the background. The results of an experiment designed to test whether grouping of achromatic surfaces depends on one or on both of these differences are here reported. Grouping was found to depend prevailingly on algebraic luminance difference in some subjects, prevailingly on absolute luminance differences in other subjects, and on both of these differences with similar probabilities in still other subjects.     

Brightness contrast: a reinterpretation of compound cue and combined cue experiments with pigeons.

A group of three pigeons was trained on a 4-ply multiple schedule: a green color and a vertical line superimposed upon an achromatic background as positive stimuli, and a red color and a horizontal line on an achromatic background as negative stimuli. The pigeons were tested with the vertical line superimposed upon different achromatic background intensities, then with the vertical line superimposed upon different green background intensities, and finally with the vertical line and its training achromatic backgfound attenuated (and unattenuated) by a neutral density filter. The gradients peaked at the luminance of the achromatic background used during training and at the equivalent luminance for the green background when it was substituted for the achromatic background. The brightness contrast, not the background luminance, was the critical variable as the neutral density filter attenuated both the line and the background equally, leaving brightness contrast unchanged; there was no response decrement to this attenuated stimulus. Two other groups of three pigeons showed that they attended to line orientation as well as to brightness contrast. The brightness contrast hypothesis was extended to explain results of attention experiments and combined cue experiments which have used line stimuli in combinations with different backgrounds.     

Measuring the meter: on the constancy of lightness scales seen against different backgrounds

The constancy of a 16-step achromatic Munsell scale was tested with regards to background variations in two experiments. In experiment 1 three groups of observers were asked to find lightness matches for targets in simultaneous lightness displays by using a 16-step achromatic Munsell scale placed on a white, black, or white-black checkered background. In experiment 2, a yellow-blue checkered background and a green-red checkered background replaced Munsell scales on the black and on the white backgrounds. Significant effects of scale background on matches were found only in experiment 1, suggesting that background luminance is a crucial factor in the overall appearance of the scale. The lack of significant differences in experiment 2, however, may stand for an overall robustness of the scale with respect to background luminance changes occurring within certain luminance ranges.     

Test of models of achromatic transparency

Consider an achromatic disk transparent on an achromatic background formed by two adjoining rectangles, with the common border of the rectangles dividing the disk in half. Current models of achromatic transparency contend that the perceived extent of transparency of the disk depends on the luminance contrast inside the disk and on the luminance contrast in the background outside the disk. Here, a model is proposed which contends that this perceived extent is determined only by the luminance contrasts inherent in the disk: inside the disk and between the disk and the background. Two experiments were designed to determine which luminance contrasts influence transparency. In the first experiment, subjects rated the perceived extent of transparency of the disk for different combinations of the luminances of the disk and of the background. The results strengthen the view that the perceived extent of transparency depends on the luminance contrasts inherent in the disk. In the second experiment, a test was made of the possibility that luminance contrasts between adjoining areas of the background outside the disk are nonessential for transparency. The results show that transparency occurred both when the areas of the background outside the transparent region adjoined one another and when they were separated, confirming that the perceived extent of transparency depended only on luminance contrasts between adjoining areas inherent in the disk.     

Perception of illumination

Abstract.— Previous research on the perception of illumination has primarily been occupied with the apparent paradox that "we must know the object colour in order to utilize the reflected light as an index of illumination, while we must know the illumination in order to use the reflected light as an index of object colour". The present study shows this paradox to exist only as a long as the traditional one-dimensional conception of achromatic colours is maintained. Two experiments are reported. In Exp. 1 it is shown that by mere variation of the luminance difference of two adjacent areas, a consistent shift in colourlshadow discrimination is obtained; i.e. within a certain range of luminance differences the observer may find it difficult to decide whether the difference is one of colour (two fields of different greyness in common illumination) or one of illumination (two fields of equal greyness under different illumination), above this range the observer perceives the differences as one of illumination. Taking the bidimensionality of achromatic colours into account in Exp. 2, results are obtained which show the brightness (alternatively termed luminosity) relations of adjacent areas to be the critical factor for the perceptual shift.     

Binocular rivalry and the contrast at contours

Binocular rivalry was recorded between various achromatic figures in or near the foveae. For a pair of intersecting contours, one in the field of each eye, it was found that the percentage of time for which a contour was dominant at the point of intersection increased with the contrast at that contour, and also with average luminance when contrast was constant. Further, for 1° circles in corresponding positions in the two fields, one darker than its surround and one lighter, the same results were obtained. Various auxiliary results, on rate of rivalry, eye-dominance, the occasional mixture of the rivalling stimuli, and binocular lustre, are given. Finally the relationship between predominance in rivalry and perceived brightness is discussed.     

Hue difference contours can be used in processing orientation information

In most studies of orientation processing, chromatic information and achromatic information have been combined or confounded. The present experiments investigated the relative sizes of tilt aftereffect induced by these two types of information. In these experiments, the tilt aftereffect is the error in adjusting a test contour to vertical, following the scanning of an inspection contour. For inspection and test contours identical except for orientation, the tilt aftereffects varied with inspection contour orientation but not with chromatic or achromatic condition. Smaller tilt aftereffects were obtained when the inspection contour was produced by a hue difference (chromatic information) and the test contour was produced by a luminance difference (achromatic information), or vice versa. These results indicate that achromatic and chromatic information is processed in a similar manner with respect to orientation. Furthermore, there is substantial, but incomplete, pooling of chromatic and achromatic orientation information.     

On the determinants of surface brightness

The brightness of an achromatic surface with luminance S on an achromatic background with luminance B varies with S, with B, and with the luminance step deltaL at the border of the surface. In agreement with previous findings indicating that the visual system can perform as a photometer, the results of the two experiments reported here show that S and B determined surface brightness independently of deltaL when the surface was adjacent to and when it was separated from the background. This finding suggests that surface brightness depends on the integration of neural signals representing magnitudes of absolute luminance. A weighted-average model of this integration is proposed.     

Lightness constancy: Ratio invariance and luminance profile

The term simultaneous lightness constancy describes the capacity of the visual system to perceive equal reflecting surfaces as having the same lightness despite lying in different illumination fields. In some cases, however, a lightness constancy failure occurs; that is, equal reflecting surfaces appear different in lightness when differently illuminated. An open question is whether the luminance profile of the illumination edges affects simultaneous lightness constancy even when the ratio invariance property of the illumination edges is preserved. To explore this issue, we ran two experiments by using bipartite illumination displays. Both the luminance profile of an illumination edge and the luminance ratio amplitude between the illumination fields were manipulated. Results revealed that the simultaneous lightness constancy increases when the luminance profile of the illumination edge is gradual (rather than sharp) and homogeneous (rather than inhomogeneous), whereas it decreases when the luminance ratio between the illumination fields is enlarged. The results are interpreted according to the layer decomposition schema, stating that the visual system splits the luminance into perceived lightness and apparent illumination components. We suggest that illumination edges having gradual and homogeneous luminance profiles facilitate the luminance decomposition process, whereas wide luminance ratios impede it.     

Luminance as a factor in the ability of achromatic gratings to interfere with MeCollough effects

McCollough effects were reduced in assessed strength by having observers view achromatic gratings that varied in space-average luminance. Whether the gratings were presented before or after induction of McCollough effects, high luminance gratings interfered with the effects more than did low luminance gratings. If considered within the context of a conditioning model, this finding may be interpreted as inconsistent with other characteristics of McCollough effects.     

Simultaneous lightness contrast with double increments

In this paper we demonstrate the existence of simultaneous lightness contrast in displays in which the target patches are both more luminant than their surrounds. These effects are not predicted by theories of lightness that assume that the highest luminance in a scene is perceived as white, and anchors all the other luminances. We show that the strength of double-increment illusions depends crucially on the luminance of both the surrounds and the target patches. Such luminance prerequisites were not met in previous studies, which explains why simultaneous contrast with incremental targets has so far been regarded as extremely weak or nonexistent.     

Spotting animals in natural scenes: efficiency of humans and monkeys at very low contrasts

The ability of monkeys to categorize objects in visual stimuli such as natural scenes might rely on sets of low-level visual cues without any underlying conceptual abilities. Using a go/no-go rapid animal/non-animal categorization task with briefly flashed achromatic natural scenes, we show that both human and monkey performance is very robust to large variations of stimulus luminance and contrast. When mean luminance was increased or decreased by 25–50%, accuracy and speed impairments were small. The largest impairment was found at the highest luminance value with monkeys being mainly impaired in accuracy (drop of 6% correct vs. <1.5% in humans), whereas humans were mainly impaired in reaction time (20 ms increase in median reaction time vs. 4 ms in monkeys). Contrast reductions induced a large deterioration of image definition, but performance was again remarkably robust. Subjects scored well above chance level, even when the contrast was only 12% of the original photographs (≈81% correct in monkeys; ≈79% correct in humans). Accuracy decreased with contrast reduction but only reached chance level -in both species- for the most extreme condition, when only 3% of the original contrast remained. A progressive reaction time increase was observed that reached 72 ms in monkeys and 66 ms in humans. These results demonstrate the remarkable robustness of the primate visual system in processing objects in natural scenes with large random variations in luminance and contrast. They illustrate the similarity with which performance is impaired in monkeys and humans with such stimulus manipulations. They finally show that in an animal categorization task, the performance of both monkeys and humans is largely independent of cues relying on global luminance or the fine definition of stimuli.     

PSYCHOPHYSICAL INVARIANTS OF ACHROMATIC COLOUR VISION: I. The multidimensionality of achromatic colour experience

L ie . I. Psychophysical invariants of achromatic colour vision. I. The multi-dimensionality of achromatic colour experience. Scand. J. Psychol ., 1969, 10, 167–175.—The literature relevant to the question of multidimensionality of achromatic colour is briefly reviewed. An experiment confirms that the achromatic scale is bidimensional, and indicates that the dimension of brightness is a direct function of the luminance of the local area inspected, while the dimension of whiteness is a direct function of luminance relations of adjacent areas in the visual field.     

PSYCHOPHYSICAL INVARIANTS OF ACHROMATIC COLOURVISION. V

LIE, I. Psychophysical invariants of achromatic colour vision. V. Brightness as a function of inducing field luminance. Scand. J. Psychol. , 1971, 12, 61–64.– Brightness (the dim-bright dimension) of achromatic colour was investigated as a function of surrounding field luminance. It was found that brightness of a test area was relatively independent of luminance level of the surrounding area until the luminance of the latter had passed well beyond that of the test area. With further increase of surrounding luminance, the brightness of the test area increased rapidly.     

A new effect of luminance gradient on achromatic simultaneous contrast

A new effect in the domain of achromatic simultaneous contrast has been observed. A middle gray region placed at the center of an area filled by a linear achromatic gradient from black (outer part) to white (inner part) is perceived as being much darker than an identical middle gray region surrounded by a reversed gradient. By using a matching task in two experiments, it has been shown that this phenomenon is much stronger than the classical achromatic simultaneous contrast effect. The new effect is interpreted in terms of thealbedo hypothesis.     

Chromatic information processing

The investigation of visual processing mediated solely by chromatic information requires conditions preventing a subject's use of the luminance differences normally accompanying a chromatic change. In Experiment 1, which involved a discriminative reaction time (RT) task, chromatic and white stimuli of the same luminance were presented on a dimmer achromatic background. Subjects were instructed to respond only to the chromatic stimuli. RT was slowest at 570 nm and somewhat faster to short wavelengths than to long wavelengths. In Experiment 2, which compared two discriminative RT tasks, RT was faster when subjects responded to color than when they responded to white. Experiments 3 and 4 demonstrated that a brighter white surround decreased the perceived brightness of chromatic stimuli as well as their perceptual similarity to white, but did not affect RT. The results are discussed in terms of the response strength of the chromatic processing channel.     

Mesopic lightness,brightness, and brightness contrast

At mesopic mean luminances, a fixed luminance contrast produces less brightness contrast than it does at photopic luminances. This suggests that lightnesses of surfaces might also be altered at low luminances. I measured lightness, brightness, and brightness contrast in CRT simulations of achromatic paper patchworks. The illuminance of the standard pattern was fixed, producing 0.12,1.2, or 12 cd/m². The illuminance on the test pattern was varied in a lightness constancy paradigm. Constant brightness contrast required more luminance contrast at lower mean luminances. Failures of lightness constancy occurred at the lowest mean luminances, but they were minor in comparison with the loss of brightness contrast in the same pattern. These results have implications for imaging applications. Often, image content falls in both the photopic and the mesopic ranges. Our results indicate that brightness contrast may decrease substantially in low-luminance regions without large changes of surface lightness.     

Edges,colour and awareness in blindsight

It remains unclear what is being processed in blindsight in response to faces, colours, shapes, and patterns. This was investigated in two hemianopes with chromatic and achromatic stimuli with sharp or shallow luminance or chromatic contrast boundaries or temporal onsets. Performance was excellent only when stimuli had sharp spatial boundaries. When discrimination between isoluminant coloured Gaussians was good it declined to chance levels if stimulus onset was slow. The ability to discriminate between instantaneously presented colours in the hemianopic field depended on their luminance, indicating that wavelength discrimination totally independent of other stimulus qualities is absent. When presented with narrow-band colours the hemianopes detected a stimulus maximally effective for S-cones but invisible to M- and L-cones, indicating that blindsight is mediated not just by the mid-brain, which receives no S-cone input, or that the rods contribute to blindsight. The results show that only simple stimulus features are processed in blindsight.     

Perceptual, oculomotor, and neural responses to moving color plaids

Moving plaids constructed from two achromatic gratings of identical luminance contrast are known to yield a percept of coherent pattern motion, as are plaids constructed from two identical chromatic (e.g. isoluminant red/green) gratings. To examine the interactive influences of chromatic and luminance contrast on the integration of visual motion signals, we constructed plaids with gratings that possessed both forms of contrast. We used plaids of two basic types, which differed with respect to the phase relationship between chromatic and luminance modulations (after Kooi et al, 1992 Perception 21 583-598). One plaid type ('symmetric') was made from component gratings that had identical chromatic/luminance phase relationships (e.g. both components were red-bright/green-dark modulation). The second plaid type ('asymmetric') was made from components that had complimentary phase relationships (i.e. one red-bright/green-dark grating and one green-bright/red-dark grating). Human subjects reported that the motion of symmetric plaids was perceptually coherent, while the components of asymmetric plaids failed to cohere. We also recorded eye movements elicited by both types of plaids to determine if they are similarly affected by these image cues for motion coherence. Results demonstrate that, under many conditions, eye movements elicited by perceptually coherent vs noncoherent plaids are distinguishable from one another. To reveal the neural bases of these perceptual and oculomotor phenomena, we also recorded the responses of neurons in the middle temporal visual area (area MT) of macaque visual cortex. Here we found that individual neurons exhibited differential tuning to symmetric vs asymmetric plaids. These neurophysiological results demonstrate that the neural mechanism for motion coherence is sensitive to the phase relationship between chromatic and luminance contrast, a finding which has implications for interactions between 'color' and 'motion' processing streams in the primate visual system.