Still motion? Motion illusions and luminance contrast

The influence of different luminance contrasts and equiluminance on illusory motion in four inducing patterns was studied: enigma, rotating snakes, pinna, and rotating-tilted-lines. At high luminance contrast the Pinna and the rotating-tilted-lines illusions are significantly stronger than at low luminance contrast, whereas the Enigma and Rotating Snakes illusions are stronger at low luminance contrast. At equiluminance along the L-M axis the strength of all illusions is greatly reduced. Data suggest that luminance contrast constitutes one important factor for their occurrence.     

Sharp-edged vs. diffuse illusory circles: The effects of varying luminance

As had been found previously, one experiment demonstrated that reducing the luminance of a pattern that induced an abrupt-edged illusory figure increased mean ratings of that illusory effect. More importantly, the same result had not been found for a pattern that induced a diffuse illusory figure; in fact, a second experiment with such patterns produced a reliable tendency in the opposite direction. These results are at variance with the suggestion that an abrupt-edged illusory effect involves merely a minor variation in the boundary definition of a diffuse illusory lightness effect.     

Effects of luminance contrast on color spreading and illusory contour in the neon color spreading effect

The present study examined whether color spreading and illusory contours in the neon color spreading effect of Ehrenstein figures are governed by different mechanisms. In the experiment, Ehrenstein figures with colored crosses inserted in the central gaps were used. There were three luminance conditions: the luminance of the Ehrenstein figures was lower than, the same as, or higher than the luminance of the background. In each condition, 16 trials (2 sets of instructions X 8 repetitions) were conducted in a random order. Subjects were required to adjust the luminance of the colored crosses according to one of the two sets of instruction given before each trial. One was to adjust the upper and lower thresholds in the luminance of the colored crosses such that their color was seen to spread out of the crosses. The other was to adjust the thresholds such that circular illusory contours were visible. It was found that illusory contours disappeared and the color spreading remained when the crosses and the Ehrenstein figures were in or nearly in isoluminance or when the Ehrenstein figures and the background were in isoluminance. These results suggest that color spreading and illusory contours are governed by different mechanisms.     

Brightness matching, brightness cancellation, and increment threshold in the Ehrenstein illusion

Matching and cancellation techniques were used to measure the relative strength of the Ehrenstein illusion in dark figures on a light background (negative contrast) and light figures on a dark background (positive contrast). Brightness enhancement on the former was shown to be maximally 0.28 log unit (relative to the detection threshold), and darkness enhancement on the latter 0.43 log unit. Values differed little with figure-ground contrast (down to a minimum of +/- 0.5), but decreased with decreasing level of illumination. The luminance increment (decrement) needed to match the illusory brightness (darkness) was similar in size to the luminance decrement (increment) needed to cancel the illusion. The increment threshold for a small test flash measured in three locations relative to the subjective contour delineating the illusion did not differ systematically. The results are compatible with a neurophysiological explanation of the Ehrenstein illusion in terms of line-induced lateral interaction in hypercomplex receptive fields.     

Dancing shapes: a comparison of luminance-induced distortions

We present an illusory display in which a grid of outlined squares is positioned in front of a moving luminance gradient. Observers perceive a strong, illusory, 'wavelike' motion of the superimposed squares. We compared luminance effects on dynamic and static aspects of this illusion. The dynamic aspect was investigated by means of a temporal gradient, which induced an illusory pulsing of the outlined squares. The static aspect was investigated in two different ways. In one experiment, the outlined squares were positioned on a spatial gradient, which caused the squares to look like trapezoid shapes. In another experiment, the squares were positioned on different luminance fields, which affected their apparent size. In all experiments, luminance settings were the same, and observers were asked to indicate the direction and strength of the induced distortions. The overall results show large agreements between the dynamic distortion and the first-mentioned static distortion, whereas different tendencies emerged for the second static distortion. In a second series of experiments, we examined these distortions for various ranges of the luminance gradient and for border gradients as well. On the basis of these data, we explored how the directions of the perceived distortions of the single-gradient displays examined in this paper could be related to each other.     

Local brightness mechanisms sketch out surfaces but do not fill them in: psychophysical evidence in the Kanizsa square.

In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous contrast. The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of both test and control figures (representing or not representing an illusory square). These observations suggest that low-level mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness. In other words, local contrast seems to generate brightness information that "sketches out" surfaces at their surrounds but does not "fill" them "in."     

Edge-induced illusory contours and visual detection: Subthreshold summation or spatial cueing?

Findings that illusory contours can facilitate visual detection of a subthreshold real line (Dresp & Bonnet, 1995) were not replicated, when line-induced instead of edge-induced illusory contour stimuli were used (Salvano-Pardieu et al., 2006). Rather, the results of the latter study supported the importance of spatial cues. The present study was designed to investigate whether spatial cueing might also facilitate detection of targets superimposed on edge-induced illusory contours. In Experiment 1, a target line was superimposed on the illusory contour of a Kanizsa square, presented between dots with a precise or distant location to the target, or on a homogeneous field (control). Detection of the target was poorest for the control, followed by the distant-dots and Kanizsa conditions, whereas it was best for precisely cueing dots. Experiment 2 replicated the conditions in Experiment 1 (Kanizsa, precisely cueing dots, and control) with additional controls for possible luminance effects. The two new conditions matched the Kanizsa condition for overall luminance and preciseness of spatial cueing without generating illusory contours. Performance was best in the dot condition and worst in the control, but the same across the Kanizsa and matched luminance conditions. In Experiments 3 and 4, the stimuli presented were matched more closely to those used by Dresp and Bonnet, but still the results confirmed those obtained in our Experiments 1 and 2. Together, these experiments strongly suggest that detection is also facilitated by spatial cueing rather than subthreshold summation, in the case of edge-induced illusory contours.     

The effect of luminance variation on the apparent position of an edge

A gray outline against a white (or black) ground appears to deviate when one of the divided regions turns into black (white). The direction of shift is not predictable on the basis of luminance profile and polarity contrast of this part of contour, called gray edge (to indicate a stepwise gradient from black to gray and from gray to white). Rather, it appears to depend on the luminance profiles of the collinear regions: A gray edge shifts toward the dark side whenever collinear with a gray line traversing a white ground. The same gray edge takes the opposite direction whenever it extends against a black ground. This rule proved to be successful in predicting the illusory convergence of the sides of a square that formed the stimuli of the first experiment, but the magnitude of the phenomenon was affected by luminance ratios and polarity contrasts of the gray edges, in agreement with the findings of the experiments on gray or blurred edge misalignment. A second experiment tested some hypotheses predicting the combined effects of two or more distorting sources. These hypotheses, suggested by the physical theory of vector sum, were partially disproved. A new model is proposed that assumes different ways of integrating local distortions. The third experiment tested predictions of how distorting pulses in opposite directions combine. The illusory misplacement of edge studied in this experiment is proposed as the underlying phenomena of the café wall illusion, the hollow square illusion, and other illusory phenomena observed with blurred areas. A connection with the induction grid phenomena is hypothesized.     

Onset of illusory figures attenuates change blindness

We examined whether the onset of a new object defined by illusory contours is detected with greater frequency than offset when neither is associated with a unique sensory transient. Observers performed a “one-shot” change detection task in which offsetting or onsetting elements of high luminance contrast circles generated the appearance or disappearance of a Kanizsa figure. Presenting “illusory figures” via this “flicker” method ensures that (1) any unique luminance transients associated with the two types of change are eliminated, and (2) the objects themselves can only be represented at a relatively high level. Results showed that offsets were detected more frequently than onsets only when they generated the onset of a Kanizsa figure. We argue that object appearance dominates object disappearance via mechanisms that operate at the level at which objects are constructed.     

Luminance gradient can break background-independent lightness constancy

A display with a luminance gradient was shown to induce a strong lightness illusion (Logvinenko, 1999 Perception 28 803-816). However, a 3-D cardboard model of this display was found to produce a much weaker illusion (less than half that in the pictorial version) despite the fact that its retinal image is practically the same. This is in line with the hypothesis that simultaneous lightness contrast is solely a phenomenon of pictorial perception (Logvinenko et al, 2002 Perception 31 73-82). The residual lightness illusion in the 3-D model can be accounted for by the fact that this model is a hybrid display. Specifically, while it is a real object, a pictorial representation (of the illumination gradient) is superimposed on it. Thus, lightness in the 3-D display is a compromise between two opposite tendencies: the background-independent lightness constancy and the lightness illusory shift induced by the luminance gradient.     

A variant of the anomalous motion illusion based upon contrast and visual latency

We examined a variant of the anomalous motion illusion. In a series of experiments, we ascertained luminance contrast to be the critical factor. Low-contrast random dots showed longer latency than high-contrast ones, irrespective of whether they were dark or light (experiments 1 -3). We conjecture that this illusion may share the same mechanism with the Hess effect, which is characterised by visual delay of a low-contrast, dark stimulus in a moving situation. Since the Hess effect is known as the monocular version of the Pulfrich effect, we examined whether illusory motion in depth could be observed if a high-contrast pattern was projected to one eye and the same pattern of low-contrast was presented to the other eye, and they were binocularly fused and swayed horizontally. Observers then reported illusory motion in depth when the low-contrast pattern was dark, but they did not when it was bright (experiment 4). Possible explanations of this inconsistency are discussed.     

Local brightness mechanisms sketch out surfaces but do not fill them in: Psychophysical evidence in the Kanizsa square

In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous con trast. The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of both test and control figures (representing or not representing an illusory square). These observations suggest that low-level mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness. In other words, local contrast seems to generate brightness information that “sketches out” surfaces at their surrounds but does not “fill” them “in.”     

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.     

The role of early visual input in the development of contour interpolation: the case of subjective contours

We tested the effect of early monocular and binocular deprivation of normal visual input on the development of contour interpolation. Patients deprived from birth by dense central cataracts in one or both eyes, and age‐matched controls, discriminated between fat and thin shapes formed by either illusory or luminance‐defined contours. Thresholds indicated the minimum amount of curvature (the fatness or thinness) required for discrimination of the illusory shape, providing a measure of the precision of interpolation. The results show that individuals deprived of visual input in one eye, but not those deprived in both eyes, later show deficits in perceptual interpolation. The deficits were shown mostly for weakly supported contours in which interpolation of contours between the inducers was over a large distance relative to the size of the inducers. Deficits shown for the unilateral but not for the bilateral patients point to the detrimental effect of unequal competition between the eyes for cortical connections on the later development of the mechanisms underlying contour interpolation.     

When figure-ground segmentation modulates brightness: the case of phantom illumination

In the phantom illumination illusion, luminance ramps ranging from black to white induce a brightness enhancement on an otherwise homogeneous dark background. The strength of the illusion was tested with regard to the extension of the brightness inducing perimeter, surrounding the target area by manipulating the number of inducers (exp. 1) and the size of the inducers (exp. 2). Participants' task was to rate the difference in brightness between the target area and the background. Results show that the illusion occurs only when the target area is not completely segregated from the background by luminance ramps; vice versa, when the target area is delimited by a continuous gradient, it appears darker than the background. These findings suggest a major role of figure-ground organization in the appearance of the illusion. This hypothesis was tested in a rating task experiment with three types of target area shapes circumscribed by four types of edges: luminance contours, illusory contours, no contours, and ambiguous contours. Illusory contours, just as luminance contours, hinder the illusion and produce a darkening of the target area. A control experiment measured the brightness of the previous stimuli without luminance ramps: all configurations resulted in a darkening of the target area. Results from all experiments suggest that figure-ground segmentation plays a major role in the determination of both illumination and lightness in stimuli with luminance gradients.     

Response times to Ehrenstein illusions of varying subjective magnitude: complementarity of psychophysical measures

The following experiments investigate the effects of contrast polarity, inducer spacing, and inducer type on three dependent variables measuring the perception of an illusory surface in Ehrenstein figures: subjective magnitude, response time, and frequency of perception. It was found that response time generally decreased when the other two behavioral indicators increased. However, it was also shown that subjective magnitude provided more discriminating measures of relatively strong illusory percepts, whereas frequency of perception and response time provided more discriminating measures of relatively weak illusory percepts. The findings generally confirm earlier work on the effect of inducer spacing and contrast polarity on the perceived strength of brightness illusions, and in particular reveal the complementarity of subjective magnitude, response time, and frequency of perception as critical measures of configurational effects in the perceptual processing of these phenomena. Thanks are due to J. E. Hoffman, T. Parks, and an anonymous referee for their insightful comments and suggestions on an earlier version of this paper.     

Visual dissociations of movement, position, and stereo depth: Some phenomenal phenomena

Helmholtz (1867) described as “irradiation” the apparently greater size of a white compared with a dark square, or disc or whatever of the same physical size. The illusory size difference is reversed at low contrasts (Weale, 1974). It is also known that rapid increases in brightness gives apparent movement (gamma movement), though there is no agreed explanation for either phenomenon.

When narrow bordering stripes are added, further systematic phenomena occur. With intensity modulation of an edge-striped grey rectangle, which has a dark stripe on the left side and a light stripe on the right (which is similar to figures used by Stuart Anstis and Brian Rogers), the entire figure shifts, with reversed motion when the background luminance is modulated. By presenting a pair of such figures, mirror reversed one to each eye and fused stereoscopically, the question may be asked: Do these illusory shifts produce stereo depth? The answer is surprising: stereo is produced-but at the cross-over with luminance of the central grey rectangle with the background the depth change is opposite to that given by normal, non-illusory, opposed lateral shifts. We interpret this anomalous stereo depth as a switch of which edges of the stripes are fused, with the change of relative contrast of the edges of the dark and light stripes as the figure-background contrast is changed.

Measures of static shift, lateral movement, and stereo depth, give somewhat different functions. These are considered in terms of different signalled positions, stereo depth, and movement. This study brings out the importance, for explaining such perceptual anomalies, of distinguishing between neural signal channel characteristics and which stimulus features from the display are selected and accepted for perception. Although conceptually clearly distinct these are all too easily confused in psycho-physical experiments.     

Lip colour affects perceived sex typicality and attractiveness of human faces

The luminance contrast between facial features and facial skin is greater in women than in men, and women's use of make-up enhances this contrast. In black-and-white photographs, increased luminance contrast enhances femininity and attractiveness in women's faces, but reduces masculinity and attractiveness in men's faces. In Caucasians, much of the contrast between the lips and facial skin is in redness. Red lips have been considered attractive in women in geographically and temporally diverse cultures, possibly because they mimic vasodilation associated with sexual arousal. Here, we investigate the effects of lip luminance and colour contrast on the attractiveness and sex typicality (masculinity/femininity) of human faces. In a Caucasian sample, we allowed participants to manipulate the colour of the lips in colour-calibrated face photographs along CIELab L* (light--dark), a* (red--green), and b* (yellow--blue) axes to enhance apparent attractiveness and sex typicality. Participants increased redness contrast to enhance femininity and attractiveness of female faces, but reduced redness contrast to enhance masculinity of men's faces. Lip blueness was reduced more in female than male faces. Increased lightness contrast enhanced the attractiveness of both sexes, and had little effect on perceptions of sex typicality. The association between lip colour contrast and attractiveness in women's faces may be attributable to its association with oxygenated blood perfusion indicating oestrogen levels, sexual arousal, and cardiac and respiratory health.     

Contrast and depth perception: effects of texture contrast and area contrast

Many objects in natural scenes have textures on their surfaces. Contrast of the texture surfaces (the texture contrast) reduces when the viewing distance increases. Similarly, contrast between the surfaces of the objects and the background (the area contrast) reduces when the viewing distance increases. The texture contrast and the area contrast were defined by the contrast between random dots, and by the contrast between the average luminance of the dot pattern and the luminance of the background, respectively. To examine how these two types of contrast influence depth perception, we ran two experiments. In both experiments two areas of random-dot patterns were presented against a uniform background, and participants rated relative depth between the two areas. We found that the rated depth of the patterned areas increased with increases in texture contrast. Furthermore, the effect of the texture contrast on depth judgment increased when the area contrast became low.     

Lightness contrast in CRT and paper-and-illuminant displays

The increased use of CRT monitors for displaying and controlling stimuli in studies of surface color poses problems of comparability with data obtained with traditional paper-and-illuminant methods. A review of comparable studies using the two methodologies revealed that CRT studies tend to report larger contrast effects. To investigate factors that may be responsible for this difference, simultaneous lightness contrast was measured using both CRT and paper-and-illuminant presentations. The spatial distribution of luminance in the whole field of view and the visual angles subtended by the displays were controlled. The CRT presentation yielded contrast effects twice as big as those measured for a paper surface in a homogeneously illuminated room. However, a paper display under Gelb lighting yielded almost exactly the same effect size as that measured in the CRT presentation. These results demonstrate that contrast effects in both modes of presentation are affected by the spatial distribution of luminance beyond the basic experimental stimuli.