A fair test of the effect of a shadow-incompatible luminance gradient on the simultaneous lightness contrast

Shadow-compatibility of simultaneous lightness contrast is discussed by Alexander D Logvinenko and Paola Bressan, with examples claiming to provide a test of the hypothesis.     

An effect of luminance contrast on high-spatial-frequency tritanopia

Luminance contrast was found to affect high-spatial-frequency tritanopia despite the S-cone coordinates of the stimuli being kept constant. This proves that a traditional account of artificial tritanopias based on the spatial resolution differences between the S-cone channel and the M- and L-cone channels is not applicable here. We suggest that the lack of spatial resolution in one of the post-receptor (rather than receptor) spatio-chromatic channels could be a cause of high-spatial-frequency tritanopia.     

Interactions between simultaneous contrast and adaptation to gradual change of luminance

Following adaptation to a field of light which was modulated by a rising ramp so that it repetitively grows gradually brighter, a steady test field of light appears to be gradually growing dimmer. In this study, if a small grey spot of constant luminance was centered in the brightening field, it appeared to be growing gradually dimmer by simultaneous contrast. This apparent dimming led to a brightening aftereffect in the spot. It was shown that this spot aftereffect had two independent components: the apparent dimming of the adapting spot produced its own aftereffect (contrast produced an aftereffect) and also the dimming aftereffect in the surround field spatially induced an aftereffect into the spot during the test period (aftereffect produced simultaneous contrast). Thus simultaneous contrast can both precede and follow successive contrast in the visual system.     

Differential effect of luminance contrast reduction and noise on motion induction

Motion perception in a region is affected by motion in the surround regions. When a physically static or flickering stimulus surrounded by moving stimuli appears to move in the direction opposite to that of the surround motion, it is referred to as motion contrast. When the centre appears to move in the same direction, it is referred to as motion assimilation. We investigated how noise and luminance contrast affect motion induction by employing static and dynamic counterphase flickering targets. The tendency of motion assimilation was found to be stronger at a high noise level than at a low noise level for both static and dynamic targets. On the other hand, a decrease of luminance contrast tended to strengthen the tendency of motion contrast. However, the addition of noise and the decrease of luminance contrast decreased the visibility of motion comparably. These results suggest that the visual system changes the mode of motion induction according to the noise level, but not the visibility.     

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.     

Effect of luminance contrast on the motion aftereffect

The effects of luminance contrast and spatial frequency on the motion aftereffect were investigated. The point of subjective equality for velocity was measured as an index of the motion aftereffect. The largest effect was observed when a low contrast grating (5%) was presented as a test stimulus after adaptation to a high contrast grating (100%) in the low spatial frequency condition (0.8 cycle deg.-1). On the whole, the effect increased with increasing adapting contrast and with decreasing test contrast or spatial frequency. Small effects were observed at high test contrasts. These results were inconsistent with those of Keck, Palella, and Pantle in 1976. Analysis showed that there was no saturation on velocity of the motion aftereffect above 5% of the contrast although Keck, et al. (1976) found that the incremental increases of the effect above 3% adapting contrast were small.     

Effects of absolute luminance and luminance contrast on visual discrimination in low mesopic environments

Recent research revealed considerable decline in visual perception under low luminance conditions. However, systematic studies on how visual performance is affected by absolute luminance and luminance contrast under low mesopic conditions (<0.5 cd/m²) is lacking. We examined performance in a simple visual discrimination task under low mesopic luminance conditions in three experiments in which we systematically varied base luminance and luminance contrast between stimulus and background. We further manipulated eccentricity of the stimuli because of known rods and cones gradients along the retina. We identified a “deficiency window” for performance as measured by d’ when luminance was < 0.06 cd/m² and luminance contrast as measured by the luminance ratio between stimulus and background was below < 1.7. We further calculated performance-based luminance as well as contrast efficiency functions for reaction times (RTs). These power functions demonstrate the contrast asymptote needed to decrease RTs and how such a decrease can be achieved given various combinations of absolute luminance and luminance contrast manipulations. Increased eccentricity resulted in slower RTs indicative of longer scan distances. Our data provide initial insights to performance-based efficiency functions in low mesopic environments that are currently lacking and to the physical mechanisms being utilized for visual perception in these extreme environments.     

Effects of absolute luminance and luminance contrast on visual search in low mesopic environments

Diverse adaptive visual processing mechanisms allow us to complete visual search tasks in a wide visual photopic range (>0.6 cd/m²). Whether search strategies or mechanisms known from this range extend below, in the mesopic and scotopic luminance spectra (<0.6 cd/m²), has yet to be addressed. Based on a study that addressed simple target discrimination in luminance environments using contrast-dependent behavioral efficiency functions, we assessed visual search in more complex-feature and conjunction-search paradigms. The results verify the previously reported deficiency windows defined by an interaction of base luminance and luminance contrast for more complex visual-search tasks. Based on significant regression analyses, a more precise definition of the magnitude of contribution of different contrast parameters. Characterized feature search patterns had approximately a 2.5:1 ratio of contribution from the Michelson contrast property relative to Weber contrast, whereas the ratio was approximately 1:1 in a serial-search condition. The results implicate near-complete magnocellular isolation in a visual-search paradigm that has yet to be demonstrated. Our analyses provide a new method of characterizing visual search and the first insight in its underlying mechanisms in luminance environments in the low mesopic and scotopic spectra.

     

Adaptation of suprathreshold contrast and luminance stimuli

The temporal and spatial properties of the difference in perceived contrast and brightness of two suprathreshold stimuli presented successively in different retinal locations were determined. Stimulus onset asynchrony (SOA) was varied and the perceived contrast or brightness of the first stimulus (S1) was measured as a function of SOA by matching the contrast or luminance of the second stimulus (S2) to that of S1. The two stimuli overlapped in time for 200 ms to allow the comparison to be made. The adjusted values for S2 could well be fitted with an exponential decay function of SOA. For luminance increments and decrements the time constant for this function was 253 ms; for checkerboards with checks of size 16 min square the time constant was 164 ms. The difference in perceived contrast was dependent on initial contrast in a nonlinear fashion. It increased with increasing check size and was independent of the mean luminance and spatial proximity of the two stimuli. The phenomenon was observed with different pattern types and with dichoptic presentation, but could only be seen when direct comparison of the two stimuli was possible.     

Illumination, background luminance, and perceptual learning in perception of achromatic colors

Plateau's experiment, in which the bisection method was introduced, is recognized as the first psychophysical experiment ever done. That experiment was replicated and extended using modem imaging devices. First, participants with professional training in visual arts painted grayscales with no control on illumination. Then, the trained participants and the nontrained participants generated grayscales using a personal computer in three different conditions: a white background of a linearized monitor, a black background of that monitor, and a white background but without linearizing the monitor. Analyses showed that with artificial illumination, scales with steps more evenly spread were produced than with natural illumination. Humans do not seem to have a natural ability to use the bisection method properly. Experts seem to be able to judge the reflectance of distal stimuli, and perceptual learning played an important role in the perception of achromatic colors as found in prior literature.     

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.     

Effects of luminance and contrast on direction of ambiguous apparent motion

A study is reported of the role of luminance and contrast in resolving ambiguous apparent motion (AM). Different results were obtained for the short-range (SR) and the long-range (LR) motion-detecting processes. For short-range jumps (7.5 min arc), the direction of ambiguous AM depended on brightness polarity, with AM only from white to white and from black to black. But for larger jumps, or when an interstimulus interval (ISI) was introduced, AM was less dependent on polarity, with white often jumping to black and black jumping to white. Two potential AMs were pitted against each other, one carried by a light stimulus and the other by a dark stimulus. The stimulus whose luminance differed most from the uniform surround captured the AM. Visual response to luminance was linear, not logarithmic. When the stimulus was modified to give continuous AM in one direction it was followed by a negative aftereffect of motion only when the spatial displacement was 1 min arc. A larger displacement (10 min arc) gave good AM but no motion aftereffect. Thus only short-range motion adapts motion-sensitive channels.     

Revisitation of the luminance conditions for the occurrence of the achromatic neon color spreading illusion

This paper develops the idea (Bressan, 1993) that neon spreading derives from the perceptual scissioning of ordinary assimilation color, a process identical to that occurring with nonillusory colors in phenomenal transparency. It is commonly held that the critical elements in achromatic neon spreading patterns must be of luminance intermediate between that of the embedding lines and of the background. The interpretation of neon spreading on the basis of color scissioning, however, predicts that neon spreading should also be observed for different luminance hierarchies, provided that these are compatible with transparency. This prediction found experimental support in the present work. The results suggest that (1) the widespread notion that chromatic and achromatic neon spreading must be mediated by separate mechanisms is unwarranted; (2) the widespread notion that color spreading in ordinary assimilation patterns and color spreading in neon patterns must be mediated by separate mechanisms is unwarranted; and (3) other than pointing to the way in which the overall organization of a scene affects the mode of color appearance, the neon spreading effect may not convey any extra theoretical relevance.     

Colour changes as a function of luminance contrast.

When spectral light increases in luminance, the hues change. Normally, long-wavelength light becomes increasingly yellow, and short-wavelength light turns blue or blue-green. This is known as the Bezold-Brücke hue shift. Less notice has been paid to the change in relative chromatic content (saturation or chromatic strength) that accompanies these shifts in hue. As luminance contrast increases from zero, chromatic strength increases to reach a maximum at a luminance that is wavelength dependent. Short-wavelength blueish light reaches this maximum at low relative luminances, whereas midspectral yellowish stimuli need several log units higher luminance. Red and green are somewhere in between. For luminances above this maximum, the chromatic content usually diminishes, and most light becomes more whitish in appearance. In this study it is demonstrated how the combined chromatic appearance of hue and chromatic strength change with intensity. Both phenomena find a common physiological interpretation in the nonlinear and nonmonotonic responses of colour-opponent P cells in the retina and lateral geniculate nucleus of the primate. A model that combines the outputs of six P-cell types accounts for observers' estimates of hue and chromatic strength.     

Assimilation of achromatic color cannot explain the brightness effects in the achromatic neon effect

If the mouths of the pacmen of a Kanizsa square are colored, for example red, then an illusory red transparent square is seen. In many visual theories such 'neon color spreading' is explained by assimilation of chromatic and achromatic color. In this paper the achromatic case was investigated. In a two-alternative forced-choice task thirty observers judged the brightness of achromatic neon figures. The results suggest that assimilation of achromatic color inside and/or outside of the illusory figures cannot explain the brightness effects seen in achromatic neon color spreading. Although these displays may produce assimilation, it appears that contrast (perhaps acting nonlocally) is a stronger influence on their perceived brightness.     

A weighted-average model of achromatic transparency

A model of achromatic transparency based on the idea that neural representations of transparency are activated by proximal contours is described. It is proposed that the weighted average of the magnitudes of the representations of transparency relative to a perceived continuous transparent surface corresponds to the judgement of the overall degree of transparency of the same surface. Tests of this weighted-average model were carried out with bistable patterns formed by two overlapping surfaces that appeared opaque where they were superimposed on the background and transparent where they were superimposed on each other (partial transparency). In agreement with predictions from the weighted-average model, the rated degrees of transparency of these two surfaces were noncomplementary and independent of background reflectance. Two experiments confirmed the contention of this model that the relevant proximal contours for the judgement of partial transparency of the two overlapping transparent surfaces in a bistable pattern correspond to the part where these surfaces are superimposed.     

Size of test patch and simultaneous contrast

Two experiments were done to find the effect on simultaneous brightness contrast of placing test patches in, as against outside, the fovea. When the display was magnified so that the test patches were outside the fovea no statistically significant effect was found. Very short exposures, to eliminate eye movements, did not appear to reduce contrast.