Simultaneous Brightness and Apparent Depth from True Colors on Grey: Chevreul Revisited

Abstract

We show that true colors as defined by Chevreul (1839) produce unsuspected simultaneous brightness induction effects on their immediate grey backgrounds when these are placed on a darker (black) general background surrounding two spatially separated configurations. Assimilation and apparent contrast may occur in one and the same stimulus display. We examined the possible link between these effects and the perceived depth of the color patterns which induce them as a function of their luminance contrast. Patterns of square-shaped inducers of a single color (red, green, blue, yellow, or grey) were placed on background fields of a lighter and a darker grey, presented on a darker screen. Inducers were always darker on one side of the display and brighter on the other in a given trial. The intensity of the grey backgrounds varied between trials only. This permitted generating four inducer luminance contrasts, presented in random order, for each color. Background fields were either spatially separated or consisted of a single grey field on the black screen. Experiments were run under three environmental conditions: dark-adaptation, daylight, and rod-saturation after exposure to bright light. In a first task, we measured probabilities of contrast, assimilation, and no effect in a three-alternative forced-choice procedure (background appears brighter on the 'left', on the 'right' or the 'same'). Visual adaptation and inducer contrast had no significant influence on the induction effects produced by colored inducers. Achromatic inducers produced significantly stronger contrast effects after dark-adaptation, and significantly stronger assimilation in daylight conditions. Grouping two backgrounds into a single one was found to significantly decrease probabilities of apparent contrast. Under the same conditions, we measured probabilities of the inducers to be perceived as nearer to the observer (inducers appear nearer on 'left', on 'right' or the 'same'). These, as predicted by Chevreul's law of contrast, were determined by the luminance contrast of the inducers only, with significantly higher probabilities of brighter inducers to be seen as nearer, and a marked asymmetry between effects produced by inducers of opposite sign. Implications of these findings for theories which attempt to link simultaneous induction effects to the relative depth of object surfaces in the visual field are discussed.

Combining Visual Contrast Information with Sound Can Produce Faster Decisions

“…Positively signed light-on-dark pattern pairs (Figure 3, graphs on left) produced shorter response times in comparison with negatively signed dark-on-light pattern pairs (Figure 3, graphs on right), despite the fact that the pattern pairs with negative contrast sign displayed moderately stronger differences in visual contrast (dC) between patterns in a pair. This effect of contrast polarity was statistically significant (Table 2), which is explained by the well-documented functional asymmetry between the so-called "on" and "off" contrast processing channels in the human brain [19][20][21][22][23][24][25][26][27][30][31][32]. One of the perceptual consequences of this functional asymmetry is that positively signed contrast configurations, processed by the "on" channels of the visual brain, produce stronger effects of figure-ground segregation [24] and relative depth [17], with shorter perceptual decision times, as confirmed by results here.…”

“…The subject was comfortably seated in front of the computer, at a distance of about 80 cm from the screen, in a semidark room (mesopic viewing condition) and adapted to surrounding conditions for about five minutes. He/she was informed that images with two abstract patterns, one on the left and one on the right, will be shown in sequences, preceded or not by a brief tone, and that his/her task was to decide as quickly as possible which of the two patterns, the left or the right one, in a given image appeared to "stand out as if it were nearer" in terms of apparent (subjective) visual depth, as previously in [17,18,21,23,24]. A response had to be delivered by pressing '1' for 'left' or '2' for 'right', and was recorded and stored in a labeled data column of an excel file.…”

“…Information 2018, 9, x FOR PEER REVIEW 7 of 11 "on" and "off" contrast processing channels in the human brain [19][20][21][22][23][24][25][26][27][30][31][32]. One of the perceptual consequences of this functional asymmetry is that positively signed contrast configurations, processed by the "on" channels of the visual brain, produce stronger effects of figure-ground segregation [24] and relative depth [17], with shorter perceptual decision times, as confirmed by results here.…”

“…The human brain's capacity to exploit combined information of visual contrast and sound in motor response behavior [11] has important implications in the context of a variety of operator tasks and in the context of human-computer interaction systems where optimal motor performance is critical [12][13][14]. The goal of this study here was to bring to the forefront the ability of individuals to effectively use visual contrast and sound for making faster perceptual decisions by taking into account the well-documented capacity of the human perceptual system to extract subjective cues of relative depth from planar (2D) object configurations on the basis of physical variations in luminance contrast [15][16][17][18][19][20][21][22][23][24][25][26][27]. As shown previously, in 2D configurations with higher-contrast and lower-contrast visual objects, those with the higher contrast will be consistently perceived as "nearer" by human observers.…”

“…In addition, the sound frequency factor was varied here to show that, in the light of Chocholle's seminal work [7], stronger contrasts combined with higher sound frequencies preceding the contrast stimulus will produce even faster forced-choice decisions, in further consistency with probability summation. The contrast factor here was varied in the context of perceptual decisions for "nearer" in response to the stronger one of two simultaneously presented image configurations [15][16][17][18][19][20][21][22][23][24][25][26][27]. The investigation takes into account that identical sounds in terms of physical intensity (dB) with higher frequencies have a higher average energy for any given section of the sound wave compared with lower frequency sounds.…”