An empirical explanation of brightness

The striking illusions produced by simultaneous brightness contrast generally are attributed to the center-surround receptive field organization of lower order neurons in the primary visual pathway. Here we show that the apparent brightness of test objects can be either increased or decreased in a predictable manner depending on how light and shadow are portrayed in the scene. This evidence suggests that perceptions of brightness are generated empirically by experience with luminance relationships, an idea whose implications we pursue in the accompanying paper.     

Transparent layer constancy.

Perceived transparency was studied as a constancy problem. In the episcotister (E-) model of scission, luminances are partitioned into layer and background components; four luminances determine values of two layer parameters that specify constancy of a transparent layer on different backgrounds. The E-model was tested in an experiment in which 12 Ss matched 24 pairs of four-luminance patterns by adjusting two luminances of the comparison pattern. Both the standard and the comparison were perceived as a transparent layer on a checkerboard. The E-model predicts matches when layer values are identical in the two patterns. One parameter was constant, constraining the adjustment along the second dimension. Obtained values corresponded well with E-predictions. Alternative models based on local luminance or average contrast ratios accounted for less variability. Results indicate that transparency models should utilize luminance, not reflectance, as the independent variable. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Brightness induction from uniform and complex surrounds: a general model

We studied the brightness induced from complex non-figural achromatic surrounds. A spatially uniform test field was surrounded by a random texture composed of two sets of dots. The luminance of each set of dots was modulated sinusoidally at 0.5 Hz. The mean luminance, phase and amplitude of modulation of each set were controlled independently so as to modulate the luminance and/or the contrast of the surround. Brightness induction was measured by a modulation nulling technique. The results were fit by a model in which the total brightness induced by a surround is equal to a weighted spatial summation of the induced effects from each point in the surround. The model incorporates local luminance gain controls in the test and surround fields and assumes that the magnitude of induction from each surround element is gain controlled by the difference between the mean luminance of the test and the individual surround elements.     

Binocular brightness: A suppression-summation trade off.

Two experiments with 22 undergraduates estimated binocular brightness of targets of large visual extent. On each trial one eye was presented with a fairly intense luminance of 800 cd/m–2, and the other eye with 1 of 12 luminances ranging from zero to 800 cd/m–2. Exp 1, using ganzfeld stimuli, produced a large amount of binocular brightness summation and very little Fechner's paradox, a decrease in binocular brightness that occurs when the luminances to the 2 eyes differ greatly. Exp 2, using a smaller target with very low spatial frequencies, produced greater Fechner's paradox than the ganzfelder, but more binocular summation and less Fechner's paradox than what is usually reported for small targets with abrupt contours. Results suggest a trade-off between suppressive and summative mechanisms involving binocular cells that are spatially tuned. (French abstract) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Use of image-based information in judgments of surface-reflectance properties

We examined how well we can recover surface-reflectance properties from shading patterns under changes in surface shape. The stimulus we used was a square surface modulated in depth by a low-pass-filtered random field and rendered by the Phong illumination model [Commun. ACM 18, 311 (1975)]. Two different surface images (target and match) were presented side by side, with either the viewing direction or the surface-normal direction rotating around the horizontal axis. The target shape was manipulated by changing the spatial spectrum, and the target reflectance was manipulated by changing the diffuse-reflection coefficient and the specular-reflection exponent (shininess) of the Phong model. The shape parameters of the match stimulus were fixed, but its reflectance parameters were under the control of subjects, who had to make the apparent reflectance of the two surfaces as similar as possible. The results showed that the constant error (difference between simulated and matched values) was large except when the two surfaces had the same shape parameters or when they differed only in scale. The pattern of the constant errors and response variabilities suggests that the judgments of the subjects were based on the similarity of the luminance histogram of the surface image. Our results demonstrate a limitation of surface-reflectance constancy for changes in shape and the importance of image-based information in reflectance judgments. The results are discussed in relation to previous studies that showed effects of spatial layout on surface-reflectance perception.     

The specific nature of the color and brightness components of the human visual evoked potential

In order to gain an insight into the electrophysiological cortical mechanisms of color discrimination and to compare the results with psychophysiological data summarized in the previous publications as the spherical model of color discrimination a problem was specified to identify color and brightness components of human evoked potentials. The experiments were carried out with alternating pairs of light flashes constituted of five colors (white and four main colors; red, blue, yellow, and green). Each of the light stimuli varied by seven brightness levels. Color and brightness components (N87 and P120, respectively) were reasonably reliably detected in all cases of substitution of stimuli with identical or different spectra. However, the latency and amplitude analysis of N87 and P120 components in these cases showed that N87 reflects not only color but also brightness information. It makes it possible to draw on the analogy between the N87 as one of the earliest components and N1 in primate cortical evoked potential and suggest that these components reflect the activity of cells receiving information directly from the lateral geniculate body. This process can be considered as the first stage of cortical analysis of chromatic and achromatic light characteristics. The brightness component P120, probably, represents the activity of cortical cells related to the analysis of nonchromatic stimuli characteristics, such as form, movement, orientation, etc. These characteristics are also based on luminance gradients and contrasts, however, in contrast to N87, these characteristics are not directly related with brightness of light.     

Skimming lists of food ingredients printed in different brightness contrasts.

76 29-72 yr. old Ss searched for particular words in lists of ingredients printed on white paper, reflectance 85%, in ink with densities of 1.3, .4, .2, and .1 (reflectances 4, 34, 53, and 68%). The print was 6-pt lower-case Univers with .6-pt leading. There were 4 sets each of 15 lists. A 4 * 4 factorial design was used that confounded list difficulty with order. In separate experiments the lighting was 40 and 2 ft-c. There were large drops (p .05). 2 Ss failed to locate any ingredients in the poor light when the density of the ink was .1. It is concluded that ingredients printed in 6-pt lower-case Univers on white paper should have an ink density of at least .4. The contrast ratio between the ink and the paper is then at least 60%; the relative brightness ratio is at least 2.5:1. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Continuous measurement of visible persistence.

In the synchrony judgment paradigm, Ss judge whether a click precedes or follows the onset of a light flash and, on other trials, whether or not a click precedes light termination. The interclick interval defines the duration of visible persistence. An elaboration of this method was developed that consisted of 2 phases: In Phase 1, the luminance of a reference stimulus was psychophysically matched to the peak brightness of the test flash. Five luminance values between .1 and 1.0 of the reference stimulus were used subsequently. In Phase 2, a random 1 of the 5 reference stimuli, a test flash, and a click were presented; the Ss judged whether the click occurred before or after the brightness of test flash reached the reference value (on onset trials) or decayed below it (on termination trials). This method was validated on 3 male graduate students with test stimuli whose luminance rose and decayed slowly in time, and then was used to trace out the precise subjective rise and decay (temporal brightness response function) of brief flashes. (40 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Reaction time to motion onset of luminance and chromatic gratings is determined by perceived speed

We measured reaction times for detecting motion onset for sinusoidal gratings whose contrast was modulated in either luminance or chromaticity, for various drift rates and contrasts. In general, reaction times to chromatic gratings were slower than to luminance gratings of matched cone contrast, but the difference in response depended critically on both contrast and speed. At high image speeds there was virtually no difference, whereas at low speeds, the difference was pronounced, especially at low contrasts. At high image speeds there was little dependence of reaction times on contrast (for either luminance or colour), whereas at low speeds the dependence was greater, particularly for chromatic stimuli. This pattern of results is reminiscent of those found for apparent speed of drifting luminance and chromatic gratings. We verified the effects of contrast on perceived speed, and went on to show that the effects of contrast on reaction times are totally predictable by the perceived speed of the stimuli, as if it were perceived rather than physical speed that determined reaction times. Our results support that idea of separate systems for fast and slow motion (with separate channels for luminance and colour at slower speeds), and further suggest that apparent speed and reaction times may be determined at a similar stage of motion analysis.     

Retention of three brightness discriminations by rats following posterior cortical lesions.

Trained 60 male Long-Evans hooded rats on 1 of 3 brightness discriminations. In 1 task, the discriminanda differed in both luminance and luminous flux. In the 2nd task, the discriminanda differed only in terms of luminous flux. In the 3rd task, the discriminanda differed only in terms of luminance. Following acquisition, 1/2 of the Ss on each task underwent removal of the striate cortex. Retention tests indicate that a discrimination based on flux cues was relatively undisturbed following striate cortex removal, whereas a discrimination based on luminance cues appeared to be permanently lost. Transfer discrimination tests indicate that deficits other than sensory impairments may follow striate ablation. Results are discussed in terms of sensory and attentional deficits which occur with striate cortex removal. (28 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Evidence for separate pathways for color and luminance detection mechanisms

We measure threshold versus contrast (TvC) functions for chromatic (red-green) and luminance sine-wave-grating stimuli for (1) the detection of luminance in the presence of color contrast and (2) the detection of color in the presence of luminance contrast. We find that, although these crossed TvC functions both display a dipperlike shape, their facilitation differs from that found for standard uncrossed dipper functions (luminance on luminance or color on color contrast). Their facilitation disappears (cross condition 1) or is reduced (cross condition 2) by randomized presentation of the phase of the test and the mask, and the remaining facilitation (cross condition 2) displays no spatial tuning. We argue that these crossed facilitatory interactions cannot be explained by detection mechanisms with common inputs from color and luminance contrast (a nonindependence of transduction), and we present evidence that instead they reflect the use of local cues in the stimuli. We also measure the luminance-luminance TvC function in the presence of a fixed suprathreshold color contrast. The results demonstrate that, even when the color contrast produces a masking of the luminance thresholds, luminance-luminance facilitation still occurs. Thus the opposing effects of masking and facilitation can occur simultaneously. Furthermore, while luminance-luminance facilitation occurs independently of color contrast, masking can be produced by either contrast. This suggests that masking and facilitation have different underlying origins. Similar results are found for the color detection thresholds in the presence of a luminance pedestal. We conclude that there are separate pathways for the detection of color and luminance contrast, each with no input from the other contrast. We suggest that the cross masking reflects divisive interactions between these pathways that is restricted to high contrasts.     

Aircraft conspicuity and flight attitude information provided by exterior paint patterns.

Experiments were conducted to determine the relative conspicuity of aircraft exterior paint patterns, and to investigate whether such paint patterns aided pilots in determining the attitude of the aircraft. The conspicuity studies, using paired comparisons of model airplanes, gave evidence that: (a) amount of red-orange fluorescent paint coverage is positively correlated with conspicuity; (b) high brightness paints should be placed on the upper surfaces of the aircraft and low brightness paints on the lower portions; (c) maximizing brightness contrasts between different parts of the aircraft surfaces does not enhance conspicuity; (d) flight attitudes, backgrounds, lighting conditions, and differences in Ss did not affect conspicuity significantly. The attitude studies in which pilots matched the model airplanes in some 1 of 15 attitudes, with 1 of 15 models mounted on a small display, indicated that the paint patterns used did not aid the pilots in making judgments of aircraft attitude. Differences in backgrounds and lighting conditions did not greatly affect Ss' ability to determine attitude. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Counter-changing luminance: A non-Fourier, nonattentional basis for the perception of single-element apparent motion.

Motion perception was studied for generalized apparent motion stimuli composed of 2 simultaneously visible elements whose luminance alternated between 2 values (only 1 element is visible at a time for standard apparent motion). It was demonstrated that 1st-order motion energy is neither necessary nor sufficient for the perception of apparent motion. Instead, it was found that counter-changing luminance--simultaneous luminance changes at 2 element locations--is the informational basis for perceiving luminance-defined apparent motion: Motion starts where luminance changes toward the background luminance value and ends where luminance changes away from the background luminance. The results were not attributable to either 2nd-order motion mechanisms (for which rectification precedes the computation of motion energy) or attention-based, 3rd-order motion mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Color-luminance interaction: data produced by oblique cross masking

Threshold-elevation (TE-) versus-mask-spatial-frequency (SF) curves and TE-versus-mask-contrast curves, produced by the oblique-masking technique, were reported for uncrossed stimuli (color-test-on-color-mask and luminance-test-on-luminance-mask) [Invest. Ophthalmol. Visual Sci. Suppl. 34, 751 (1993) and Vision. Res. 23, 873 (1983)]. The technique minimizes the artifacts that are due to spatial phase effects, spatial beats, spatial probability summation, and local cues. My goal was to measure these curves for crossed stimuli (color-test-on-luminance-mask and luminance-test-on-color-mask) by this oblique-masking technique and to compare the curves with those reported in previous studies. For this purpose threshold contrasts were measured by a yes-no procedure with randomized double staircases. Test targets were vertical spatially localized (D6) patterns, and masks were oblique sinusoidal patterns; both the test and the mask were presented simultaneously, for 2 s (Gaussian window), on a color monitor interfaced with an ATVista system and a Powell achromatizing lens. The test SF's were 0.125, 0.5, 2, 4, and 8 cycles per degree (cpd); mask SF's were 0.031-16 cpd; and mask contrasts were 6.25%-50%. Furthermore, the Red-Green channel was defined by the minimum flicker and the hue cancellation techniques. Results show mostly masking effect (TE > 1) at contrasts above threshold; sometimes, separability (TE = 1) and above-threshold facilitation (TE < 1) effects were also observed, depending on the test SF, the mask SF, the mask contrast, and the subject. In general, the magnitudes of TE's are smaller and the TE-versus-mask-SF curves are slightly narrower for the oblique-cross-masking conditions than those for the respective oblique uncross masking. In addition, the TE-versus-mask-contrast curves for the crossed conditions are mostly shallower than those for the respective uncrossed conditions. Furthermore, mostly the color-luminance asymmetry (color masks luminance more than luminance masks color) is found, in mild form, for SF's > or = 0.5 cpd. For the lower SF of 0.125 cpd, there is either a lack of asymmetry or a very mild asymmetry of the opposite kind (luminance masks color slightly more than color masks luminance) seems to prevail. In general, the oblique-masking data shows mild asymmetry and reduced facilitation; both are consistent with reduced local cues, similar to those shown by randomized phase data, thus making the data suitable for SF analysis; moreover, at high contrast, the masking data are consistent with those reported in previous studies.     

Red-green chromatic discrimination with variegated and homogeneous stimuli

Chromatic discrimination thresholds were measured under conditions which yielded fine and degraded discrimination steps. Discrimination was assessed by identification of the location of one of four homogeneous equiluminant stimuli arranged in a square or with pseudoisochromatic (PIC) figures using the stimulus design of Regan, Reffin and Mollon (Vis Res 1994; 34: 1279-1299). Stimuli were presented on CRT monitors and specified in units of cone trolands. They were viewed within a surround metameric to the equal energy spectrum. L troland threshold versus retinal illuminance (TVR) functions were measured by four-alternative spatial forced-choice staircase procedures for (1) a four 1 x 1 degree equiluminant stimuli arranged in a square and (2) 'C' shaped pseudoisochromatic figures in which the observer had to identify the gap location. The 'C' was constructed of spatially discrete patches of varying size and luminance to ensure that the observer's responses depended on chromatic signals. The TVR functions appeared V-shaped and were similar for the two paradigms. The minimum occurred near the L excitation of equal-energy white. The PIC stimuli yielded poorer discrimination with the TVR function being displaced by approximately 0.5 log unit. Discrimination for stimuli degraded by luminance and spatial noise presented within an achromatic appearing surround is sharpest near the chromaticity metameric to the equal energy spectrum.     

Stereoacuity and colour contrast

We have measured the contrast dependence of stereoacuity using both horizontally and vertically oriented, isoluminant (red-green) and isochromatic (yellow-black), 0.5 c/deg Gabor patches. For comparison, contrasts were computed in multiples of detection threshold, where detection threshold was defined as the contrast required for the stimulus to be simultaneously detectable in each eye. Disparity thresholds (1/stereoacuity) for vertical chromatic Gabors were higher than those for vertical luminance Gabors by a factor of between 4 and 9 depending on contrast, and declined less steeply with contrast. Disparity thresholds for horizontal chromatic Gabors were very high (130-210 min arc) compared with horizontal luminance Gabors (by a factor of between 9 and 17) and were only measurable at contrasts above 10 times simultaneous monocular detection threshold. These results support the view that chromatic stereoscopic processing is less precise than luminance stereoscopic processing, and that there is a special deficit in the processing of disparity with horizontally oriented chromatic stimuli. The implications of these results for the role of colour vision in stereopsis are discussed.     

Pattern-reversal electroretinogram in response to chromatic stimuli: II. Monkey

We have recorded steady-state PERGs from five macaque monkeys in response to red-green plaid patterns reversed sinusoidally in contrast. The patterns had either a pure luminance contrast (red-black, green-black, yellow-black), pure red-green color contrast, or a variable amount of luminance and color contrast. By varying the relative luminance of the red-to-total luminance (color ratio) of red-green patterns, a value could be obtained at which the PERG amplitude was either minimum or locally maximum, and the phase was most lagged. This value was very similar to that producing equiluminance in human observers, and was considered to be equiluminance for the monkey. The phase of the PERG to chromatic stimulus was systematically lagged compared with that of luminance stimuli, by an amount corresponding to about 10-20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 80 ms for color contrast compared with 63 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. As a function of temporal frequency, the chromatic PERG function was clearly low-pass with a cutoff around 15 Hz, whereas that to luminance was double-peaked and extended to higher temporal frequencies, around 30 Hz. For both luminance and chromatic stimuli, the amplitude of PERGs increases with increasing stimulus contrast. By summing vectorially the luminance and chromatic responses of appropriate contrasts, we were able to predict with accuracy the response as a function of color ratio. In two monkeys, the optic chiasm was sectioned sagittally causing total degeneration of ganglion cells in the nasal retina, without affecting the temporal retina (verified by histology). In these animals, there was a strong response to both luminance and chromatic patterns in the temporal retinae, but none to either type of pattern in the nasal retinae, suggesting that the PERG to both luminance and chromatic stimuli arises from the inner-retinal layers. Electrophysiological studies suggest that the PERG to chromatic stimuli is probably associated with the activity of P-cells. P-cells may also make a major contribution to the PERG of luminance stimuli, although M-cells may also participate. The above findings on normal monkeys all agree with those reported in the accompanying paper for humans (Morrone et al., 1994), so similar conclusions can probably be extended to human PERG.     

Appearance of colored patterns: pattern-color separability

We have measured how color appearance of square-wave bars varies with stimulus strength and spatial frequency. Observers adjusted the color of a uniform patch to match the color appearance of the bars in square-wave patterns. We used low-to-moderate square-wave patterns, from 1 to 8 cycles per degree (c/deg). The matches are not photoreceptor matches but rather are established at more central neural sites. The signals at the putative central sites obey several simple regularities. The cone contrast of the uniform patch is proportional to square-wave stimulus strength (color homogeneity) and additive with respect to the superposition of equal-frequency square waves containing different colors (color superposition). We use the asymmetric matches to derive, from first principles, three pattern-color-separable appearance pathways. The matches are explained by two spectrally opponent, spatially low-pass mechanisms and one spectrally positive, spatially bandpass mechanism. The spectral mechanisms that we derive are similar to luminance and opponent mechanisms that are derived with entirely different experimental methods.     

Study of the Stevens exponential function for brightness perception

Study of the brightness scaling function within the range from 2.10(-2) to 3.5.10(3) lk revealed that the total curve of brightness estimation was not exactly Stevens power function within this range: the exponent's value and its deviation was variable in different light regions. Presumably, the luminance's region with minimal value of exponent is the most information for brightness perception. There seems to exist a certain interrelationship between both Veber--Fechner's and Stevens' laws.     

Decoupling Stimulus Duration From Brightness in Metacontrast Masking: Data and Models.

A brief target that is visible when displayed alone can be rendered invisible by a trailing stimulus (metacontrast masking). It has been difficult to determine the temporal dynamics of masking to date because increments in stimulus duration have been invariably confounded with apparent brightness (Bloch's law). In the research reported here, stimulus luminance was adjusted to maintain constant brightness across all durations. Increasing target duration yielded classical U-shaped masking functions, whereas increasing mask duration yielded monotonic decreasing functions. These results are compared with predictions from 6 theoretical models, with the lateral inhibition model providing the best overall fit. It is tentatively suggested that different underlying mechanisms may mediate the U-shaped and monotonic functions obtained with increasing durations of target and mask, respectively. (PsycINFO Database Record (c) 2010 APA, all rights reserved)