Brightness versus apparent contrast. 1: Incremental and decremental disks with varying diameter.

This study describes the matching of the brightness and of the apparent contrast of foveal disks, presented as an increment or decrement with varying diameter against a 300 cd.m-2 background. If the brightness in the centre of the disks is matched with a constant reference brightness, the well-known spatial Broca-Sulzer phenomenon is obtained. This effect is not found if the apparent contrast of the disks is matched instead. All matching results and detection thresholds indicate that luminance increments and decrements are processed asymmetrically by the visual system: for decrements the threshold curve and curves of isobrightness as well as apparent iso-contrast are all shifted towards larger disk radii relative to those for increments. Two nonlinear single-channel models are tested against the data.     

Symmetry between the visual B- and D-systems and equivalence of center and surround: Studies of light increment and decrement in retinal and geniculate neurons of the cat

The dual center surround organization of retinal and geniculate neurons in two antagonistic subsystems B and D, having on-center or off-center receptive fields and signalling brightness or darkness respectively, has been studied by local light increments and decrements. Intensity response functions obtained by the introduction and withdrawal of small center spots either brighter or darker than a common homogeneous field are similar in a given neuron, but the phasic responses are stronger in on-center neurons than in off-center neurons. Center size increments and decrements, however, lead to equal excitations in the B- and D-system, respectively, provided that both luminance steps start from the same level and are of equal size on a linear scale. Decrementing and incrementing the surrounding luminance of the same optimal center spots lead to equal surround responses in the two subsystems if the two luminance steps terminate at the same level. This lateral activation is elicited by light decrement in the B-system and by light increment in the D-system. Center and surround responses within a given subsystem are of comparable amplitude, but generally slightly stronger responses are elicited by optimal center increments (decrements) than by the equivalent surround decrements (increments) which lead to the same spatial contrast for B-(D-) neurons. The symmetry relations between the B- and D-system and the equivalence relations between center and surround in each subsystem hold for retinal and geniculate neurons. The difference between center and surround response latencies is about 9 ms in both subsystems at the retinal and 14 ms at the geniculate level. Stimulus response functions of on- and off-center neurons are unified on the basis of linear relative luminance scales.     

Texture segregation in chromatic element-arrangement patterns.

An element-arrangement pattern is composed of two types of elements arranged differently in different regions of a pattern. Rapid texture segregation depends on spontaneously discriminating the difference in the arrangement of the elements. Five experiments investigated the perceived segregation of patterns composed of two types of squares arranged in vertical stripes in the top and bottom regions and in a checkerboard arrangement in the middle region. The squares were either equal in luminance and different in hue or equal in hue and different in luminance. The rated similarities of the two hues in a pattern failed to predict perceived segregation. For a given background luminance, the perceived segregation was predicted by the square-root of the sum of the squares of the differences in the outputs of the L - M + S and L + M - S opponent channels, where L, M, and S were the cone contrasts of the long-, medium-, and short-wavelength receptors. The perceived similarity of the two hues in a pattern was not affected by the background luminance but was a function of cone excitations instead. For patterns differing in hue and equal in luminance, perceived segregation was an inverse function of the background luminance. A white background decreased the perceived segregation, but a black background did not. The effect of background luminance was not on the discrimination of the individual hues. The two hues making up a texture pattern were clearly distinguishable on a white background. A white background interfered with the discrimination of the vertical and diagonal columns of squares that distinguished the texture regions. For patterns differing in luminance and equal in hue, black and white backgrounds decreased the perceived segregation. The results indicate that adapting to an achromatic luminance distant from the luminance of the squares increased the Weber threshold for discriminating luminance differences, but did not increase the Weber threshold for discriminating hue differences. The experiments also revealed that luminance was the primary factor affecting perceived segregation and that perceived brightness is secondary. The results are consistent with the hypothesis that perceived segregation in element-arrangement patterns is primarily a function of the differences in the outputs of relatively early filtering mechanisms that encode pattern differences prior to the specification of the element shapes and their properties.     

Photometric Compliance of Tablet Screens and Retro-Illuminated Acuity Charts As Visual Acuity Measurement Devices

Mobile technology is increasingly used to measure visual acuity. Standards for chart-based acuity tests specify photometric requirements for luminance, optotype contrast and luminance uniformity. Manufacturers provide some photometric data but little is known about tablet performance for visual acuity testing. This study photometrically characterised seven tablet computers (iPad, Apple inc.) and three ETDRS (Early Treatment Diabetic Retinopathy Study) visual acuity charts with room lights on and off, and compared findings with visual acuity measurement standards. Tablet screen luminance and contrast were measured using nine points across a black and white checkerboard test screen at five arbitrary brightness levels. ETDRS optotypes and adjacent white background luminance and contrast were measured. All seven tablets (room lights off) exceeded the most stringent requirement for mean luminance (≥ 120 cd/m²) providing the nominal brightness setting was above 50%. All exceeded contrast requirement (Weber ≥ 90%) regardless of brightness setting, and five were marginally below the required luminance uniformity threshold (L_min/L_max ≥ 80%). Re-assessing three tablets with room lights on made little difference to mean luminance or contrast, and improved luminance uniformity to exceed the threshold. The three EDTRS charts (room lights off) had adequate mean luminance (≥ 120 cd/m²) and Weber contrast (≥ 90%), but all three charts failed to meet the luminance uniformity standard (L_min/L_max ≥ 80%). Two charts were operating beyond manufacturer’s recommended lamp replacement schedule. With room lights on, chart mean luminance and Weber contrast increased, but two charts still had inadequate luminance uniformity. Tablet computers showed less inter-device variability, higher contrast, and better luminance uniformity than charts in both lights-on and lights-off environments, providing brightness setting was >50%. Overall, iPad tablets matched or marginally out-performed ETDRS charts in terms of photometric compliance with high contrast acuity standards.     

Colour and brightness coding in the central nervous system: theoretical aspects and visual evoked potentials to homogeneous red and green stimuli

We designed visual evoked potentials experiments to study the differential aspects of colour and brightness coding in man. The substitution of equally bright red and green stimuli for a background yellow was investigated and compared with different luminance increments and decrements of red and green. A dominant N87 component was found for a colour change from yellow to brighter red colours, which was less pronounced for green and absent for yellow luminance changes. It is also absent for pure red luminance increments and green luminance changes, but reappears with red luminance decrements or red-offset. The data are discussed within the framework of a new concept of how the visual system fuses red-green information and black-white border information. Retinal X-cells can transmit colour and high spatial frequency achromatic information simultaneously by encoding only the presence of edges (a.c.) for the black-white stimuli and the presence of both edges (a.c.) and uniform areas of colour (d.c.) for red-green stimuli. Phylogenetically this kind of information transmission enables colour vision to be implemented in a retina such as the cat's by adding only a second class of cones. Barlow's economy principle will be violated for colour in the periphery, but restored early in the striate cortex where there is an early decoding of the combined chromatic and achromatic information by the concentric double opponent cells. The N87 behaviour correlates with the proposed discharge of peripheral X-type cells, but not with the discharge of cortical double opponent concentric or simple cells, which no longer respond to homogeneous colour stimuli. It is suggested that N87 may be generated by geniculate afferents in the dendritic arborization of cortical cells, reflecting the behaviour of peripheral units, and thus the violation of the economy principle, rather than the next step in cortical processing. The early cortical restoration of the economy principle is supported by the absence of any further dissociated behaviour for colour and brightness in later components.     

Brightness contrast inhibits color induction: evidence for a new kind of color theory

A gray region can be made to look colored by a colored surround. This phenomenon, chromatic induction, depends on color differences around the boundary of the region. We performed experiments on chromatic induction with small, initially achromatic, targets on nine different colored surrounds ranging in color from blue to red. Using scaling of saturation as our measure of perceived color strength, we found that chromatic induction is at its maximum when the brightness contrast at the boundary between target and surroundings is minimal. This implies that the neural mechanism in the cerebral cortex that mediates the appearance of brightness at a boundary inhibits the activity of chromatic mechanisms at that same boundary. Observers matched the apparent brightness and luminance of each of the colored surrounds. For surround colors where brightness and luminance matches differ, brightness contrast, not luminance contrast, controls chromatic induction. These new findings, taken together with other evidence, require a new theory of color appearance that includes mutually inhibitory interactions between color and brightness mechanisms that are sensing color and brightness contrast at visual boundaries.     

Use of light intensifiers in improving acuity in retinitis pigmentosa

An English Electric Valve Co. P8073 light intensifier was set up with a viewing system to assess the performance of retinitis pigmentosa sufferers at low luminance levels. Six subjects had their acuity measured at different luminance levels using a light intensifier initially and subsequently using direct vision. All subjects showed an improvement in acuity using the intensifier compared with direct vision at levels of illumination ranging from 0.25 cd m-2 to 20 cd m-2.     

Brightness versus apparent contrast. 3: Blurred disks and concentric cosine gratings.

Matching either the centre-brightness or the apparent contrast of incremental 1-deg disks, with varying edge-sharpness, yields quite different results. These suggest that the maximum brightness gradient at the edge determines apparent-contrast perception. However, no significant differences are found in matching the brightness maxima, the brightness minima, or the apparent contrast of concentric cosine gratings with varying spatial frequency. This means that spatial frequency influences both brightness extremes in the same way, and that apparent contrast is a function of the difference between the brightness extremes. The grating results therefore deviate from the results obtained with sharp as well as blurred disks. These contradictions complicate the construction of a model which unifies suprathreshold disk and grating perception.     

用显示器测量办公亮度环境下的人眼对比度敏感视觉特性

对比度敏感是描述人眼视觉系统空间特性的主要指标之一,对比度敏感函数是反映不同条件下的对比度敏感与空间频率之间的关系。人眼对比度敏感数据的测量受到环境亮度较大的影响,为了研究常用办公环境条件下的人眼对比度敏感情况,对6位青年在环境亮度分别为153,312,470 cd/m2和暗室条件下,在距离为2米处观测11种空间频率的矩形光栅进行测量,光栅用显示器进行显示,其平均亮度分别为60和90 cd/m2。实验结果表明,对于相同频率的光栅,人眼对比度敏感程度随着环境亮度的增加而减小,而且人眼在暗室环境下比在办公环境条件下对亮度光栅更敏感;但是在观测平均亮度为60cd/m2的光栅时,人眼特殊地对在环境亮度为312 cd/m2的条件下更敏感。     

用显示器测量办公亮度环境下的人眼对比度敏感视觉特性

对比度敏感是描述人眼视觉系统空间特性的主要指标之一,对比度敏感函数是反映不同条件下的对比度敏感与空间频率之间的关系。人眼对比度敏感数据的测量受到环境亮度较大的影响,为了研究常用办公环境条件下的人眼对比度敏感情况,对6位青年在环境亮度分别为153,312,470 cd/m2和暗室条件下,在距离为2米处观测11种空间频率的矩形光栅进行测量,光栅用显示器进行显示,其平均亮度分别为60和90 cd/m2。实验结果表明,对于相同频率的光栅,人眼对比度敏感程度随着环境亮度的增加而减小,而且人眼在暗室环境下比在办公环境条件下对亮度光栅更敏感;但是在观测平均亮度为60cd/m2的光栅时,人眼特殊地对在环境亮度为312 cd/m2的条件下更敏感。     

Vernier acuity predicted from changes in the light distribution of the retinal image

If two thin bars of different luminance are placed side by side, their joint spatial position in a Vernier alignment task is determined simply by their relative luminances. The threshold luminance contrast difference required to produce a just detectable change in spatial position corresponds to a spatial shift of 5-20 arcsec in the centroid of the retinal light distribution, depending upon contrast relative to the background. This technique may be used to measure acuity with a display that has a spatial resolution considerably worse than the Vernier offset threshold. We have also extended the centroid technique to components that differ both in wavelength and luminance. Colour was found to make no essential difference to the task. Taking into account the spread of light in the retinal image, the manifest contrast thresholds are equivalent to threshold intensity increments between adjacent foveal receptors of less than 1% comparable to the values reported by Hecht and Mintz for dark line detection.     

综述: 初级视皮层的亮度信息加工

亮度(luminance)是最基本的视觉信息.与其他视觉特征相比,由于视神经元对亮度刺激的反应较弱,并且许多神经元对均匀亮度无反应,对亮度信息编码的神经机制知之甚少.初级视皮层部分神经元对亮度的反应要慢于对比度反应,被认为是由边界对比度诱导的亮度知觉(brightness)的神经基础.我们的研究表明,初级视皮层许多神经元的亮度反应要快于对比度反应,并且这些神经元偏好低的空间频率、高的时间频率和高的运动速度,提示皮层下具有低空间频率和高运动速度通路的信息输入对产生初级视皮层神经元的亮度反应有贡献.已经知道初级视皮层神经元对空间频率反应的时间过程是从低空间频率到高空间频率,我们发现的早期亮度反应是对极低空间频率的反应,与这一时间过程是一致的,是这一从粗到细的视觉信息加工过程的第一步,揭示了处理最早的粗的视觉信息的神经基础.另外,初级视皮层含有偏好亮度下降和高运动速度的神经元,这群神经元的活动有助于在光照差的环境中检测高速运动的低亮度物体.     

Spatial dissociation of early and late colour evoked components

Evoked potentials to the primary colours red, green, yellow and blue were recorded and compared with those evoked by white. The unpatterned 10° × 13° stimuli were generated with the aid of a colour monitor. Activity was depicted with 5 electrodes, the central electrode 5 cm above the inion and two on each side 5 and 10 cm apart from the central electrode.With equally bright colour stimuli a previously described early negative colour-dominated component N87 was localized in all subjects at the central occipital electrode while the following positivity P100 was clearly more lateralized to the peripheral electrodes. With half-field stimulation N87 showed a similar — paradoxical — lateralization to the ipsilateral electrodes as has been demonstrated for pattern reversal.The existence and localization of N87 could be confirmed also for blue colours, its amplitude independent of the blue luminance, its latency decreasing for definite additional brightness increments and decrements. The N87 to blue was of the same latency as the N87 components to other colours.N87 is mainly generated foveally and parafoveally, its amplitude saturates with stimuli larger than 6–8° in diameter.     

Aposematic coloration, luminance contrast, and the benefits of conspicuousness

Many organisms use warning, or aposematic, coloration to signaltheir unprofitability to potential predators. Aposematicallycolored prey are highly visually conspicuous. There is considerableempirical support that conspicuousness promotes the effectivenessof the aposematic signal. From these experiments, it is welldocumented that conspicuous, unprofitable prey are detectedsooner and aversion learned faster by the predator as comparedwith cryptic, unprofitable prey. Predators also retain memoryof the aversion longer when prey is conspicuous. The presentstudy focused on the elements of conspicuousness that conferthese benefits of aposematic coloration. Drawing on currentunderstanding of animal vision, we distinguish 2 features ofwarning coloration: high chromatic contrast and high brightness,or luminance, contrast. Previous investigations on aposematicsignal efficacy have focused mainly on the role of high chromaticcontrast between prey and background, whereas little researchhas investigated the role of high luminance contrast. Usingthe Chinese mantid as a model predator and gray-painted milkweedbugs as model prey, we found that increased prey luminance contrastincreased detection of prey, facilitated predator aversion learning,and increased predator memory retention of the aversive response.Our results suggest that the luminance contrast component ofaposematic coloration can be an effective warning signal betweenthe prey and predator. Thus, warning coloration can even evolveas an effective signal to color blind predators.     

Virtual retinal display system with nodal point image of a laser beam: construction example and evaluation of subjective brightness perception]

We investigated the perception of brightness for red monochromatic laser light. For this purpose, a modified virtual retinal display (VRD) was constructed. The modification involved projecting the laser beam into the eye. In our VRD, the laser beam pivots in the nodal point of the eye (badal system). The displayed image therefore does not depend on the refractive state of the eye. Brightness perception was assessed by means of psychophysical experiments. The results of these experiments indicate that perception of brightness at 652 nm increases more rapidly with increasing physical stimulus than does the perception of white light (colour temperature 2'935 K). At a wavelength of 652 nm, an optical power of 79 nW is required to produce an image subtending 2 degrees of equal brightness, as is perceived with an image of about 2'900 cd/m2 subtending 2 degrees.     

Receiver operating characteristic analysis. Application to the study of quantum fluctuation effects in optic nerve of Rana pipiens.

Receiver operating characteristic (ROC) analysis of nerve messages is described. The hypothesis that quantum fluctuations provide the only limit to the ability of frog ganglion cells to signal luminance change information is examined using ROC analysis. In the context of ROC analysis, the quantum fluctuation hypothesis predicts (a) the detectability of a luminance change signal should rise proportionally to the size of the change, (b) detectability should decrease as the square root of background, an implication of which is the deVries-Rose law, and (c) ROC curves should exhibit a shape particular to underlying Poisson distributions. Each of these predictions is confirmed for the responses of dimming ganglion cells to brief luminance decrements at scotopic levels, but none could have been tested using classical nerve message analysis procedures.     

Color and luminance influence, but can not explain, binocular rivalry onset bias

When an observer is presented with dissimilar images to the right and left eye, the images will alternate every few seconds in a phenomenon known as binocular rivalry. During sustained viewing, the timing of these switches appears to be unpredictable. Recent research has suggested that the initial 'onset' period of rivalry is not random and may be different in its neural mechanism than subsequent dominance periods. It is known that differences in luminance and contrast have a significant influence on the average dominance during sustained rivalry and that perception of luminance can vary between individuals and across the visual field. We therefore investigated whether perception of luminance contrast plays a role in onset rivalry. Observers viewed rival targets of equal brightness for brief presentations in eight locations of the near periphery and reported the color that was first dominant in each location. Results show that minimizing differences in brightness and contrast yields a stronger pattern of onset dominance bias and reveals evidence of monocular dominance. The results suggest that both contrast and monocular dominance play a role in onset dominance, though neither can fully explain the effect.     

Detection symmetry and asymmetry

Experiments were performed on the detection symmetry and asymmetry of incremental and decremental disks, as a function of both disk diameter and duration. It was found that, for a background luminance of 300 cd.m-2, thresholds of dynamic (briefly presented) foveal disks are symmetrical for all diameters, and that thresholds of quasi-statically presented disks are symmetrical for large diameters only. Threshold curves of quasi-static incremental and decremental disks are mutually shifted along the log area axis. In order to obtain a better insight into the underlying detection mechanism, additional experiments were performed with halfwave-rectified concentric cosine gratings, i.e. with either incremental or decremental phases. Threshold curves of these gratings proved to be identical, that is independent of polarity, and of the same shape as those obtained with fullwave cosine gratings, though 0.3 log unit shifted in amplitude. The discrepancy between threshold curves of quasi-static disks (asymmetry) and halfwave-rectified gratings (symmetry) is discussed in terms of a multiple-channel theory.     

Age-related changes in pattern visual evoked potentials: differential effects of luminance,contrast and check size

We recorded visual evoked potentials (VEPs) to checkerboard pattern-reversal stimulation in 109 normal subjects (51 males and 59 females; aged 19–84 years) in order to study the aging effect on the multiple channels of the visual system in humans. Transient VEPs to 3 check sizes (15′, 30′ and 50′) were obtained by monocular stimulation. Two test conditions were employed: (1) a high luminance (180 cd/m²) and a low luminance (11 cd/m²) both with a fixed contrast (90%), and (2) a high contrast (85%) and a low contrast (10%) both at a fixed luminance (57 cd/m²). The major features of our results included: (1) the presence of a curvilinear relationship between P100 latency and age for all conditions, while the P100 amplitude did not show any such aging effect, (2) the age-latency function was similar between the two luminance conditions, while it was different between the two contrast conditions, and (3) the differential age effect on the P100 latency caused by changes in contrast depended on the check size. These results suggest that age-related changes in the human visual system are not uniform, but rather are different in the specific functional subdivisions. It is thus hypothesized that aging may differentially influence the separate channels of the human visual system.     

Age-related changes of reaction time and p300 for low-contrast color stimuli: Effects of yellowing of the aging human lens

We measured reaction time (RT), P300, and subjective evaluation for color Landolt-Cs with a gray color background presented on a CRT display. Seven young and 7 elderly subjects (mean ages: 21.6 and 68.4 years, respectively) participated, and the young subjects wore glasses with filters simulating spectral transmittance of an aging human lens as a test condition. The results for young subjects not wearing the filters showed that RT and P300 latency are constant among different test colors. In contrast, the results for elderly subjects showed that RT and P300 varied substantially depending upon the test colors and RT and P300 latency became longer than those of young subjects, particularly for gray and blue stimuli. In addition, the results for the young subjects with filters showed tendencies similar to those in elderly subjects. These results indicate that the yellowing of the human lens strongly influences reaction time and cognition time for color targets, suggesting that wearing the filters enables the young to simulate RT qualitatively as well as visibility of the elderly because both the simulated filter and the aging human lens modify the effective luminance, effective luminance contrast and effective color difference between the color target and the background on the retina. We also found that the reciprocal of RT and P300 latency could be expressed in a multiple regression model consisting of effective luminance, effective luminance contrast, effective color difference and age. Absolute values of RT and P300 latency in young subjects with filters, however, did not quantitatively coincide with those of the elderly subjects. There were differences of RT and P300 latency between the young with filters and the elderly, indicating that higher order age-related delay could be involved.