Regular occurance of simultaneous brightness contrast in the mesopic region

Clear evidence of electrophysiological effects causing simultaneous contrast phenomena in the visual system of animals has lately been presented. These findings incite to subjective perceptual psychophysiological experiments and give some directions concerning the interpretation and generalizations of the results. Psychophysical measurements have been made of the visual response to sinusoidal, spatially varying stimuli in the mesopic region well above contrast threshold. The results are presented with the characteristic properties, object contrast, spatial frequency, and average luminance of the physical luminance distribution as parameters. Some regular features of the fundamental input-output relations of the visual system are elucidated. Non-linear as well as quasi-linear processes are predicted to be prevalent.     

Mach band type lateral inhibition in different sense organs

Experiments were done on the skin with shearing forces, vibrations, and heat stimuli and on the tongue with taste stimuli to show that the well known Mach bands are not exclusively a visual phenomenon. On the contrary, it is not difficult to produce areas of a decreased sensation magnitude corresponding to the dark Mach bands in vision. It is shown on a geometrical model of nervous interaction that the appearance of Mach bands for certain patterns of stimulus distribution is correlated with nervous inhibition surrounding the area of sensation. This corroborates the earlier finding that surrounding every area transmitting sensation there is an area simultaneously transmitting inhibition.     

Eye movements and the enhancement of edges

A mathematical examination of retinal photochemistry leads to a hypothesis for Mach band phenomena based on eye movements. This retinal model suggests why minimally distinct borders fade under eye fixation and agrees qualitatively with subjective measures of border contrast as a function of overall field luminance.Deceased     

Effects of mean luminance changes on human contrast perception: contrast dependence, time-course and spatial specificity

Background

When we are viewing natural scenes, every saccade abruptly changes both the mean luminance and the contrast structure falling on any given retinal location. Thus it would be useful if the two were independently encoded by the visual system, even when they change simultaneously. Recordings from single neurons in the cat visual system have suggested that contrast information may be quite independently represented in neural responses to simultaneous changes in contrast and luminance. Here we test to what extent this is true in human perception.

Methodology/Principal Findings

Small contrast stimuli were presented together with a 7-fold upward or downward step of mean luminance (between 185 and 1295 Td, corresponding to 14 and 98 cd/m²), either simultaneously or with various delays (50–800 ms). The perceived contrast of the target under the different conditions was measured with an adaptive staircase method. Over the contrast range 0.1–0.45, mainly subtractive attenuation was found. Perceived contrast decreased by 0.052±0.021 (N = 3) when target onset was simultaneous with the luminance increase. The attenuation subsided within 400 ms, and even faster after luminance decreases, where the effect was also smaller. The main results were robust against differences in target types and the size of the field over which luminance changed.

Conclusions/Significance

Perceived contrast is attenuated mainly by a subtractive term when coincident with a luminance change. The effect is of ecologically relevant magnitude and duration; in other words, strict contrast constancy must often fail during normal human visual behaviour. Still, the relative robustness of the contrast signal is remarkable in view of the limited dynamic response range of retinal cones. We propose a conceptual model for how early retinal signalling may allow this.     

Correlation versus gradient type motion detectors: the pros and cons

Visual motion contains a wealth of information about self-motion as well as the three-dimensional structure of the environment. Therefore, it is of utmost importance for any organism with eyes. However, visual motion information is not explicitly represented at the photoreceptor level, but rather has to be computed by the nervous system from the changing retinal images as one of the first processing steps. Two prominent models have been proposed to account for this neural computation: the Reichardt detector and the gradient detector. While the Reichardt detector correlates the luminance levels derived from two adjacent image points, the gradient detector provides an estimate of the local retinal image velocity by dividing the spatial and the temporal luminance gradient. As a consequence of their different internal processing structure, both the models differ in a number of functional aspects such as their dependence on the spatial-pattern structure as well as their sensitivity to photon noise. These different properties lead to the proposal that an ideal motion detector should be of Reichardt type at low luminance levels, but of gradient type at high luminance levels. However, experiments on the fly visual systems provided unambiguous evidence in favour of the Reichardt detector under all luminance conditions. Does this mean that the fly nervous system uses suboptimal computations, or is there a functional aspect missing in the optimality criterion? In the following, I will argue in favour of the latter, showing that Reichardt detectors have an automatic gain control allowing them to dynamically adjust their input–output relationships to the statistical range of velocities presented, while gradient detectors do not have this property. As a consequence, Reichardt detectors, but not gradient detectors, always provide a maximum amount of information about stimulus velocity over a large range of velocities. This important property might explain why Reichardt type of computations have been demonstrated to underlie the extraction of motion information in the fly visual system under all luminance levels.     

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。     

Glossiness and Perishable Food Quality: Visual Freshness Judgment of Fish Eyes Based on Luminance Distribution

Background

Previous studies have reported the effects of statistics of luminance distribution on visual freshness perception using pictures which included the degradation process of food samples. However, these studies did not examine the effect of individual differences between the same kinds of food. Here we elucidate whether luminance distribution would continue to have a significant effect on visual freshness perception even if visual stimuli included individual differences in addition to the degradation process of foods.

Methodology/principal findings

We took pictures of the degradation of three fishes over 3.29 hours in a controlled environment, then cropped square patches of their eyes from the original images as visual stimuli. Eleven participants performed paired comparison tests judging the visual freshness of the fish eyes at three points of degradation. Perceived freshness scores (PFS) were calculated using the Bradley-Terry Model for each image. The ANOVA revealed that the PFS for each fish decreased as the degradation time increased; however, the differences in the PFS between individual fish was larger for the shorter degradation time, and smaller for the longer degradation time. A multiple linear regression analysis was conducted in order to determine the relative importance of the statistics of luminance distribution of the stimulus images in predicting PFS. The results show that standard deviation and skewness in luminance distribution have a significant influence on PFS.

Conclusions/significance

These results show that even if foodstuffs contain individual differences, visual freshness perception and changes in luminance distribution correlate with degradation time.     

VARIATION OF VISUAL DETECTION OVER THE 24-HOUR PERIOD IN HUMANS

A circadian rhythm for visual sensitivity has been intensively assessed in animals. This rhythm may be due to the existence of a circadian clock in the mammalian eye, which could account for fluctuating sensitivity to light over the day in certain species. However, very few studies have been devoted to the human visual system. The present experiment was designed to assess a possible rhythm of visual sensitivity using a psychophysical method over the whole 24h period. Twelve subjects underwent visual detection threshold measures in a protocol that allowed one point every 2h. The results show that the visual detection threshold changes over the 24h period, with high thresholds in the morning, a progressive decrease over the day and the early night, and an increase during the last part of the night. These data suggest that a circadian rhythm influences visual sensitivity to mesopic luminance in humans. (Chronobiology International, 17(6), 795–805, 2000)     

The perceived image: efficient modelling of visual inhomogeneity.

When we fixate on a certain location in a scene, the image that our visual system provides us with is not like a snapshot of the scene. Because of visual inhomogeneity, the image we perceive is sharp and clean only around the fixation location, and it gradually blurs and loses detail with increasing distance from that location. In this paper, a procedure is presented that allows one to obtain the luminance distribution in the perceived image of a scene. The procedure is based on a stack distribution model of spatial visual processing, with parameters obtained experimentally. An application of this procedure is presented which allows an explanation of data on the decrease of visual acuity with eccentricity. The advantages of this procedure for taking visual inhomogeneity into account when seeking explanations for perceptual phenomena are also discussed.     

Characteristics of the flux dimension cue in apparent motion correspondence.

It has been shown that element flux and size (but not luminance) serve as correspondence cues in the apparent motion visual system. Results are now presented of a study of the characteristics of the flux cue. It was found that flux rather than luminance is used by the system even when the size of the elements is greater than the size limit of Ricco's law. There were interactions between the apparent motion processing of the size and flux dimensions, beyond the obvious dependence of flux on size: positively correlated size and flux differences between elements have a greater effect on correspondence than do negatively correlated differences. Finally, when comparing the fluxes of different elements, the apparent motion system uses relative flux (above or below background) rather than absolute flux (relative to zero).     

Pre-Exposure to Moving Form Enhances Static Form Sensitivity

Background

Motion-defined form can seem to persist briefly after motion ceases, before seeming to gradually disappear into the background. Here we investigate if this subjective persistence reflects a signal capable of improving objective measures of sensitivity to static form.

Methodology/Principal Findings

We presented a sinusoidal modulation of luminance, masked by a background noise pattern. The sinusoidal luminance modulation was usually subjectively invisible when static, but visible when moving. We found that drifting then stopping the waveform resulted in a transient subjective persistence of the waveform in the static display. Observers'' objective sensitivity to the position of the static waveform was also improved after viewing moving waveforms, compared to viewing static waveforms for a matched duration. This facilitation did not occur simply because movement provided more perspectives of the waveform, since performance following pre-exposure to scrambled animations did not match that following pre-exposure to smooth motion. Observers did not simply remember waveform positions at motion offset, since removing the waveform before testing reduced performance.

Conclusions/Significance

Motion processing therefore interacts with subsequent static visual inputs in a way that can improve performance in objective sensitivity measures. We suggest that the brief subjective persistence of motion-defined forms that can occur after motion offsets is a consequence of the decay of a static form signal that has been transiently enhanced by motion processing.     

Mathematical principles in afferent visual neurons: Differentiation,integration and transient proportionality related to receptive fields and shift-effect

The dual reciprocal and antagonistic organization of B- and D-neurons of the afferent visual system is obtained using differentiation and integration as mathematical equivalents of visual information processing by an impulse frequency code. The spatial and temporal derivatives lead to the transient responses. A constant and a time-dependent term proportional to the luminance distribution describe the sustained response components and the shift-effect of retinal on- and off-center ganglion cells. Receptive field properties of lateral geniculate cells and their antagonistic shift-effect are obtained by passing the retinal output, i.e. the difference between B- and D-neurons' activity, once again through the same operations. However, the factor of proportionality is applied to the retina alone. The surprisingly small difference between retinal and geniculate receptive field properties on the one hand and the dramatic change from a synergistic to an antagonistic shift-effect on the other hand are thereby explained. The theory offers an understanding of a a possible functional significance of the shift-effect as a mechanism of transientrestoration of visual information, which prevents the system from total fading by means of shifts of the retinal image, normally produced by eye movements.     

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.     

Visual sensitivity of ground squirrels to spatial and temporal luminance variations

Summary We have investigated the visual sensitivity of the California ground squirrel (Speromphilus beecheyi) to spatial and temporal luminance patterns. Spatial contrast sensitivity functions were determined in behavioral discrimination experiments in which the stimuli were sinusoidally-modulated luminance gratings. These squirrels were found to be maximally sensitive to spatial frequencies of about 0.7 cycles/ degree (c/d), and they are unable to discriminate gratings whose frequencies exceed 4 c/d. Similar results were obtained in electrophysiological experiments when the visually evoked cortical potential (VECP) was recorded from anesthetized squirrels. A third experiment involved tests of the ability of ground squirrels to discriminate square-wave gratings of much higher luminance (340 cd/m²). The finest gratings which were discriminable at this luminance level did not exceed 3.9–4.3 c/d and, thus, we conclude that the maximal spatial resolution of the California ground squirrel is about 4 c/d (corresponding to a bar separation of 7.5). In another behavioral experiment the abilities of ground squirrels to discriminate sinusoidally flickering lights (mean luminance = 3.4 cd/m²) was measured. The results show that ground squirrels are maximally sensitive to lights flickering at a rate of about 18 Hz, and that the highest rates that are still discriminable are slightly above 60 Hz.Abbreviations c/d cycles/degree - CFF critical flicker frequency - VECP visually evoked cortical potential This research was supported by Grant EY 00105 from the National Eye Institute. We thank David Birch who participated in some preliminary behavioral experiments and Kenneth Long who provided the histological material from which measurements of receptor spacing were made.     

Temporal fidelity in the visual system.

The temporal properties of the visual system have been analyzed by recording the ERG and its isolated components in response to light stimuli whose luminance was varied sinusoidally. The performance of the visual system to periodic light stimuli was tested in human subjects psychophysically. The comparison of the results in control conditions and after administration of drugs that specifically block the hyperpolarization activated current (Ih) suggest that the inner rectifying properties of the inner segment membrane of rods is involved in a process of temporal differentiation of the visual signals whereby high frequency components of the response especially relevant for the visual performance are enhanced. It is proposed that the temporal fidelity of the visual system is the results of an elaboration starting at early level of the signal generated by the phototransductive cascade.     

Preferential distribution of excitation energy into photosystem I of desiccated samples of the lichen Cladonia impexa and the isolated lichen-alga Trebouxia pyriformis

Abstract The effect of desiccation on distribution of excitation energy between the two photosystems has been studied in the lichen Cladonia impexa Harm., in the green alga Trebouxia pyriformis Archibald, isolated from Cladonia squamosa; and in the non-lichen green alga Scenedesmus obliquus, strain D₃. The method used was to compare the low temperature fluorescence emission of samples equilibrated with air with different humidity prior to freezing in liquid nitrogen. Desiccation of Cladonia and Trebouxia caused a pronounced increase of the height of the far red fluorescence emission band, F 715, over the short wave bands, F685 and F697; the ratio between the two short wave bands remained essentially constant. Upon rewetting, these species regained normal fluorescence emission properties, indicating that they are desiccation-tolerant. Scenedesmus, which was used as a desiccation intolerant species, also showed an increase of the far red fluorescence band over the two short wave bands upon desiccation, but the original fluorescence spectrum was not restored upon rewetting. These results are interpreted as showing that desiccation of tolerant species such as Cladonia and Trebouxia causes a preferential energy distribution into photosystem I. We tentatively believe that desiccation induces conformational changes within the chloroplast thylakoids, thereby controlling distribution of energy between the two photosystems. Furthermore, this change in energy distribution may be of ecological significance as the mechanism by which desiccated lichens or algae avoid photo-dynamic destruction of the photosynthetic apparatus when photosynthesis is inhibited under dry conditions. By a preferential distribution of absorbed energy into photosystem I, the organisms avoid the formation of strong, harmful oxidants in photosystem II when photosynthesis is inhibited. It is suggested that β-carotene associated with the far red-absorbing chlorophyll a fraction of the reaction center antenna of photosystem I is the final sink for excess excitation energy in dry, desiccation-tolerant lichens and algae.     

Mach bands in the lateral eye of Limulus

Patterns of optic nerve activity were computed for stationary step patterns of illumination from theoretical models of lateral inhibiton based on revised Hartlin-Ratliff equations. The computed response patterns contain well-defined Mach bands which match closely in amplitude and shape those recorded from single optic nerve fibers of the Limulus lateral eye. Theory and experiment show that the amplitude of the Mach bands is reduced by in inhibitory nonlinearity, the width of the Mach bands is approximately equal to the lateral dimension of the inhibitory field, but the shapes of the Mach bands are poor indices of the precise configuration of the inhibitory field. Theorems are proved establishing the equivalence of Mach-band patterns for models of different dimensions and a uniqueness condition for solutions of the piecewise linear model.     

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.     

Endogenous circadian rhythms in pigment composition induce changes in photochemical efficiency in plant canopies

There is increasing evidence that the circadian clock is a significant driver of photosynthesis that becomes apparent when environmental cues are experimentally held constant. We studied whether the composition of photosynthetic pigments is under circadian regulation, and whether pigment oscillations lead to rhythmic changes in photochemical efficiency. To address these questions, we maintained canopies of bean and cotton, after an entrainment phase, under constant (light or darkness) conditions for 30–48 h. Photosynthesis and quantum yield peaked at subjective noon, and non‐photochemical quenching peaked at night. These oscillations were not associated with parallel changes in carbohydrate content or xanthophyll cycle activity. We observed robust oscillations of Chl a/b during constant light in both species, and also under constant darkness in bean, peaking when it would have been night during the entrainment (subjective nights). These oscillations could be attributed to the synthesis and/or degradation of trimeric light‐harvesting complex II (reflected by the rhythmic changes in Chl a/b), with the antenna size minimal at night and maximal around subjective noon. Considering together the oscillations of pigments and photochemistry, the observed pattern of changes is counterintuitive if we assume that the plant strategy is to avoid photodamage, but consistent with a strategy where non‐stressed plants maximize photosynthesis.