Sustained pupillary constrictions mediated by an L- and M-cone opponent process

AbstractWe show that irradiance-coding alone cannot explain the sustained pupillary constrictions evoked by chromatic and luminance variations in a stimulus. For example, stimulus modulations that decremented the contrasts in L- and M-cones as well as rods and melanopsin photoreceptors produced sustained constrictions rather than the predicted dilations. Although the sustained responses are unidirectional, we confirm that they are at least partially mediated by an L- and M-cone opponent interaction. We discuss the implications of sustained unidirectional chromatic responses in view of the function of the pupil to improve the clarity of vision

Digital color image processing and psychophysics within the framework of a human visual model

Journal ArticleA three-dimensional homomorphic model of human color vision based on neurophysiological and psychophysical evidence is presented. This model permits the quantitative definition of perceptually important parameters such as brightness. saturation, huo and strength. By modelling neural interaction in the human visual system as three linear filters operating on perceptual quantities, this model accounts for the automatic gain control properties of the eye and for brightness and color contrast effects. In relation to color contrast effects, a psychophysical experiment was performed. It utilized a high quality color television monitor driven by a general purpose digital computer. This experiment, based on the cancellation by human subjects of simultaneous color contrast illusions, allowed the measurement of the low spatial frequency part of the frequency responses of the filters operating on the two chromatic channels of the human visual system. The experiment is described and its results are discussed. Next, the model is shown to provide a suitable framework in which to perform digital images processing tasks. First, applications to color image enhancement are presented and discussed in relation to photographic masking techniques and to the handling of digital color images. Second, application of the model to the definition of a distortion measure between color images (in the sense of Shannon's rate-distortion theory), meaningful in terms of human evaluation, is shown. Mathematical norms in the "perceptual" space defined by the model are used to evaluate quantitatively the amount of subjective distortion present in artificially distorted color presented. Results of a coding experiment yielding digital color images coded at an average bit rate of 1 bit/pixel are shown. Finally conclusions are drawn about the implications of this research from the standpoints of psychophysics and of digital image processing

Chromatic noise perception in digital photography

A model was built to predict chromatic noise perception in digital photography. The model includes the orthogonal opponent color space Y C1C2, and sets of optimized contrast sensitivity functions. Past research on opponent colors, contrast sensitivity functions at threshold and suprathreshold matching has been reviewed. Some historical opponent color spaces have been investigated in terms of transformation and application. Three psychophysical experiments were performed to build the model. Through the equi-luminance plane experiment three equi-luminance planes corresponding to three luminance levels were determined. Method of adjustment was applied for subjects to adjust chromatic noise image until it is least perceptible. Based on results from the first experiment, the orthogonal opponent color space Y C1C2 was developed. The Y C1C2 space shows optimal performance compared with OPP space, with the respect of separating luminance information from chromatic channels and vice versa. The threshold experiment measured contrast threshold for the three cardinal axes and the two diagonal axes of the new opponent color space with three frequency bands and three luminance levels. The QUEST procedure was applied for observers to choose which one of the two side-by-side-displayed stimuli has noise. The supra-threshold experiment was to measure contrast sensitivity above threshold. Method of adjustment was used for observers to adjust the noise contrast of the test stimuli to match the contrast of the parallel displayed achromatic anchor stimuli, which had three times threshold contrast. Sets of optimized CSFs were obtained by empirical modeling on experiment data from the threshold and supra-threshold experiments. The five-parameter band-pass CSF was fitted to model achromatic noise. As to chromatic noise, the six-parameter low-pass CSF was optimized to model chromatic noise. The fact that threshold CSFs and suprathreshold CSFs have similar shape suggests one set of CSFs may be applicable for both cases

Color constancy for landmark detection in outdoor environments

European Workshop on Advanced Mobile Robots (EUROBOT), 2001, Lund (Suecia)This work presents an evaluation of three color constancy techniques applied to a landmark detection system designed for a walking robot, which has to operate in unknown and unstructured outdoor environments. The first technique is the well-known image conversion to a chromaticity space, and the second technique is based on successive lighting intensity and illuminant color normalizations. Based on a differential model of color constancy, we propose the third technique, based on color ratios, which unifies the processes of color constancy and landmark detection. The approach used to detect potential landmarks, which is common to all evaluated systems, is based on visual saliency concepts using multiscale color opponent features to identify salient regions in the images. These regions are selected as landmark candidates, and they are further characterized by their features for identification and recognition.This work was supported by the project 'Navegación autónoma de robots guiados por objetivos visuales' (070-720).Peer Reviewe

Using equiluminance settings to determine the cardinal chromatic directions for individuals

Color information is processed by the retina and lateral geniculate along principal dimensions known as the cardinal directions of color space. Individual differences impacting the direction of these cardinal axes exist within the normal population and are influenced by individual variation in lens density, macular pigment, photopigment opsins, photoreceptor optical density, and relative cone numbers. The same factors that influence color differences also impact luminance. We modeled and empirically tested how well tilts in the equiluminant plane are correlated with rotations in the cardinal axes. Our results show that – especially in the SvsLM axis – cardinal axis rotation can be partially predicted by luminance settings

The time-course of colour vision

Four experiments are presented, each investigating temporal properties of colour vision processing in human observers. The first experiment replicates and extends an experiment by Stromeyer et al. (1991). We look for a phase difference between combined temporal modulations in orthogonal directions in colour space, which might null the often-claimed latency of signals originating from the short-wavelength sensitive cones (S-cones). We provide another estimate of the magnitude of this latency, and give evidence to suggest that it originates early in the chromatic pathway, before signals from S-cones are combined with those that receive opposed L- and M-cone input. In the second experiment we adapt observers to two stimuli that are matched in the mean and amplitude of modulation they offer to the cone classes and to the cardinal opponent mechanisms, but that differ in chromatic appearance, and hence their modulation of later colour mechanisms. Chromatic discrimination thresholds after adaptation to these two stimuli differ along intermediate directions in colour space, and we argue that these differences reveal the adaptation response of central colour mechanisms. In the third experiment we demonstrate similar adaptation using the same stimuli, measured with reaction times rather than thresholds. In the final experiment, we measure the degree to which colour constancy is achieved as a function of time in a simulated stimulus environment in which the illuminant changes periodically. We find that perfect constancy is not achieved instantaneously after an illuminant chromaticity shift and that constancy of colour appearance judgements increases over several seconds

Detecting salient cues through illumination-invariant color ratios

This work presents a novel technique for embedding color constancy into a saliency-based system for detecting potential landmarks in outdoor environments. Since multiscale color opponencies are among the ingredients determining saliency, the idea is to make such opponencies directly invariant to illumination variations, rather than enforcing the invariance of colors themselves. The new technique is compared against the alternative approach of preprocessing the images with a color constancy procedure before entering the saliency system. The first procedure used in the experimental comparison is the well-known image conversion to chromaticity space, and the second one is based on successive lighting intensity and illuminant color normalizations. The proposed technique offers significant advantages over the preceding two ones since, at a lower computational cost, it exhibits higher stability in front of illumination variations and even of slight viewpoint changes, resulting in a better correspondence of visual saliency to potential landmark elements.This work was supported by the project 'Sistema reconfigurable para la navegación basada en visión de robots caminantes y rodantes en entornos naturales.' (00). The authors would like to the support obtained from the Forschungszentrum Informatik and Institut für Prozessrechentechnik, Automation und Robotik , Karlsruhe University, Germany. This work is partially supported by the Spanish Science and Technology Directorate, in the scope of the project “Reconfigurable system for vision-based navigation of legged and wheeled robots in natural environments (SIRVENT)”, grant DPI2003-05193-C02-01.Peer Reviewe

Chromatic sensitivity: effects of light level and selective photoreceptor adaptation

When the light reflected from an object differs in spectral composition to the surrounding background these spectral differences are reflected in the excitation levels produced in each class of photoreceptor. The ability to see colours and to notice small colour differences is strongly affected by both the spectral composition and luminance level of the adapting light. Knowledge of the limits of colour detection is important in setting safety standards and guidelines in visually demanding workplaces, as varying conditions of illumination and chromatic adaptation are often encountered in different working environments. It is therefore of both fundamental and practical interest to be able to predict accurately how a human observer’s chromatic detection performance changes with both light level and chromatic adaptation. The Colour Assessment and Diagnosis (CAD) test was employed to measure colour detection threshold ellipses under different states of chromatic adaptation and background light levels. The advantage of this new technique is that it isolates the use of colour signals by embedding the isoluminant chromatic stimulus in dynamic luminance noise. These measured threshold variations were analysed in terms of changes in L-, Mand S-cone excitation levels required for threshold in different colour directions. Models based on the measured chromatic threshold data are proposed that are capable of reconstructing entire detection ellipses. These models were based on experiments where observers had colour thresholds measured around a series of different chromatic adaptation points, over a range of light levels (typically from 0.3 to 31 cd m-2), and additionally away from the adaptation point. The findings reveal the independent adaptation states of individual cone classes on measured thresholds, i.e., the threshold in a given cone class depends only on the signal produced by the background in that cone class and is independent of the adaptation state of the other cone classes and hence independent of chromaticity and light level. The effect of adapting different areas of the peripheral retina when thresholds are measured foveally was also investigated. No long range retinal interactions were observed. The results show that the adaptation state of the periphery has no effect on colour detection thresholds made in central vision. Variations in L-, M- and S-cone contrasts curves were simulated to assess the influence that detection ellipse size and ellipse orientation have on them. This revealed a correlation between the L-cone contrast curve gradient and the corresponding ellipse orientation. This was additionally shown to correlate with the central 2.8° mean value of macular pigment optical density, hence providing a new method of estimating macular pigment level from colour detection ellipses. Steady state pupil sizes were analysed with the rod and cone excitations that produced them. These data indicate that when chromatically adapted, the steady state pupil size correlates strongest with the S-cone signal, and is independent of the actual chromaticity and luminance levels involved. Cone signal-to-noise ratios were extracted from repeated threshold measurements for a series of colour directions. Analysis of these revealed the existence of a constant signal-to-noise ratios over the full range of colour directions tested relative to a whitish background. The results show that as the cone contrast level increases in a particular cone class so does the associated noise