Achromatic Mechanisms Research Articles

Binocular integration of chromatic and luminance signals.

Much progress has been made in understanding how the brain combines signals from the two eyes. However, most of this work has involved achromatic (black and white) stimuli, and it is not clear if the same processes apply in color-sensitive pathways. In our first experiment, we measured contrast discrimination ("dipper") functions for four key ocular configurations (monocular, binocular, half-binocular, and dichoptic), for achromatic, isoluminant L-M and isoluminant S-(L+M) sine-wave grating stimuli (L: long-, M: medium-, S: short-wavelength). We find a similar pattern of results across stimuli, implying equivalently strong interocular suppression within each pathway. Our second experiment measured dichoptic masking within and between pathways using the method of constant stimuli. Masking was strongest within-pathway and weakest between S-(L+M) and achromatic mechanisms. Finally, we repeated the dipper experiment using temporal luminance modulations, which produced slightly weaker interocular suppression than for spatially modulated stimuli. We interpret our results in the context of a contemporary two-stage model of binocular contrast gain control, implemented here using a hierarchical Bayesian framework. Posterior distributions of the weight of interocular suppression overlapped with a value of 1 for all dipper data sets, and the model captured well the pattern of thresholds from all three experiments.

Randomised trial of three treatments for amblyopia: Vision therapy and patching, perceptual learning and patching alone.

Active vision therapy for amblyopia shows good results, but there is no standard vision therapy protocol. This study compared the results of three treatments, two combining patching with active therapy and one with patching alone, in a sample of children with amblyopia. Two protocols have been developed: (a) perceptual learning with a computer game designed to favour the medium-to-high spatial frequency-tuned achromatic mechanisms of parvocellular origin and (b) vision therapy with a specific protocol and 2-h patching. The third treatment group used patching only. Fifty-two amblyopic children (aged 4-12 years), were randomly assigned to three monocular treatment groups: 2-h patching (n = 18), monocular perceptual learning (n = 17) and 2-h patching plus vision therapy (n = 17). Visual outcomes were analysed after 3 months and compared with a control group (n = 36) of subjects with normal vision. Visual acuity (VA) and stereoacuity (STA) improved significantly after treatment for the three groups with the best results for patching plus vision therapy, followed by monocular perceptual learning, with patching only least effective. Change in the interocular difference in VA was significant for monocular perceptual learning, followed by patching. Differences in STA between groups were not significant. For VA and interocular differences, the final outcomes were influenced by the baseline VA and interocular difference, respectively, with greater improvements in subjects with poorer initial values. Visual acuity and STA improved with the two most active treatments, that is, vision therapy followed by perceptual learning. Patching alone showed the worst outcome. These results suggest that vision therapy should include monocular accommodative exercises, ocular motility and central fixation exercises where the fovea is more active.

Poster Session I: Effect of flicker adaptation on perception of small spots presented with AOSLO.

Previous work suggests the detection of small spots is mediated by a mixture of chromatic and achromatic mechanisms. We tested whether exposure to spatially-uniform chromatic or luminance flicker affected detection thresholds for 543 nm increments delivered through an AOSLO. Heterochromatic flicker photometry was used to determine isoluminant settings for the red and green primaries of a DLP display; this isoluminant red-green mixture provided the 2.1° background upon which 23 arcmin (N=4) or 3 arcmin (N=2) stimuli were presented for 100ms. The projector background was modulated to produce isoluminant chromatic flicker or isochromatic luminance flicker at 3.75 or 30 Hz. The time-averaged luminance and chromaticity for all adaptation conditions were equivalent. For each condition, data collection was preceded by 2 minutes of preadaptation, followed by alternating windows of stimulus delivery (1 sec, steady background) and top-up adaptation (3 sec). Thresholds for all flicker conditions were compared to data obtained on a static background. For 23 arcmin spots, we found reduced sensitivity in the 3.75 Hz chromatic and luminance flicker conditions, but no adaptation effect was observed for 3 arcmin flashes or for 30 Hz flicker of either type. Our data suggest that raster-scanned, AO-corrected stimuli are susceptible to flicker adaptation, but that proximity to a flickering edge may be an important factor governing the effects of contrast adaptation on small spot detection.

Poster Session II: Alternating orientation of the chromatic pattern VEP improves signal even in the absence of contrast adaptation.

The visual evoked potential (VEP) to chromatic pattern reversal is greatly reduced compared to VEPs to pattern onsets. Chromatic pattern onsets produce large and stereotypical waveforms that reliably differ from standard achromatic pattern reversal VEP waveforms used in clinical applications. Rapid contrast adaptation for sustained chromatic but not transient achromatic mechanisms has been suggested as one explanation for these observations. Here we first examined changes in the magnitude of response during recordings to reversing and onset grating patterns that preferentially modulate the L-M, S, and achromatic pathways. Given the evidence for both chromatic and achromatic orientation-selective mechanisms, we then hypothesized that contrast adaptation may be reduced by changing the orientation of the pattern for each reversal or onset. VEPs were recorded for 60 s with 2 onsets/reversals per second using both fixed and alternating (horizontal/vertical) orientations. FFT amplitudes for 6-second windows did not reveal evidence of adaptation for chromatic or achromatic onsets or reversal patterns over the 60-second recording period. Despite this, alternating pattern orientation increased the signal for all chromatic but not achromatic conditions. Although alternating the orientation for reversals increased the signal, the onset responses were still larger, even for non-alternating orientations. Mechanisms other than contrast adaptation must be invoked to explain the results.

Contributed Session II: Attentional Modulation of Early Cortical Chromatic Responses

Prior research demonstrates that attention can modulate the amplitude of achromatic (black and white) pattern reversal stimuli. Similarly, MRI research has shown attentional modulation of BOLD responses to both chromatic and achromatic stimuli. However, prior research has also demonstrated that the chromatic onset visual evoked potential (VEP) is robust to attentional modulation (e.g., Highsmith & Crognale, 2010; Wang & Wade, 2011) with either spatially contiguous or spatially disparate manipulations. It is possible that more demanding attentional tasks than those that have been used previously, may reveal attentional modulation of the chromatic VEP. Here we report the results of experiments in which we recorded chromatic and achromatic VEPs while subjects performed multiple object tracking (MOT) tasks with several levels of difficulty. Steady-state onset responses were recorded at 3hz for stimuli modulated along the chromatic and achromatic axes. Our preliminary results suggest that the increased attentional demand provided by MOT can reveal a clear, but idiosyncratic modulation of chromatic VEP responses. The question remains whether or not chromatic and achromatic mechanisms share similar attentional gain processes as reflected in the VEP. We are currently characterizing the nature of these attentional gain mechanisms.

Reliability in perimetric multichannel contrast sensitivity measurements

BackgroundIn this study, the reliability of perimetric contrast sensitivity measurements favouring the achromatic, the red‐green and the blue‐yellow post‐receptorial mechanisms was analysed.MethodsA new technique, multichannel ATD perimetry, provides spatial and temporal stimuli favouring the detection by an achromatic mechanism (A), from a magno or parvocellular origin or by a red‐green (RG) chromatic mechanism, with a parvocellular origin or a blue‐yellow (BY) mechanism, with a koniocellular origin. The repeatability and reproducibility of contrast sensitivity measurements with these stimuli were studied in a group of 40 healthy subjects. The analysis was carried out on 21 testing points within a 60° by 40° fovea‐centred region of the visual field.ResultsThe within‐observer repeatability for the four mechanisms studied is either good or excellent when the intra‐class correlation coefficient (ICC) can be calculated. For the remaining points, the Friedman's test finds that the measurements are repeatable. The between‐observer reproducibility was either excellent or good in cases where the ICC was applied and according to the Friedman's test all results were reproducible.ConclusionsThe results obtained showed good repeatability and reproducibility with A, RG and BY stimuli, although with BY stimuli repeatability is slightly worse. Future studies on the diagnostic validity of this device are based on the fact that changes of sensitivity can be compared by means of a visual single task, contrast sensitivity measurement and using a common metric.

An experimental method for the assessment of color simulation tools.

The Simulcheck method for evaluating the accuracy of color simulation tools in relation to dichromats is described and used to test three color simulation tools: Variantor, Coblis, and Vischeck. A total of 10 dichromats (five protanopes, five deuteranopes) and 10 normal trichromats participated in the current study. Simulcheck includes two psychophysical tasks: the Pseudoachromatic Stimuli Identification task and the Minimum Achromatic Contrast task. The Pseudoachromatic Stimuli Identification task allows determination of the two chromatic angles (h(uv) values) that generate a minimum response in the yellow–blue opponent mechanism and, consequently, pseudoachromatic stimuli (greens or reds). The Minimum Achromatic Contrast task requires the selection of the gray background that produces minimum contrast (near zero change in the achromatic mechanism) for each pseudoachromatic stimulus selected in the previous task (L(R) values). Results showed important differences in the colorimetric transformations performed by the three evaluated simulation tools and their accuracy levels. Vischeck simulation accurately implemented the algorithm of Brettel, Viénot, and Mollon (1997). Only Vischeck appeared accurate (similarity in huv and L(R) values between real and simulated dichromats) and, consequently, could render reliable color selections. It is concluded that Simulcheck is a consistent method because it provided an equivalent pattern of results for huv and L(R) values irrespective of the stimulus set used to evaluate a simulation tool. Simulcheck was also considered valid because real dichromats provided expected huv and LR values when performing the two psychophysical tasks included in this method.

A hundred years of color studies in insects: with thanks to Karl von Frisch and the workers he inspired.

and colleagues (Hempel de Ibarra et al. 2014). Lars Chittka and colleagues specifically remind us that whilst bees and other insects see in color, in complex natural environments color perception can be a challenging cue due to the large changes in the spectral quality of light in different foraging environments (Chittka et al. 2014). Anne Leonard and Pavel Masek (2014) discuss the latest evidence that bees can use multimodal information combining both color and olfactory cues to help reduce uncertainty in complex foraging conditions. Indeed there is currently a lot of interest in how bees make decisions in complex environments. Aurore Avargues-Weber and Martin Giurfa (2014) discuss that a variety of important factors like conditioning method can influence bee perception, possibly through cognitive ‘top-down’ attention-type mechanisms. Further evidence of complex decision making in bees comes from a nice study on bumblebees by Lisa Evans and Nigel Raine (2014) who show that the way in which some bees make errors may be of adaptive advantage for selecting novel rewarding colors in natural conditions. Other studies show us that since the time of von Frisch color processing has been tested in a wide variety of different insect species. Motohiro Wakakuwa and colleagues (2014) discuss photoreceptor sensitivity in the leafhopper (Nephotettix cincticeps) which shows some behavioral sensitivity to long wavelength ‘infrared’ radiation. Francismeire Telles and colleagues (2014) show in the hummingbird hawkmoth (Macroglossum stellatarum) that the behavioral sensitivity to blue wavelength light is much higher than predicted from the retinal photoreceptor sensitivity, which may be related to the innate preference of hawkmoths for blue flowers. Besides bees, flies and butterflies have been well studied. Klaus Lunau (2014) provides a timely review on the color vision of flies which is a system generating a lot of interest at present. Michiyo Kinoshita About 100 years ago it was often thought the world might appear pretty dull for insects until the publication of Karl von Frisch in 1914. this ground breaking study by von Frisch revealed that free flying honeybees could be trained with sucrose solution to visit a colored card, and choose this stimulus correctly in tests when placed amongst a variety of grey cards. the logic of the experiment, done at a time when the spectral sensitivities of bee photoreceptors were unknown, was that if bees were only using an achromatic mechanism to find the conditioned, colored stimulus associated with a sucrose reward then the bees would make mistakes for grey cards having similar brightness values to the colored stimulus. However, the bees reliably chose the colored stimulus amongst many shades of grey (von Frisch 1914), and the eyes of the next generations of researchers were opened to the refinement and diversity of color perception in bees and other insects (Kuhn 1927; Daumer 1956; Menzel 1967; Helversen 1972; Giurfa 2004). this current year of 2014 also marks the 50th anniversary of the first publication of the spectral sensitivities of individual photoreceptor cells in the honeybee by Autrum and von Zwehl (1964), which further helped to establish honeybees as a classic model for research into insect color vision (Peitsch et al. 1992; Briscoe and Chittka 2001). In this special issue of The Journal of Comparative Physiology A, leading scientists in the field have teamed up to pay tribute to the important start Karl von Frisch gave to the field of neuroethology and insect vision with his publication in 1914. Basic physiology and ecology of bee color vision is summarized by Natalie Hempel de Ibarra

The contribution of single and double cones to spectral sensitivity in budgerigars during changing light conditions

Bird colour vision is mediated by single cones, while double cones and rods mediate luminance vision in bright and dim light, respectively. In daylight conditions, birds use colour vision to discriminate large objects such as fruit and plumage patches, and luminance vision to detect fine spatial detail and motion. However, decreasing light intensity favours achromatic mechanisms and eventually, in dim light, luminance vision outperforms colour vision in all visual tasks. We have used behavioural tests in budgerigars (Melopsittacus undulatus) to investigate how single cones, double cones and rods contribute to spectral sensitivity for large (3.4°) static monochromatic stimuli at light intensities ranging from 0.08 to 63.5 cd/m². We found no influences of rods at any intensity level. Single cones dominate the spectral sensitivity function at intensities above 1.1 cd/m², as predicted by a receptor noise-limited colour discrimination model. Below 1.1 cd/m², spectral sensitivity is lower than expected at all wavelengths except 575 nm, which corresponds to double cone function. We suggest that luminance vision mediated by double cones restores visual sensitivity when single cone sensitivity quickly decreases at light intensities close to the absolute threshold of colour vision.

Simulating Images Seen by Patients with Inhomogeneous Sensitivity Losses

We aim to simulate how colored images are perceived by subjects with local achromatic and chromatic contrast sensitivity losses in the visual field (VF). The spatiochromatic corresponding pair algorithm, introduced in a previous article (J Opt Soc Am (A) 2004;21:176-186), has been implemented with a linear model of the visual system. Spatial information is processed separately by the chromatic and achromatic mechanisms by means of a multiscale model, with sensors selective to frequency, orientation, and spatial position, whose mechanism-dependent relative weights change with the spatial location of the image. These weights have been obtained from perimetric data from a patient with Leber's Hereditary Optic Neuropathy and an age-matched sample of normal subjects, using achromatic, red-green, and blue-yellow gratings of different spatial frequencies. Distortion contours for each mechanism have been derived from the images simulating the perception of these subjects at different locations in the VF. The images simulating the perception of normal subjects at different locations of the VF show a fast decrease in image quality with eccentricity. The same analysis carried out with the Leber's Hereditary Optic Neuropathy patient reveals worse overall image quality throughout the VF, plus a color vision defect resembling red-green dichromacy at fovea and trichromatic anomaly in the rest of the VF. In the present article, we show that implementing the algorithm with a spatial vision model that considers the changes in contrast sensitivity with spatial location of the stimulus may reveal the local effects that are suffered, in general, by pathological subjects, and that are ignored by simpler spatial vision models.

Orientation based saliency and visual search in the archer fish

Event Abstract Back to Event Orientation based saliency and visual search in the archer fish Alik Mokeichev1, 2*, Ronen Segev2, 3 and Ohad Ben-Shahar1, 2 1 Ben Gurion University of the Negev, Department of Computer Science, Israel 2 Ben Gurion University of the Negev, Zlotowski Center for Neuroscience, Israel 3 Ben-Gurion University of the Negev, Department of Life Sciences, Israel Orientations based visual saliency effects have been extensively demonstrated in behavioral experiments with humans and other primates, and in many theoretical models have been tightly linked to known properties of the anatomical and functional organization of early visual cortex. Employing a novel behavioral methodology in experiments with the archer fish, a proficient hunter with remarkable visual abilities (Schlegel and Schuster, 2008), we observed orientation saliency effects similar to those observed with human subject (Mokeichev, Segev and Ben-Shahar, 2010). Such findings challenge our current conceptions of the necessitous physiological hardware for processing orientations saliency. In other experiments with human subjects we exploited our unique methodological approach to find interactions between chromatic and achromatic mechanisms in processing of orientation saliency. These finding, again, challenge the general view of largely independent processing streams of chromatic and achromatic information at early stages of the visual pathways. We conjecture that both of these studies suggest orientation-based saliency processing to constitute a fundamental (and abstract) building block for efficient visual information processing. Acknowledgements This work was supported in part by the DFG and the Israel Science Foundation, as well as the Zlotowski Center for Neuroscience, the Paul Ivanier Robotics Center, the Frankel Fund at Ben-Gurion University. References Schlegel, T., and Schuster, S.(2008). Small circuits for large tasks: high-speed decision-making in archerfish. Science 319, 104-106 Mokeichev, A., Segev, R., and Ben-Shahar, O. (2010). Orientation saliency without visual cortex and target selection in archer fish. PNAS 107, 16726–16731. Keywords: archer fish, visual saliency, orientation saliency, visual search, orientation pop out, color pop out, visual information processing, orientation-based texture segregation Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Invited Symposium (only for people who have been invited to a particular symposium) Topic: Sensory: Vision Citation: Mokeichev A, Segev R and Ben-Shahar O (2012). Orientation based saliency and visual search in the archer fish. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00069 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 04 Jul 2012; Published Online: 07 Jul 2012. * Correspondence: Mr. Alik Mokeichev, Ben Gurion University of the Negev, Department of Computer Science, Be'er Sheva, 84105, Israel, mokeiche@cs.bgu.ac.il Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Alik Mokeichev Ronen Segev Ohad Ben-Shahar Google Alik Mokeichev Ronen Segev Ohad Ben-Shahar Google Scholar Alik Mokeichev Ronen Segev Ohad Ben-Shahar PubMed Alik Mokeichev Ronen Segev Ohad Ben-Shahar Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

X-ray Time Lags in TeV Blazars

We use Monte Carlo/Fokker–Planck simulations to study the X-ray time lags. Our results show that soft lags will be observed as long as the decay of the flare is dominated by radiative cooling, even when acceleration and cooling time scales are similar. Hard lags can be produced in the presence of a competitive achromatic particle energy loss mechanism if the acceleration process operates on a time scale such that particles are slowly moved towards higher energy while the flare evolves. In this type of scenario, the γ-ray/X-ray quadratic relation is also reproduced.

視覚系における色覚メカニズムと明るさメカニズムの動的相互作用, II: 2刺激光法による測定

視覚系における色覚メカニズムと明るさメカニズムの動的相互作用, II: 2刺激光法による測定

The achromatic mechanisms do not combine cone signals additively: a new experimental approach

Many standard models of color vision include a “luminance” mechanism that combines signals from the L and M cones additively (e.g., Boynton, 1979; Guth, 1991; Lennie, Pokorny, & Smith, 1993). Experiments that employ “equiluminant” stimuli generally assume linearity, explicitly or implicitly. The luminance mechanism is often studied using moderate to high frequency flicker or motion stimuli. However, brief, weak equichromatic flashes are also generally believed to be detected by this mechanism (Eskew, McLellan, & Giulianini, 1999; Krauskopf, Williams, & Heeley, 1982). The present experiments were undertaken to determine if the mechanism that detects “white” Gaussian blobs, on a gray background, combines the L and M cones linearly, using a new approach that was developed to study the linearity of mechanisms detecting S-cone isolating flashes (Giulianini & Eskew, 2007). A test flash is embedded in masking noise (flickering lines covering the test region). In different runs, the noise consists of L-cone modulations only, M cone modulations only, or the superposition of the two cone component noises (L&M noise). If the detection mechanism combines its L and M cone inputs linearly, the threshold elevation produced by the compound L&M noise is predictable from a specific combination of the elevations produced by the two component noises. Previous results with this method found that S cone mechanisms are nonlinear, but that the ‘green’ mechanism is remarkably linear. In the present study, we measured the masking effect of 8 different compound noises and their associated L and M components. The results show that the mechanism that detects achromatic flashes does not combine its L and M cone inputs linearly. This result, along with others using flicker detection (Stockman & Plummer, 2005; Stromeyer, Chaparro, Tolias, & Kronauer, 1997), calls into question the generality of the idea of equiluminance.

Effect of chromatic mechanisms on the detection of mesopic incremental targets at different eccentricities

Spectral sensitivity functions for the threshold detection of mesopic incremental targets were compared for different target eccentricities (10, 20, and 30 degrees ) and for different mesopic backgrounds (0.1, 0.5 and 1.0 cd m(-2)). Relative responsivities of achromatic mechanisms (L + M and rods) and chromatic mechanisms (S and /L-M/) were estimated for each eccentricity and background. Chromatic mechanisms contribute significantly to detection but their effect is lower at 30 degrees . A new contrast metric (C(CHC2)) is introduced to account for the selective adaptation of the photoreceptors and the effects of the chromatic mechanisms i.e. broadening of the range of spectral sensitivity with multiple local maxima and yellow sub-additivity of detection performance. The C(CHC2) metric is compared with the achromatic contrast metric of the MOVE model (C(MOVE)). For the same target, C(CHC2) generally predicts a higher visibility level than C(MOVE). However, in accordance with visual observations, for grey or yellowish incremental targets appearing at the eccentricities of 20 and 30 degrees , the visibility predicted by C(CHC2) is less than the visibility predicted by C(MOVE).

Avian colour vision: Effects of variation in receptor sensitivity and noise data on model predictions as compared to behavioural results

Colour vision models require measurement of receptor noise and the absorbance of visual pigments, oil droplets, and ocular media. We have studied how variation in these parameters influences colour matching, spectral sensitivity, and colour discrimination predictions in four bird species. While colour match predictions are sensitive to variation in visual pigment and oil droplet absorbance data, discrimination predictions are mostly sensitive to variation in receptor noise. Ocular media transmittance influences only modelled spectral sensitivities at short wavelengths. A comparison between predicted and measured spectral sensitivities in domestic fowl and duck revealed large discrepancies, likely because of influences from achromatic mechanisms.

Egg colour mimicry in the common cuckooCuculus canorusas revealed by modelling host retinal function

Some parasite cuckoo species lay eggs that, to the human eye, appear to mimic the appearance of the eggs of their favourite hosts, which hinders discrimination and removal of their eggs by host species. Hitherto, perception of cuckoo-host egg mimicry has been estimated based on human vision or spectrophotometry, which does not account for what the receivers' eye (i.e. hosts) actually discriminates. Using a discrimination model approach that reproduces host retinal functioning, and museum egg collections collected in the south of Finland, where at least six different races of the European cuckoo (Cuculus canorus) coexist, I first assess whether the colour design of cuckoo eggs of different races maximizes matching for two favourite avian hosts, viz. the redstart (Phoenicurus phoenicurus) and the pied wagtail (Motacilla alba). Second, I assess the role of nest luminosity on host perception of mimicry by the same two hosts. Phoenicurus-cuckoo eggs showed a better chromatic matching with the redstart-host eggs than other cuckoo races, and in most cases can not be discriminated. Sylvia-cuckoo eggs, however, showed better achromatic matching with redstart-host eggs than Phoenicurus-cuckoo eggs. Also, Motacilla-cuckoo eggs showed poorer chromatic and achromatic matching with pied wagtail-host eggs than Sylvia-cuckoo eggs. Nest luminosity affected chromatic and achromatic differences between cuckoo and host eggs, although only minimally affected the proportion of cuckoo eggs discriminated by chromatic signals. These results reveal that cuckoo races as assessed by humans do not entirely match with host perception of matching and that achromatic mechanisms could play a main role in the discrimination of cuckoo eggs at low-light levels.

Amplitude of the transient visual evoked potential (tVEP) as a function of achromatic and chromatic contrast: Contribution of different visual pathways

We investigated how the stimulation mode influences transient visual evoked potentials (tVEP) amplitude as a function of contrast of achromatic and isoluminant chromatic gratings. The chromatic stimulation probed only responses to the red-green axis. Visual stimuli were monocularly presented in a 5 degrees diameter circle, achromatic and chromatic horizontal gratings, 1 Hz pattern reversal stimulation, and achromatic and chromatic gratings, 300 ms onset per 700 ms offset stimulation. For the achromatic pattern reversal stimulation, a double slope function describes how the P100 amplitude varied as a function of log contrast which had a limb at low-to-medium contrasts and another limb at high contrasts. For the achromatic onset/offset stimulation, C2 amplitude saturated at the highest contrast tested and a single straight line described how it changed along most of the contrast range. Both presentation modes for chromatic gratings resulted in amplitude versus log contrast relations which were well described by single straight lines along most of the contrast range. The results may be interpreted as if at 2 cpd, achromatic pattern reversal stimulation evoked the activity of at least two visual pathways with high and low contrast sensitivity, respectively, while achromatic onset/offset stimulation favored the activity of a pathway with high contrast sensitivity. The neural activity in the M pathway is the best candidate to be the high contrast mechanism detected with pattern reversal and pattern onset/offset VEPs. The activity of color opponent pathways such as the P and K pathways either combined or in isolation seems to be responsible for VEPs obtained with isoluminant chromatic gratings at both presentation modes. When the amplitudes of chromatic VEPs were plotted in the same contrast scale as used for achromatic VEPs, chromatic contrast thresholds had similar values to those of the achromatic mechanism with high contrast sensitivity.

The discoloration illusion

The discoloration illusion, a new visual phenomenon, is described. This phenomenon originates from the juxtaposition of eight chromatic parallel contours on a white background, creating a luminance gradient and enclosing a light red region. Under these conditions, the inner region appears white: the light red discolors and appears white with both surface color and luminous qualities. In two experiments, the discoloration illusion was (i) compared with the coloration effect of the watercolor illusion, obtained when the number of adjacent contours was reduced to at least two, and (ii) tested under several conditions useful for understanding the roles of the luminance gradient profile. The results suggest that discoloration is not a lightness illusion and does not depend on simultaneous contrast or on achromatic mechanisms, but more likely on chromatic mechanisms that, through the luminance chromatic gradient, provide cues about the interactions of light and surface and model the volume by depicting lights and shades. The discoloration illusion suggests a possible neural scenario where multiple juxtaposed contours may stimulate neurons, selective for different asymmetric luminance profiles and signaling not only the unilateral belongingness of the boundaries and the coloration effect but also the volumetric and the illumination effects.

Effect of a yellow filter on brightness evaluated by asymmetric matching: measurements and predictions

We found that yellow filters reduce the mean brightness of a set of coloured samples significantly less with blue surrounds than with black, achromatic or yellow ones, in comparison with the naked eye and less than a yellow surround in comparison with a luminance-matched grey filter. The predictions of Hunt's model and Guth's ATD95 agree with these results. The yellow filter is actually capable of significantly increasing the brightness with the blue surround both in comparison with the naked eye (although only for our younger observer) and with the neutral filter. We show that in some samples the yellow filter causes global response increments in the chromatic mechanisms that compensate the response reduction in the achromatic mechanism, resulting in increased brightness. Although global chromatic response increments may arise from any chromatic mechanism, the blue–yellow mechanism determines in most cases the final result, even when the response of the red–green mechanism decreases.