Chromatic Stimuli Research Articles

Contrast gain shapes visual time

Traditionally time perception has been considered the product of a central, generic, cognitive mechanism. However, evidence is emerging for a distributive system with modality-specific sensory components (Morrone et al., 2005; Johnston et al., 2006). Here we show that fast contrast adaptation, which can be observed in the retina, induces a change in apparent duration. The perceived duration of a subsecond interval containing a 50% luminance contrast drifting pattern is compressed when it follows a high (90%) as compared to a low (10%) contrast interval. The duration effect cannot be attributed to changes in latency at onset relative to offset, can be dissociated from the effect of contrast context on apparent speed or apparent contrast per se and it occurs in a retinocentric frame of reference. The temporal compression is limited to high drift temporal frequencies (≥10 Hz) and is not observed for equiluminant chromatic stimuli. This pattern of results indicates a major role for the magnocellular pathway in the neural encoding and representation of visual time.

Color-Related Signals in the Primate Superior Colliculus

Color is important for segmenting objects from backgrounds, which can in turn facilitate visual search in complex scenes. However, brain areas involved in orienting the eyes toward colored stimuli in our environment are not believed to have access to color information. Here, we show that neurons in the intermediate layers of the monkey superior colliculus (SC), a critical structure for the production of saccadic eye movements, can respond to isoluminant color stimuli with the same magnitude as a maximum contrast luminance stimulus. In contrast, neurons from the superficial SC layers showed little color-related activity. Crucially, visual onset latencies were 30-35 ms longer for color, implying that luminance and chrominance information reach the SC through distinct pathways and that the observed color-related activity is not the result of residual luminance signals. Furthermore, these differences in visual onset latency translated directly into differences in saccadic reaction time. The results demonstrate that the saccadic system can signal the presence of chromatic stimuli only one stage from the brainstem premotor circuitry that drives the eyes.

Investigating neuromagnetic brain responses against chromatic flickering stimuli by wavelet entropies.

BackgroundPhotosensitive epilepsy is a type of reflexive epilepsy triggered by various visual stimuli including colourful ones. Despite the ubiquitous presence of colorful displays, brain responses against different colour combinations are not properly studied.Methodology/Principal FindingsHere, we studied the photosensitivity of the human brain against three types of chromatic flickering stimuli by recording neuromagnetic brain responses (magnetoencephalogram, MEG) from nine adult controls, an unmedicated patient, a medicated patient, and two controls age-matched with patients. Dynamical complexities of MEG signals were investigated by a family of wavelet entropies. Wavelet entropy is a newly proposed measure to characterize large scale brain responses, which quantifies the degree of order/disorder associated with a multi-frequency signal response. In particular, we found that as compared to the unmedicated patient, controls showed significantly larger wavelet entropy values. We also found that Renyi entropy is the most powerful feature for the participant classification. Finally, we also demonstrated the effect of combinational chromatic sensitivity on the underlying order/disorder in MEG signals.Conclusions/SignificanceOur results suggest that when perturbed by potentially epileptic-triggering stimulus, healthy human brain manages to maintain a non-deterministic, possibly nonlinear state, with high degree of disorder, but an epileptic brain represents a highly ordered state which making it prone to hyper-excitation. Further, certain colour combination was found to be more threatening than other combinations.

The study of chromatic and achromatic VEP in the first year of life

Abstract Purpose To study chromatic and achromatic VEP responses in the first year of life. Methods In 30 babies aged 2 to 12 months VEP to chromatic and achromatic stimuli were binocularly recorded. Chromatic VEP were recorded to isoluminant red‐green and blue‐yellow stimulus. The stimulus was a circle composed of horizontal sinusoidal gratings with 90 % chromatic contrast. Two stimulus sizes (7 deg and 21 deg) were used. The stimulus was presented in an onset‐offset mode (on – 300ms, off – 700 ms). Achromatic VEP were recorded to black and white checkerboard (50′ square size), which was presented in pattern reversal (reversal VEP) and onset‐offset (onset VEP) mode. VEP were recorded from Oz (mid occipital) position and the reference was at Fz. Results Chromatic VEP responses were present in all babies, except youngest two (2 months old). The positive wave (P) amplitude to red‐green and blue‐yellow stimulus increased with age when using both stimulus sizes. Achromatic VEP responses were present in all babies. P100 wave (pattern reversal stimulation) showed longer latency in youngest babies (p=0.045), whereas its amplitude did not change throughout the first year of life. C1 wave (onset‐offset stimulation) showed longer latency in youngest babies (p=0.0039), whereas its amplitude decreased with increasing age (p=0.0087). Conclusion Chromatic VEP responses can be recorded in babies after the age of 3 months and show marked maturational changes throughout the first year of life.

Does the Chromatic Mach bands effect exist?

The achromatic Mach bands effect is a well-known visual illusion, discovered over a hundred years ago. This effect has been investigated thoroughly, mainly for its brightness aspect. The existence of Chromatic Mach bands, however, has been disputed. In recent years it has been reported that Chromatic Mach bands are not perceived under controlled iso-luminance conditions. However, here we show that a variety of Chromatic Mach bands, consisting of chromatic and achromatic regions, separated by a saturation ramp, can be clearly perceived under iso-luminance and iso-brightness conditions. In this study, observers' eye movements were recorded under iso-brightness conditions. Several observers were tested for their ability to perceive the Chromatic Mach bands effect and its magnitude, across different cardinal and non-cardinal Chromatic Mach bands stimuli. A computational model of color adaptation, which predicted color induction and color constancy, successfully predicts this variation of Chromatic Mach bands. This has been tested by measuring the distance of the data points from the "achromatic point" and by calculating the shift of the data points from predicted complementary lines. The results suggest that the Chromatic Mach bands effect is a specific chromatic induction effect.

Dynamics of chromatic visual system processing differ in complexity between children and adults

Measures of chromatic contrast sensitivity in children are lower than those of adults. This may be related to immaturities in signal processing at or near threshold. We have found that children's VEPs in response to low contrast supra-threshold chromatic stimuli are more intra-individually variable than those recorded from adults. Here, we report on linear and nonlinear analyses of chromatic VEPs recorded from children and adults. Two measures of signal-to-noise ratio are similar between the adults and children, suggesting that relatively high noise is unlikely to account for the poor clarity of negative and positive peak components in the children's VEPs. Nonlinear analysis indicates higher complexity of adults' than children's chromatic VEPs, at levels of chromatic contrast around and well above threshold.

The perception of speed based on L-M and S-(L+M) cone opponent processing

We have measured perceived speed and speed discrimination thresholds for stimuli that selectively activate the L-M, S-(L +M) cone opponent and L +M (luminance) post-receptoral pathways. For low speeds and low contrasts speed discrimination thresholds for L-M and S-(L +M) are similar but are higher and have a greater dependency upon contrast than those for luminance motion. These differences between chromatic and luminance speed perception can be eliminated when stimuli are equated with respect to their individual motion detection thresholds (MDTs). For fast moving gratings speed perception based upon L-M, S-(L+M) and L+M signals is similar in terms of threshold performance and contrast dependency. These results are consistent with the view that there are separate mechanisms for the analysis of chromatic and luminance motion, the relative contributions of which may change as a function of stimulus contrast and speed. The similarity in performance for S-(L+M) and L+M chromatic stimuli across a range of stimulus parameters suggests that signals derived from the two cone opponent pathways can be used equally well. Our results argue against the idea that speed perception is compromised when it is based upon information derived from the S-(L+M) cone opponent pathway.

Chromatic Contrast Sensitivity During Optokinetic Nystagmus, Visually Enhanced Vestibulo-ocular Reflex, and Smooth Pursuit Eye Movements

Recently we showed that sensitivity for chromatic- and high-spatial frequency luminance stimuli is enhanced during smooth-pursuit eye movements (SPEMs). Here we investigated whether this enhancement is a general property of slow eye movements. Besides SPEM there are two other classes of eye movements that operate in a similar range of eye velocities: the optokinetic nystagmus (OKN) is a reflexive pattern of alternating fast and slow eye movements elicited by wide-field visual motion and the vestibulo-ocular reflex (VOR) stabilizes the gaze during head movements. In a natural environment all three classes of eye movements act synergistically to allow clear central vision during self- and object motion. To test whether the same improvement of chromatic sensitivity occurs during all of these eye movements, we measured human detection performance of chromatic and luminance line stimuli during OKN and contrast sensitivity during VOR and SPEM at comparable velocities. For comparison, performance in the same tasks was tested during fixation. During the slow phase of OKN we found a similar enhancement of chromatic detection rate like that during SPEM, whereas no enhancement was observable during VOR. This result indicates similarities between slow-phase OKN and SPEM, which are distinct from VOR.

‘Gamma’ band oscillatory response to chromatic stimuli in volunteers and patients with idiopathic Parkinson’s disease

The signal structure of the responses to equiluminant chromatic and achromatic (contrast) stimuli was studied in normal volunteers and patients with mild to moderate idiopathic Parkinson’s disease. Visual stimuli were full-field (14 × 16 deg) achromatic or equiluminant (red–green or blue–yellow) sinusoidal gratings at 2 c/deg and 90% contrast presented in onset–offset mode. The signal was processed offline by DFT and factor analysis was performed in the frequency domain. The conventional VEPs to chromatic onset stimuli showed a monophasic negative wave, while the response to offset stimuli was comparable in shape to the on-/offset achromatic responses; latencies were longer and amplitudes higher than those of responses to contrast stimulation. In patients, latencies were longer than in controls after achromatic and (to a lesser extent) red–green stimulations, but not after blue–yellow stimulation; amplitudes were comparable in all stimulus conditions. In healthy subjects, two non-overlapping factors accounted for the ∼2–30.0 Hz and ∼25.0–50.0 Hz signal components (representative of the low-frequency VEP and gamma oscillatory responses, respectively); the frequency of the ∼25.0–50.0 Hz factor was lower after color than after contrast stimulation. The same factor structure was identified in patients, but the peak frequency of the factor on gamma activity was higher than in controls and did not vary with color-opponent stimulation. These observations indicate that stimulus-related gamma activity originates in cortex irrespective of the activated (magno-, parvo-, or konio-cellular) visual pathway, consistent with the suggested role in the phase coding of neuronal activities. Some dopaminergic modulation of gamma activity is conceivable.

Color properties of movement detectors in the Carassius gibelio (Bloch, 1782) tectum opticum studied by selective stimulation of different cone types

and takes part in orienting responses revealed in rapid eye, head, and/or body movemements. This function can proceed without taking into account information about color, and the superior colliculus in mammals is color-blind (Michael, 1973). In the frog, the system carrying chromatic information also originates as a channel separate from the tectal pathway, providing the main information needed for the frog’s reactions to moving objects (Maximov et al., 1985). Color blindness of movement detectors projecting to the fish tectum was proved in color-matching experiments for both orientation-selective (Maximova, 1999) and direction-selective (Maximova et al., 2005) units. Recent experiments using selective stimulation of different cone types revealed new features of these projections. The color vision of Prussian carp (Carassius gibelio Bloch, 1782) adults is determined by three types of cones: long-wavelength (L), middle-wavelength (M), and short-wavelength (�), con taining three A2-based visual pigments that absorb maximally at about 622, 545, and 434 nm, respectively (Maximova et al., 2005). Color opponent cells were presented at different levels of the fish visual system, and single-unit extracellular recordings from the C. gibelio tectum opticum were made in order to examine the possible role of action potential timing in coding chromatic stimuli.

Hue and saturation shifts from spatially induced blackness

We studied changes in the color appearance of a chromatic stimulus as it underwent simultaneous contrast with a more luminous surround. Three normal trichromats provided color-naming descriptions for a 10 cd/m2 monochromatic field while a broadband white annulus surround ranged in luminance from 0.2 cd/m2 to 200 cd/m2. Descriptions of the chromatic field included Red, Green, Blue, Yellow, White, and Black or their combinations. The naming frequencies for each color/surround were used to calculate measures of similarity among the stimuli. Multidimensional scaling analysis of these subjective similarities resulted in a four-dimensional color space with two chromatic axes, red-green and blue-yellow, and two achromatic axes, revealing separate qualities of blackness/lightness and saturation. Contrast-induced darkening of the chromatic field was found to be accompanied by shifts in both hue and saturation. Hue shifts were similar to the Bezold-Brücke shift; shifts in saturation were also quantified. A stage model is proposed to account for the relationships among blackness induction and the inherent nonlinearities in chromatic and achromatic processing.

Simple reaction times to chromatic stimuli: Luminance and chromatic contrast

This paper examines the effect of colour on reaction times for variations in both luminance and chromatic contrast. Results confirm the idea that reaction times are determined by a cone-opponent mechanism: Reaction times generated in response to S-cone isolating stimuli are the longest, whereas the shortest reaction times are generated by L—M-cone isolating stimuli. In addition, an asymmetry between ON and OFF opponent channels is observed for stimuli modulated on the blue—yellow axis. Reaction time (RT) is influenced by hue and chromatic contrast at isoluminant condition. In the isoluminant condition, RT decreases as luminance contrast increases. At luminance contrasts of approximately 20%, RT approaches an asymptotic value, thus becoming independent of colour for further luminance contrast increases. This asymptotic value is achieved for lower luminance contrasts as chromatic contrast increases.

Chromatic discrimination capacity in the ceramics sorting process

Nowadays, the most of the ceramic tile companies keep on sorting their production visually by means of specialized staff (classifiers). The know how of these classifiers comes from their own practical experience developed in the working place, not having at one's disposal accurate references that indicate the procedures to systematically use tools for color measurement. Therefore, the criteria employed by each of them are not the same. The evident improvement that could be achieved thanks to uniform criteria justifies the search for objective procedures to carry out the visual classification process. In this paper we propose homogeneous criteria for color classification. In this sense, we put forward a series of experiments, executed by specialized (classifiers) and non-specialized observers, based on a specific set of achromatic samples. The discrimination capacity of the classifiers is determined by the adequate arrangement of the samples according to a greyscale, establishing an objective procedure for their sorting. Moreover, using particular samples as patterns, we obtain a group of the chromatic stimulus that is useful to know the acceptance interval in the determination of the chromatic color-differential thresholds of a set of observers.

FMRI measurements of color in macaque and human

We have used fMRI to measure responses to chromatic and achromatic contrast in retinotopically defined regions of macaque and human visual cortex. We make four observations. Firstly, the relative amplitudes of responses to color and luminance stimuli in macaque area V1 are similar to those previously observed in human fMRI experiments. Secondly, the dorsal and ventral subdivisions of macaque area V4 respond in a similar way to opponent (L--M)-cone chromatic contrast suggesting that they are part of a single functional area. Thirdly, we find that macaque area V4, like area V1, responds preferentially to chromatic contrast compared to luminance contrast and the degree of preference is strongly influenced by the temporal frequency of the stimulus. Finally, we observe that while macaque V4d is a region on the dorsal surface of the macaque visual cortex that responds robustly to chromatic stimuli, human chromatic responses to identical stimuli are largely confined to the ventral surface suggesting a fundamental difference in the topographical organization of higher visual areas between humans and macaques.

Parvocellular and Magnocellular Contributions to the Initial Generators of the Visual Evoked Potential: High-Density Electrical Mapping of the “C1” Component

The C1 component of the VEP is considered to index initial afference of retinotopic regions of human visual cortex (V1 and V2). C1 onsets over central parieto-occipital scalp between 45 and 60 ms, peaks between 70 and 100 ms, and then resolves into the following P1 component. By exploiting isoluminant and low-contrast luminance stimuli, we assessed the relative contributions of the Magnocellular (M) and Parvocellular (P) pathways to generation of C1. C1 was maximal at 88 ms in a 100% luminance contrast condition (which stimulates both P and M pathways) and at 115 ms in an isoluminant chromatic condition (which isolates contributions of the P pathway). However, in a 4% luminance contrast condition (which isolates the M pathway), where the stimuli were still clearly perceived, C1 was completely absent. Absence of C1 in this low contrast condition is unlikely to be attributable to lack of stimulus energy since a robust P1-N1 complex was evoked. These data therefore imply that C1 may be primarily parvocellular in origin. The data do not, however, rule out some contribution from the M system at higher contrast levels. Nonetheless, that the amplitude of C1 to P-isolating isoluminant chromatic stimuli is equivalent to that evoked by 100% contrast stimuli suggests that even at high contrast levels, the P system is the largest contributor. These data are related to intracranial recordings in macaque monkeys that have also suggested that the initial current sink in layer IV may not propagate effectively to the scalp surface when M-biased stimuli are used. We also discuss how this finding has implications for a long tradition of attention research that has used C1 as a metric of initial V1 afference in humans. C1 has been repeatedly interrogated for potential selective attentional modulations, particularly in spatial attentional designs, under the premise that modulation of this component, or lack thereof, would be evidence for or against selection at the initial inputs to visual cortex. Given the findings here, we would urge that in interpreting C1 effects, a consideration of the dominant cellular contributions will be necessary. For example, it is plausible that spatial attention mechanisms could operate primarily through the M system and that as such C1 may not always represent an adequate dependent measure in such studies.

Chromatic VEP in colour deficient children

Abstract Purpose To compare chromatic VEP response to isoluminant red‐green stimulus in children with congenital red‐green colour deficiency with a control group of 30 children with normal colour vision. Methods 15 children (7‐18 years) with congenital colour vision deficiency (8 in deutan and 7 in protan axis) and 30 healthy children (7‐19 years) were included in the study. Colour vision was assessed with Ishihara plates, Nagel Anomaloscope, Mollon‐Reffin Minimalist test, Farnsworth‐Munsell D‐15 saturated and desaturated test and Farnsworth‐Munsell hue 100 test. VEP were recorded to isoluminant red‐green stimulus. The stimulus was a 7 deg large circle composed of horizontal sinusoidal gratings, with spatial frequency 2 cycles/deg and 90 % chromatic contrast. VEP were recorded from Oz (mid occipital) position. Children were tested binocularly. Latency and amplitude of positive (P) and negative (N) wave were measured and so was mean amplitude (N‐P wave). Results N wave was present in 24/30 children with normal colour vision (110 ± 25.1 ms; 9.7 ± 4.8 μV) and only in 1/15 child with colour vision deficiency (93 ms; 3.2 μV). P wave was present in 30/30 children with normal colour vision (138 ± 21.1 ms; 21.1 ± 13.5 μV) and in 13/15 children with colour vision deficiency (131.9 ± 6.1 ms; 19.4 ± 10.7 μV). In healthy children waveform changed from predominantly positive to negative wave with increasing age, whereas in colour deficient children no obvious waveform changes were observed. Conclusion VEP response to isoluminant chromatic stimulus showed different characteristics in children with congenital colour vision deficiency compared to children with normal colour vision. manca.tekavcic‐pompe@guest.arnes.si

Genetic heterogeneity in autosomal dominant retinitis pigmentosa with low-frequency damped electroretinographic wavelets

To define molecular and ophthalmic features of a rare phenotype in autosomal dominant (ad) retinitis pigmentosa (RP). A 32-year-old woman (proband) with adRP and the low-frequency damped electroretinographic (ERG) wavelet phenotype and her mother were studied with optical coherence tomography (OCT), chromatic perimetry and ERG. A previously reported adRP patient with this ERG phenotype (Lam et al) was also studied with OCT. Genotype in the two families was determined with DNA sequencing. ERGs from the proband were identical to those reported previously. Chromatic perimetry and ERG stimulus intensity series indicated that there can be severely reduced rod function in addition to substantial cone dysfunction. A heterozygous deletion in peripherin/RDS (Met152del3 atGAA) was present in the patient and the affected mother. There were foveal cystoid changes and pericentral splitting of the inner nuclear layer. ONL thickness and vision tapered with eccentricity, and 'blind' regions without discernible ONL showed a thickened, delaminated inner retina. Similar OCT findings were present in the reported adRP patient with this ERG; the patient was heterozygous for a 4-bp deletion (Leu107del4 ctGAGT) in PRPF31. The low-frequency damped ERG wavelet phenotype is genetically heterogeneous. Inner retinal structural abnormalities are also present in this rare disease expression.

Smooth Pursuit Eye Movements to Isoluminant Targets

At slow speeds, chromatic isoluminant stimuli are perceived to move much slower than comparable luminance stimuli. We investigated whether smooth pursuit eye movements to isoluminant stimuli show an analogous slowing. Beside pursuit speed and latency, we studied speed judgments to the same stimuli during fixation and pursuit. Stimuli were either large sine wave gratings or small Gaussians blobs moving horizontally at speeds between 1 and 11 degrees /s. Targets were defined by luminance contrast or color. Confirming prior studies, we found that speed judgments of isoluminant stimuli during fixation showed a substantial slowing when compared with luminance stimuli. A similarly strong and significant effect of isoluminance was found for pursuit initiation: compared with luminance targets of matched contrasts, latencies of pursuit initiation were delayed by 50 ms at all speeds and eye accelerations were reduced for isoluminant targets. A small difference was found between steady-state eye velocities of luminance and isoluminant targets. For comparison, we measured latencies of saccades to luminance and isoluminant stimuli under similar conditions, but the effect of isoluminance was only found for pursuit. Parallel psychophysical experiments revealed that different from speed judgments of moving isoluminant stimuli made during fixation, judgments during pursuit are veridical for the same stimuli at all speeds. Therefore information about target speed seems to be available for pursuit eye movements and speed judgments during pursuit but is degraded for perceptual speed judgments during fixation and for pursuit initiation.

Chromatic and achromatic stimulus discrimination of long wavelength (red) visual stimuli by the honeybee Apis mellifera

It has long been assumed that bees cannot see red. However, bees visit red flowers, and the visual spectral sensitivity of bees extends into wavelengths to provide sensitivity to such flowers. We thus investigated whether bees can discriminate stimuli reflecting wavelengths above 560 nm, i.e., which appear orange and red to a human observer. Flowers do not reflect monochromatic (single wavelength) light; specifically orange and red flowers have reflectance patterns which are step functions, we thus used colored stimuli with such reflectance patterns. We first conditioned honey bees Apis mellifera to detect six stimuli reflecting light mostly above 560 nm and found that bees learned to detect only stimuli which were perceptually very different from a bee achromatic background. In a second experiment we conditioned bees to discriminate stimuli from a salient, negative (un-rewarded) yellow stimulus. In subsequent unrewarded tests we presented the bees with the trained situation and with five other tests in which the trained stimulus was presented against a novel one. We found that bees learned to discriminate the positive from the negative stimulus, and could unambiguously discriminate eight out of fifteen stimulus pairs. The performance of bees was positively correlated with differences between the trained and the novel stimulus in the receptor contrast for the long-wavelength bee photoreceptor and in the color distance (calculated using two models of the honeybee colors space). We found that the differential conditioning resulted in a concurrent inhibitory conditioning of the negative stimulus, which might have improved discrimination of stimuli which are perceptually similar. These results show that bees can detect long wavelength stimuli which appear reddish to a human observer. The mechanisms underlying discrimination of these stimuli are discussed.

The contribution of color to global motion processing

This study investigates the contribution of color vision to global motion. We present evidence demonstrating that performance on a global motion task attains similar levels for both types of chromatic (L/M-cone opponent and S-cone opponent) and luminance stimuli at suprathreshold contrasts. We show, however, that the motion thresholds for isoluminant chromatic stimuli are luminance based, on the grounds that they are masked by luminance noise but robust to chromatic noise. Detection thresholds, on the other hand, are chromatic in origin (masked by chromatic but not luminance noise), indicating that there is no luminance artifact in the stimulus. We suggest that for color vision at isoluminance the global motion task is based on the integration of many local, luminance-based signals.