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PubMed1993

Development of infant contrast sensitivity to chromatic stimuli.

We have monitored the development of contrast sensitivity to equiluminant red-green chromatic patterns by monitoring visual evoked potentials (VEPs) in 13 infants. The results confirm our previous report [Morrone, Burr and Fiorentini, Proceedings of the Royal Society B, 242 (1990a)] that, before 7-8 weeks of age, there was no response to purely chromatic stimuli, while at the same age luminance stimuli of 20% contrast produced reliable responses. At all ages (even before the onset of a chromatic response) the colour mixture to yield equiluminance was similar to that of adults, suggesting that the relative proportion and efficacy of medium- and long-wave cones is similar for infants as for adults. For both luminance and chromatic stimuli, amplitude increased roughly linearly with log-contrast, so sensitivity thresholds could be predicted by linear extrapolation to the abscissa. Detailed contrast sensitivity curves were measured for four infants at various ages. The results show that luminance and chromatic contrast sensitivity develop independently at different rates, probably reflecting differential development of postreceptoral neural mechanisms.

Age Factors↗
PubMed1995

Detection of blue under chromatic adaptation: the effects of stimulus size and eccentricity.

We measured thresholds for the perception of blue under chromatic adaptation to white, green, yellow or red at the eccentricities of 0-70 deg in the temporal visual field of four subjects. We used a series of stimulus sizes at each eccentricity, without a prior assumption of any peripheral size-scaling factor. The CIE 1976 UCS (u',v') chromaticity coordinates corresponding to blue perception were subtracted from the chromaticity coordinates of the adaptation field in order to obtain the threshold differences (du',dv') in chromaticity coordinates. Spatial scaling factors for the perception of blue were obtained by non-linear regression (E2 + 5 deg) refers to the eccentricity at which stimulus diameter had to be doubled in order to maintain performance found at the eccentricity of 2.5 deg. E2 for the perception of blue tint varied from 1.2 to 36 deg depending on the state of chromatic adaptation and subject. For the perception of blue tint in yellow three subjects and for the perception of blue tint in red one subject had no spatial scaling factor that would make performance independent of eccentricity. Thus, spatial scaling does not always work.

Adaptation, Ocular↗
PubMed1994

Motion of chromatic stimuli: first-order or second-order?

This paper measures the minimum velocity required to discriminate the direction of motion (the lower threshold of motion--LTM) for patterns which consisted of spatial variations in luminance, chromaticity or luminance contrast in an attempt to distinguish between the underlying directionally-selective mechanisms. The characteristics of these patterns can be defined as first-order/Fourier stimuli (luminance and chromatic gratings) or second-order/non-Fourier stimuli (contrast gratings or "beats"). Measurements for each pattern were made at durations ranging from 0.015 to 0.96 sec and at contrasts of 0.5 log units above detection threshold and 1.5 log units above the threshold for detecting the stationary pattern. Observers were able to discriminate the direction of motion in luminance gratings and high contrast chromatic gratings at all durations above 0.015 sec. The direction of motion of beats and low contrast chromatic gratings was indiscriminable until they had been presented for at least 0.12 sec. This was taken to indicate the existence of a fast-acting and a slow-acting system dealing with the first- and second-order patterns respectively. When defined on this basis, the chromatic stimulus acts as a first-order (luminance) pattern at high contrasts and a second-order (beat) pattern at low contrasts.

Color Perception↗
PubMed1995

Reversals of the colour-depth illusion explained by ocular chromatic aberration.

Although many colour-depth phenomena are predictable from the interocular difference in monocular chromatic diplopia caused by the eye's transverse chromatic aberration (TCA), several reports in the literature suggest that other factors may also be involved. To test the adequacy of the optical model under a variety of conditions, we have determined experimentally the effects of background colour on perceived monocular chromatic diplopia and perceived depth (chromostereopsis). A Macintosh colour monitor was used to present red, blue, and green test stimuli which were viewed monocularly or binocularly (haploscopically) through 1.78 mm artificial pupils. These apertures were displaced nasally and temporally from the visual axis under controlled conditions to induce a variable degree of TCA. Monocular chromatic diplopia and binocular chromostereopsis were measured for red and blue targets, and also for red and green targets, presented on either a black background or on a background which was composed of the sum of the targets' spectral composition (e.g. red and blue presented on magenta; red and green presented on yellow). In all cases, chromatic diplopia and chromostereopsis were found experimentally to reverse in sign with this change in background. Furthermore, we found that a given coloured target could be located in different depth planes within the same display when located on different background colours. These seemingly paradoxical results could nevertheless be explained by a simple model of optical TCA without the need to postulate additional factors or mechanisms.

Adult↗
PubMed1996

Contribution of chromatic mechanisms to the production of small-field optokinetic nystagmus (OKN) in normals and strabismics.

The contribution of chromatic mechanisms to motion processing is currently debated. Although OKN as a measure of motion processing has been employed for chromatic stimuli, the contribution of chromatic mechanisms to the production of OKN has not been assessed directly. We measured voluntary-pursuit and involuntary-stare OKN responses to drifting patterns defined by colors modulated in a cone based color space which isolates luminance and color mechanisms. Results show that OKN responses are reduced for isoluminant stimuli, particularly for the stare conditions. The greatest reduction in responses occur for isoluminance stimuli that isolate the S cones. These results suggest that the chromatic mechanisms may have reduced input to the neural substrate that produces involuntary-stare OKN or that stare OKN has different temporal characteristics when stimulated by chromatic and luminance mechanisms. We also measured OKN asymmetries in several convergent strabismics in response to isoluminant and luminance patterns and compared their response to previous reports of horizontal OKN asymmetries in normal neonate infants. The results question the validity of either the modeling of OKN asymmetries in strabismus as incomplete development or the comparison of adult eye movement records evaluated with criteria for OKN with psychophysical forced choice evaluation of ocular drift in infants.

Adult↗
PubMed2006

Layered double hydroxides as potential chromate scavengers.

The LDH of Ni with Fe, having the formula Ni(1-x)Fe(x)(OH)2(A(n-))(x/n)yH2O (A = NO3-, Cl-; x = 0.25, 0.33), scavenges CrO4(2-) ions from solution throughout the concentration range examined (0.00625-0.25 N). The CrO4(2-) uptake capacity is independent of the anion in the starting LDH but is higher when x = 0.25 (3.60 meq g(-1)) as compared to x = 0.33 (2.40 meq g(-1)). These values are higher than those observed for control compounds beta-Ni(OH)2 (1.86 meq g(-1)) and FeO(OH) (1.26 meq g(-1)), which do not have any interlayer chemistry, showing that chromate uptake takes place by its incorporation in the interlayer region by a stoichiometric anion-exchange reaction, rather than by adsorption. Nevertheless, the interaction between the LDH and the chromate ions is weak. The weak interaction is due to the mismatch between the symmetry of the chromate ions and the symmetry of the interlayer site, which introduces turbostratic disorder in the chromate-intercalated LDHs. The chromate ions can be completely leached out by soaking the LDH in a sodium carbonate solution.

Journal Article↗
PubMed2004

Chromatic edges, surfaces and constancies in cerebral achromatopsia.

We tested achromatopsic observer, MS, on a number of tasks to establish the extent to which he can process chromatic contour. Stimuli, specified in terms of cone-contrast, were presented in a three-choice oddity paradigm. First we show that MS is able to discriminate the magnitude of chromatic and luminance contrast, but performance is inferior to that of normal observers. Moreover, MS can discriminate isoluminant borders of different chromatic composition. These abilities are not the result of unintended luminance differences and are abolished when chromatic borders are masked by sharp luminance change. In simple displays, local cone-contrast signals can make a significant contribution to surface colour appearance in normal observers. In more complex displays, the perception of a surface's colour becomes largely independent of the local contrast to its background, via processes presumed to be similar to the edge integration and anchoring stages of Land's Retinex algorithm. We show that in simple displays the percepts of both MS and normal observers are dominated by local chromatic-contrast. But, although the percepts of normal observers change in line with the predictions of retinex theory in more complex displays, those of MS do not, remaining dominated by local contrast signals. We conclude that MS has lost the ability to perform edge integration and that this loss is closely related to his absence of colour experience.

Adult↗
PubMed2005

Chromatic properties of the colour-shading effect.

The 'colour-shading effect' describes the phenomenon whereby chromatic variations affect the magnitude of perceived shape-from-shading in luminance patterns. A previous study showed that in mixed colour-plus-luminance sine-wave plaids, impressions of depth in the luminance component were enhanced by non-aligned chromatic components, and suppressed by aligned chromatic components [Nature Neuroscience 6 (2003) 641-644]. Here we examine the chromatic determinants of these effects. Colour contrast was defined along the cardinal axes of colour space in order to isolate the L-M and S-(L+M) post-receptoral chromatic mechanisms. We found no difference in the potency of L-M-only and S-(L+M)-only gratings, either for enhancing or suppressing perceived depth. Moreover, the magnitude of depth-suppression was no different for any combination of depth-enhancing and depth-suppressing cardinal directions. Finally we tested whether the visual system carried the assumption that natural shading is tinged with blue, by measuring perceived depth in a colour-plus-luminance grating that was made to appear either bright-yellow/dark-blue or bright-blue/dark-yellow. However there was no difference in the magnitude of depth-suppression between conditions, suggesting that the visual system does not make any assumption about the colour of natural shading. Taken together, the results suggest that while the colour-shading effect is highly sensitive to colour contrast, it is agnostic with respect to colour direction.

Color Perception↗
PubMed2005

The detection of motion in chromatic stimuli: pedestals and masks.

This study seeks to clarify the reasons for some of the differences in the published data on chromatic motion perception, and to provide further support for the existence of a low-level motion mechanism sensitive to purely chromatic change. Observers discriminated the direction of motion of displaced sinusoidal gratings in the presence of a static grating mask (or pedestal). Each component of the stimulus was independently described in cardinal colour space and calibrated for subjective equiluminance using multiple methods. The motion structure, stimulus size, temporal frequency, contrast, relative phase and chromatic properties were all varied parametrically and the data cast in terms of predictions made by two different theoretical approaches to the test-mask combination. The vast majority of the data were well explained by a low-level motion mechanism sensitive to the motion of foveally-placed chromatic stimuli. Data consistent with either higher-level motion perception or a luminance-like signal were found outside the fovea and when the stimulus properties did not otherwise favour chromatic motion perception. There was some explanation of inconsistencies in previously published data and a strong suggestion that previous results showing pedestal-like behaviour for these stimulus combinations were a special case rather than a general result.

Color Perception↗
PubMed2006

The loci of achromatic points in a real environment under various illuminant chromaticities.

Under colored illumination, the achromatic point (the point in the chromaticity diagram seen as colorless) shifts toward the chromaticity of the illuminant. This investigation measured the loci of achromatic points for various intensities of a test field presented in a real rather than a simulated environment, lit by illuminants of various chromaticities. The achromatic point varied markedly with the intensity level of the test field: for dim test fields it was close to the surround chromaticity, but for high luminance test fields it was almost invariant with the surround chromaticity. The varying achromatic settings imply a variation in the relative effectiveness of the different cone types, but this variation originates in the postreceptoral system rather than at the photoreceptors themselves: flicker photometric sensitivity was almost independent of the illuminant in all cases. Nor does the variation take the simple form of a sensitivity-scaling coefficient; such a model can not predict the observed dependence of the achromatic setting on test intensity. The data could, however, be modeled with a scheme in which the log of the relative cone weight implicit in the achromatic setting depends almost linearly on (1) the log of the relative cone excitation by the illuminant and (2) the log of the test field intensity.

Adaptation, Ocular↗
PubMed2001

Uncertainty, attentional capacity and chromatic mechanisms in visual search.

Two general questions were investigated using a visual search task. First, we asked whether effects of target uncertainty on reaction time varied with the discriminability of the target and distractors. Second, a higher order chromatic mechanism model was tested against a flexible model in which the signals in cardinal color-opponent mechanisms are combined through an attentional process. The models were tested by measuring the effects of target uncertainty on search time. A regression analysis indicated that the magnitude of the uncertainty effect was approximately constant in logarithmic units as a function of the chromatic difference between the target and distractors. The constant magnitude of the uncertainty effect suggested that an attentional capacity limit was exceeded when observers were required to monitor several chromatic mechanisms at several locations. The results of experiments 3 and 4 suggested that search for chromatic targets among distractors was mediated by diagonally tuned higher order chromatic mechanisms, rather than by signals in cardinal color-opponent mechanisms that were combined through an attentional mechanism.

Adult↗
PubMed2001

Chromatic and achromatic defects in patients with progressing glaucoma.

To evaluate the pattern of losses associated with glaucomatous injury in patients with progressing glaucoma, functional losses were examined in 14 patients with progressing glaucoma using tests for which detection should be selectively mediated by one of three psychophysical mechanisms. Red-on-white increments, blue-on-white increments and critical flicker frequency were used to isolate the responses of the red-green chromatic mechanism, the blue-on chromatic mechanism, and the high-frequency flicker achromatic mechanism. For our 3.1 degrees circular stimuli, chromatic defects were found in a greater number of the patients with glaucoma than were achromatic defects. We evaluated these defects in terms of two existing hypotheses: preferential loss and reduced redundancy. The greater sensitivity to glaucomatous injury of chromatic tests, compared to achromatic tests, found in this and other studies and the apparent discrepancy between anatomical and psychophysical studies can be parsimoniously explained by differences in cortical summation of ganglion cell responses for the chromatic and achromatic pathways.

Aged↗
PubMed2001

Dependency of reaction times to motion onset on luminance and chromatic contrast.

We measured reaction times for detecting the onset of motion of sinusoidal gratings of 1 c/deg, modulated in either luminance or chromatic contrast, caused to move abruptly at speeds ranging from 0.25 to 10 deg/s (0.25-10 Hz). At any given luminance or chromatic contrast, RTs varied linearly with temporal periodicity (r2 congruent with 0.97), yielding a Weber fraction of period. The value of the Weber fraction varied inversely with contrast, differently for luminance and chromatic contrast. The results were well simulated with a simple model that accumulated change in contrast over time until a critical threshold had been reached. Two crucial aspects of the model are a second-stage temporal integration mechanism, capable of accumulating information for periods of up to 2 s, and contrast gain control, different for luminance than for chromatic stimuli. The contrast response for luminance shows very low semi-saturating contrasts and high gain, similar to LGN M-cells and cells in MT; that for colour shows high semi-saturating contrasts and low gain, similar to LGN P-cells. The results suggest that motion onset for luminance and chromatic gratings are detected by different mechanisms, probably by the magno- and parvo-cellular systems.

Contrast Sensitivity↗
PubMed2001

Visual evoked potentials elicited by chromatic motion onset.

Visually Evoked Potentials (VEPs) were recorded in response to the onset of chromatic and luminance motion gratings of 1 cpd and luminance 40 cd m(-2) subtending a 7 degrees field. At slow speeds (< or =2 cycles s(-1)) the motion onset response exhibits a clear amplitude minimum at isoluminance. Over the Michelson contrast range tested (0.05-0.75) the chromatic response at 2 cycles s(-1) possesses a linear response function compared to the saturating function of the luminance response and the contrast dependency of the former is a factor of 5-6 times greater than for the latter. These differences are suggestive of different neural substrates for the chromatic and luminance motion VEPs at slow speeds. At 10 cycles s(-1) the chromatic motion onset VEP exhibits no amplitude minimum at isoluminance and becomes more like its luminance counterpart in terms of its saturating contrast response function. Furthermore, the contrast dependency of the chromatic and luminance responses differs by only a factor of 1.6 at this faster rate. These findings are consistent with the idea of separate motion mechanisms that operate at fast and slow speeds, the latter having separate channels for colour and luminance motion.

Adult↗
PubMed2001

Luminance and chromatic cues in a spatial integration task.

These experiments explore the way in which cues provided by luminance and chromatic contrast interact in the spatial integration of elements. The stimuli were composed of bidimensional and isotropic Gauss functions. The elements were placed so that when experimentally manipulating the separations between the lines, subjects could generate an oriented percept from the elements sharing luminance or chromaticity. Results showed that, in most cases, grouping elements that share chromatic content is possible, in spite of variations in luminance content. Grouping elements as a function of luminance is more difficult when chromaticity alternates from one element to another. Lastly, if competing groupings are generated, the stimulus is structured as a function of chromatic content and not of luminance content.

Color Perception↗
PubMed2002

The influence of stimulus chromaticity on the isoluminant motion-onset VEP.

Motion-onset visual evoked potentials (VEPs) were elicited by low spatial frequency chromatic isoluminant gratings presented in a central 7 degrees circular field. The chromatic composition of the stimuli was varied so as to modulate along different axes in colour space. For slow speeds (<5 degrees/s) changing the chromatic axis induced large response differences between the S- and L/M-cone VEPs. At faster speeds (5-12 degrees/s) the effects were not as marked. A dichotomy between the slow and fast responses was also shown to exist in terms of their contrast dependencies, the former exhibiting a stronger dependency on contrast than the latter. These findings suggest that neural substrates with chromatic sensitivity are involved in the generation of S- and L/M-cone mediated motion-onset VEPs at low velocities. At higher velocities, responses are generated by different mechanisms that possess little or no chromatic sensitivity.

Adult↗
PubMed1997

Chromatic and monochromatic optical resolution in the rainbow trout.

The modulation transfer function due to measured longitudinal chromatic aberration was calculated for the otherwise unaberrated eye of the adult rainbow trout (Oncorhynchus mykiss) under daylight conditions assuming light absorption by single retinal cone pigments, and by photopic mechanisms involving interaction between cones. The adult trout eye, with its large immobile pupil, is limited by chromatic aberration to resolution much lower than the diffraction limit, consistent with the low acuity reported for fish. This low resolution can be considered a design trade-off cost of a bright image. The measured monochromatic modulation transfer function is similar to that calculated due to chromatic aberration alone, showing that these independent aberrations are approximately balanced in the fish eye. The effect of changes in receptor length, pigment density, water depth, and pupil size upon the chromatic resolution was calculated. The calculated chromatic modulation transfer function will hold approximately for other teleost eyes with lens larger than about 1 mm.

Animals↗
PubMed1998

Chromatic masking in the (delta L/L, delta M/M) plane of cone-contrast space reveals only two detection mechanisms.

The post-receptoral mechanisms that mediate detection of stimuli in the (delta L/L, delta M/M) plane of color space were characterized using noise masking. Chromatic masking noises of different chromaticities and spatial configurations were used, and threshold contours for the detection of Gaussian and Gabor tests were measured. The results do not show masking that is narrowly-selective for the chromaticity of the noise. On the contrary, our findings suggest that detection of these tests is mediated only by an opponent chromatic mechanism (a red-green mechanism) and a non-opponent luminance mechanism. These results are not consistent with the hypothesis of multiple chromatic mechanisms mediating detection in this color plane [1].

Color Perception↗