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PubMed2005

Saccadic latencies for achromatic and chromatic targets.

The purpose of this experiment was to compare saccadic latencies for supra-threshold achromatic and chromatic targets of equivalent contrast. Two experiments were performed. In the first experiment, subjects made saccades to horizontal and vertical chromatic (red, green, and blue) targets. The luminance of these targets was matched to the luminance of the white background. In the second experiment, subjects made saccades to horizontal and vertical achromatic targets whose luminance contrast was matched to the chromatic contrast of the targets in the first experiment using the CIE L*a*b* color space. In the first experiment, the saccadic latencies did not vary significantly (p = 0.074) for the different target colors. However, in the second experiment the mean latency for achromatic targets (268.6 ms +/- 53.1) varied significantly from the pooled latency for color targets (318.4 ms +/- 75.1).

Adult↗
PubMed2000

Asymmetries in the time-course of chromatic adaptation and the significance of contrast.

The time-course of chromatic adaptation was determined as a function of the spectral content of the adaptation-light and of image-contrast. The adaptation-lights varied along the chromatic cardinal axes or one of their intermediate axes in an equiluminant plane in colour-space. We found an asymmetry in the initial time-course of adaptation (0.2-10 s) in that adaptation to middle-wavelength light was significantly faster than adaptation to short- and long-wavelength light. The asymmetry was only observed in the presence of a spatially complex pattern. It was fully supported by luminance and chromatic contrast and present under haploscopic stimulus conditions.

Adaptation, Physiological↗
PubMed2000

Development of chromatic and luminance detection contours using the sweep VEP.

This study measured the development of contrast-sweep VEP thresholds to a range of chromatic and luminance stimuli. Subjects were 14-32 week-old infants (n=21) and three adults. Stimuli were 1 c/d sine gratings reversed at 5.6 Hz. Chromaticity was varied from the L-M axis to an achromatic axis. VEP thresholds when plotted in L- and M-cone contrasts showed that: (1) VEP thresholds did not consistently locate the psychophysical isoluminance match under the same stimulus conditions; (2) About 50% of the data were described by independent chromatic and luminance mechanism, however, thresholds were limited by the cone contrast of the stimulus, phase cancellation between visual mechanisms, and the proper sampling of thresholds in L- and M-cone contrast space and; (3) No significant changes occurred in VEP detection contours across ages but suprathreshold VEP amplitudes suggested complex developmental changes. Data from previous studies showed similar results.

Adult↗
PubMed2001

Spatial neural modulation transfer function of human foveal visual system for equiluminous chromatic gratings.

To determine the spatial modulation transfer function (MTF) of the human foveal visual system for equiluminous chromatic gratings we measured contrast sensitivity as a function of retinal illuminance for spatial frequencies of 0.125-4 c/deg with equiluminous red-green and blue-yellow gratings. Contrast sensitivity for chromatic gratings first increased with luminance, obeying the Rose-DeVries law, but then the increase saturated and contrast sensitivity became independent of light level, obeying Weber's law. Critical retinal illuminance (I(c)) marking the transition point between the laws was found to be independent of spatial frequency at 165 phot. td. According to our detection model of human spatial vision the MTF of the retina and subsequent neural visual pathways (P(c)) is directly proportional to radicalI(c). Hence, P(c) is independent of spatial frequency, reflecting the lack of precortical lateral inhibition for equiluminous chromatic stimuli in spatiochromatically opponent retinal ganglion cells and dLGN neurons.

Adult↗
PubMed2001

Investigating sources of variability of monochromatic and transverse chromatic aberrations across eyes.

Schematic eye models have typically been used to explain the average monochromatic and chromatic imaging properties of the eye. Both monochromatic aberrations and transverse chromatic aberration are known to vary widely across subjects. However, to our knowledge, the ability of schematic eye models to predict these individual variations has not been tested experimentally. We used a spatially resolved refractometer to measure the monochromatic aberrations and the optical transverse chromatic aberration (oTCA) in a group of 15 eyes. By recording the 1st and 4th Purkinje images for five directions of gaze, we also estimated the tilt, misalignment of ocular surfaces (front surface of the cornea and back surface of the lens) and off-axis position of the fovea (angle alpha), as well as pupil centration. We conclude that, contrary to expectations none of those factors are major contributors to the variability in monochromatic aberrations and oTCA in this group of eyes. Simulations show that corneal curvature and corneal conicity are also unlikely to account for the observed relation between monochromatic aberrations and oTCA. Our results suggest an important contribution of corneal irregularities to those aberrations.

Adult↗
PubMed2003

The spatial tuning of chromatic adaptation.

A key question in colour research is how the colour and spatial analysis of an image interact. Traditionally, colour and form analysis have been regarded as parallel and separate processes, and documented effects of image complexity on chromatic adaptation have been attributed to a temporal integration process during eye movements. Evidence is presented here for a spatial mechanism, which tunes chromatic adaptation to the luminance structure (spatial frequency and orientation) of an image. This in turn suggests a close cooperation between colour and form analysis during chromatic adaptation. The results are discussed in relation to the "segregated pathway hypothesis" and the role of spatial aspects for the computation of colour constancy and adaptation to natural scenes.

Adaptation, Ocular↗
PubMed1997

Responses to chromatic and luminance contrast in glaucoma: a psychophysical and electrophysiological study.

Increasing anatomical evidence indicates that large retinal ganglion cells (M-cells) are preferentially damaged in primary open angle glaucoma (OAG), while the smaller ganglion cells (P-cells) are relatively spared. In 13 patients with defined OAG and modest visual field defects, we evaluated the responses to stimuli that are expected to involve primarily the function of the P-pathway and compared them with those of control subjects. The psychophysical contrast sensitivity (CS), the PERG and the VEPs were measured for red-green gratings of pure chromatic contrast, as well as yellow-black gratings of pure luminance contrast. As compared with controls, OAG patients had reduced CS for both luminance and chromatic contrast stimuli by about 6 dB. PERGs and VEPs to luminance stimuli were little affected, whereas those to chromatic stimuli were both reduced in amplitude and delayed. These results indicate that visual dysfunction in glaucoma is not selective for the M-pathway, and that responses to equiluminant colour-contrast stimuli may be of diagnostic value.

Aged↗
PubMed1998

Temporal analysis of the topographic ERG: chromatic versus achromatic stimulation.

The topographic electroretinogram evoked by multi-focal exchange of black and white or red and green stimuli was analysed into linear and non-linear Wiener kernels. The first-order (temporally linear) response showed a biphasic waveform which inverted as the luminance ratio of the exchanged colours passed through unity (established both psychophysically and photometrically). A short latency non-linearity which was dependant on luminance contrast was observed in both chromatic and achromatic ERG. However, in the chromatic second-order response, a long-latency non-linearity, foveally prominent, with a distinct skew in power towards the nasal retina, appeared around the isoluminant point, between the points of silent substitution for the L and M-cone types. Modelling of the second-order responses showed that over a wide range of luminance ratios, the chromatic ERG is well described by a linear combination of the achromatic (contrast-dependent) component and the response at isoluminance. The difference in second-order response between coloured and black and white stimulation, at the same luminance contrast, showed that the long-latency non-linearity is recorded when the red and green cone types are operating out of phase and peaks in amplitude at a green/red luminance ratio of 0.8. This interpretation was confirmed by the lack of the long-latency non-linearity in colour-anomalous subjects (whether deficient in the L or the M-cone type). A marked similarity exists between the properties of the long-latency non-linearity and the frequency-doubled response generated in the ganglion cells of the magnocellular pathway.

Color Perception↗
PubMed1998

When is a background equivalent? Sparse chromatic context revisited.

Jenness and Shevell (Vision Res 1995;35:797-805) reported that a red background with white dots scattered on it has a different influence on a target's apparent colour than an equivalent uniform background. We show that this finding depends on what one considers an equivalent background. Jenness and Shevell averaged the chromaticity and luminance of the background with the dots, and 'superimposed' the target onto this new background. This changed the luminance and chromaticity of both the target and the surround. We show that if only the surround is changed, it is irrelevant whether the latter is red with white dots scattered over it, or a uniform field with the same space averaged chromaticity and luminance. Our findings are consistent with a local contrast mechanism that has a limited spatial resolution.

Adaptation, Ocular↗
PubMed1998

Red-green chromatic discrimination with variegated and homogeneous stimuli.

Chromatic discrimination thresholds were measured under conditions which yielded fine and degraded discrimination steps. Discrimination was assessed by identification of the location of one of four homogeneous equiluminant stimuli arranged in a square or with pseudoisochromatic (PIC) figures using the stimulus design of Regan, Reffin and Mollon (Vis Res 1994; 34: 1279-1299). Stimuli were presented on CRT monitors and specified in units of cone trolands. They were viewed within a surround metameric to the equal energy spectrum. L troland threshold versus retinal illuminance (TVR) functions were measured by four-alternative spatial forced-choice staircase procedures for (1) a four 1 x 1 degree equiluminant stimuli arranged in a square and (2) 'C' shaped pseudoisochromatic figures in which the observer had to identify the gap location. The 'C' was constructed of spatially discrete patches of varying size and luminance to ensure that the observer's responses depended on chromatic signals. The TVR functions appeared V-shaped and were similar for the two paradigms. The minimum occurred near the L excitation of equal-energy white. The PIC stimuli yielded poorer discrimination with the TVR function being displaced by approximately 0.5 log unit. Discrimination for stimuli degraded by luminance and spatial noise presented within an achromatic appearing surround is sharpest near the chromaticity metameric to the equal energy spectrum.

Adult↗
PubMed1999

Morphology of transient VEPs to luminance and chromatic pattern onset and offset.

Characteristics of the visual evoked response to chromatic and luminance-modulated stimuli reflect the activity of underlying neural mechanisms, although selective neuronal activity depends upon stimulus parameters. In the present study, the behaviour of the transient visual evoked response to low spatial and temporal frequency chromatic stimuli is investigated at a range of colour luminance ratios. Our results show that the response to pattern-offset may be used in addition to the pattern-onset response as part of the signature of the evoked response to luminance-modulated or isoluminant chromatic stimuli.

Adult↗
PubMed2004

The primacy of chromatic edge processing in normal and cerebrally achromatopsic subjects.

The local chromatic contrast between surfaces in a visual scene plays an important role in theories of color perception. Our studies of cerebral achromatopsia suggest that this contrast signal is computed independently of the more complex processes such as edge integration and anchoring. We report a study in which we attempted to determine whether local-contrast signals also drove behavior in normal subjects. We sought to reduce the role of edge integration and anchoring by using stimuli whose background varied very gradually in color from top to bottom. The local chromatic contrast of patches relative to such backgrounds depends upon the position at which they are presented. It is therefore possible for patches with identical spectral composition to have opposite contrasts. We constructed stimuli in which two of three vertically arranged discs had the same contrast while the third had opposite contrast. The stimuli were also constructed so that the contrast-odd disc and one of the other two had identical spectral composition while the third disc had different composition. We used these stimuli in an attentional task where, after a brief delay, a letter discrimination target was presented in the location of one of the discs. Attention should automatically be attracted to the odd disc in such a display. Normal observers were faster at making the letter discrimination when the target appeared at the contrast-odd as opposed to spectrally odd location. We conclude that local chromatic contrast, but not raw spectral composition, is accessible to normal observers at an appropriate stage in visual processing to drive attention.

Adult↗
PubMedDate not supplied

Spatial structure of chromatically opponent receptive fields in the human visual system.

This study investigates the receptive-field structure of mechanisms operating in human color vision, by recording visual evoked potentials (VEPs) to multiharmonic gratings modulated either in luminance or color (red-green). Varying the Fourier phase of the harmonics from 0 deg to 90 deg produced a family of stimulus profiles that varied from lines to edges. The stimuli were contrast reversed to elicit steady-state VEPS, and also randomly jittered (at a higher temporal frequency than the contrast reversal) to ensure that the evoked response resulted from the polarity reversal, rather than from local variation of luminance or color. Reliable VEPs were recorded from both luminance and chromatic stimuli at all phases, suggesting that the mechanisms sensitive to chromatic contrast and those sensitive to luminance contrast have both symmetric and asymmetric receptive fields. Contrast thresholds estimated by extrapolation of the contrast response curves were very similar to psychophysical thresholds for phase discrimination, suggesting that the VEP response is generated by mechanisms mediating phase discrimination. The results support the idea that human color mechanisms have receptive fields with a variety of spatial symmetries (including odd- and even-symmetric fields) and that these mechanisms may contribute to phase discrimination of chromatic stimuli in a similar way to what has been suggested for luminance vision.

Color Perception↗
PubMedDate not supplied

Perceptual classification of chromatic modulation.

We measured the regions of the equiluminant plane that are exploited by observers during a Yes/No detection task. The signal was a 640-ms Gaussian modulation (sigma(t) = 160 ms) of a Gaussian spatial patch (sigma(s) = 2.4 deg) presented in chromatically bivariate uniform noise. One component of the noise was along the direction axial with the signal in color space, the other perpendicular. Four signal directions were tested: along cardinal LM and S axes and two intermediate directions to which the cardinal axes were equally sensitive. The distribution of noise chromaticities from each trial was correlated with the observers' responses and the presence and absence of the signal to build a classification image of the distribution of chromaticities on which the decision of the observer was based. The images show a narrowly selective peak in the signal direction flanked by regions with a broader selectivity. These results raise the possibility that detection judgments are mediated by both linear and nonlinear mechanisms with peak sensitivities between the cardinal directions.

Adult↗
PubMedDate not supplied

Chromatic and contrast selectivity in color contrast adaptation.

We used color contrast adaptation to examine the chromatic and contrast selectivity of central color mechanisms. Adaptation to a field whose color varies along a single axis of color space induces a selective loss in sensitivity to the adapting axis. The resulting changes in color appearance are consistent with mechanisms formed by different linear combinations of the cone signals. We asked whether the visual system could also adjust to higher-order variations in the adapting stimulus, by adapting observers to interleaved variations along both the L versus M and the S versus LM cardinal axes. The perceived hue of test stimuli was then measured with an asymmetric matching task. Frequency analysis of the hue shifts revealed weak but systematic hue rotations away from each cardinal axis and toward the diagonal intermediate axes. Such shifts could arise if the adapted channels include mechanisms with narrow chromatic selectivity, as some physiological recordings suggest, but could also reflect how adaptation alters the contrast response function. In either case they imply the presence of more than two mechanisms within the chromatic plane. In a second set of measurements, we adapted to either the L versus M or the S versus LM axis alone and tested whether the changes in hue could be accounted for by changes in relative contrast along the two axes. For high contrasts the hue biases are larger than the contrast changes predict. This dissociation implies that the contrast and hue changes are not carried by a common underlying signal, and could arise if the contrast along a single color direction is encoded by more than one mechanism with different contrast sensitivities or if different subsets of channels encode contrast and hue. Such variations in contrast sensitivity are also consistent with physiological recordings of cortical neurons.

Adaptation, Ocular↗
PubMedDate not supplied

Chromatic and luminance interactions in spatial contrast signals.

We report VEP studies which delineate interactions between chromatic and luminance contrast signals. We examined responses to sinusoidal luminance gratings undergoing 4-Hz square-wave contrast reversal, upon which standing gratings with various admixtures of luminance and chromatic contrast were alternately superimposed and withdrawn. The presence of the standing grating induced a VEP component at the fundamental frequency of the contrast-reversal grating. This VEP component appeared without any appreciable lag, and did not vary in amplitude over the 4 s during which the standing grating was present. The observed fundamental response differed from the fundamental component that would be expected from the known interaction between the luminance component of the standing grating with the modulated grating (Bodis-Wollner et al., 1972; Bobak et al., 1988), in three ways: (1) The fundamental response was not nulled for standing gratings that were isoluminant or near-isoluminant. (2) The chromatic dependence of the fundamental response implied an S-cone input to the interaction. (3) No single mechanism (driven by a linear combination of cone signals) could account quantitatively for the size of this response, particularly when the standing grating strongly modulated two cones in phase.

Adult↗
PubMed2005

Chromate nanorods/nanobelts: general synthesis, characterization, and properties.

A general synthesis route to a family of single-crystal chromate nanorods/nanobelts has been established. The effects of pH and surfactant on phase and morphology of these microcrystalline materials have been investigated. The physical properties of the as-synthesized chromate nanocrystals such as dielectric, electrochemical, UV-vis absorbance, and photoluminescent properties have also been studied. The present general synthesis of various low-dimensional chromate nanomaterials provides useful information on the possible synthesis of other microcrystalline transition metal oxysalts.

Journal Article↗
PubMed2002

Performance of a simple UV LED light source in the capillary electrophoresis of inorganic anions with indirect detection using a chromate background electrolyte.

Light emitting diodes (LEDs) are known to be excellent light sources for detectors in liquid chromatography and capillary electromigration separation techniques, but to date only LEDs emitting in the visible range have been used. In this work, a UV LED was investigated as a simple alternative light source to standard mercury or deuterium lamps for use in indirect photometric detection of inorganic anions using capillary electrophoresis with a chromate background electrolyte (BGE). The UV LED used had an emission maximum at 379.5 nm, a wavelength at which chromate absorbs strongly and exhibits a 47% higher molar absorptivity than at 254 nm when using a standard mercury light source. The noise, sensitivity and linearity of the LED detector were evaluated and all exhibited superior performance to the mercury light source (up to 70% decrease in noise, up to 26.2% increase in sensitivity, and over 100% increase in linear range). Using the LED detector with a simple chromate-diethanolamine background electrolyte, limits of detection for the common inorganic anions, Cl-, NO3-, SO4(2-), F- and PO4(3-) ranged from 3 to 14 microg L(-1), using electrostatic injection at -5 kV for 5 s.

Journal Article↗