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PubMed2000

What covariance mechanisms underlie green/red equiluminance, luminance contrast sensitivity and chromatic (green/red) contrast sensitivity?

In order to investigate the mechanisms underlying green/red equiluminance matches in human observers and their relationship to mechanisms subserving luminance and/or chromatic (green/red) contrast sensitivity, we tested 21 human subjects along these dimensions at 16 different spatial and temporal frequencies (spatial frequency, 0.25-2 c/deg; temporal frequency, 2-16 Hz) and applied factor analysis to extract mechanisms underlying the data set. The results from our factor analysis revealed separate sources of variability for green/red equiluminance, luminance sensitivity and chromatic sensitivity, thus suggesting separate mechanisms underlying each of the three main conditions. When factor analysis was applied separately to green/red equiluminance data, two temporally-tuned factors were revealed (factor 1, 2-4 Hz; factor 2, 8-16 Hz), suggesting the existence of separate mechanisms underlying equiluminance settings at low versus high temporal frequencies. In addition, although the three main conditions remained separate in our factor analysis of the entire data set, our correlation matrix nonetheless revealed systematic correlations between equiluminance settings and luminance sensitivity at high temporal frequencies, and between equiluminance settings and chromatic sensitivity at low temporal frequencies. Taken together, these data suggest that the high temporal frequency factor underlying green/red equiluminance is governed predominantly by luminance mechanisms, while the low temporal frequency factor receives contribution from chromatic mechanisms.

Color Perception↗
PubMed2000

Chromatic and luminance contributions to a hyperacuity task.

Displacement thresholds with incremental chromatic and luminance edges were measured on different backgrounds. Above 3% luminance contrast, thresholds were always similar. At luminance contrasts below 3%, luminance edges could not be detected, but chromatic edges were still visible. At these low contrasts displacement thresholds for chromatic edges increased to a high level. We interpret these data in terms of multiple mechanisms; above 3% contrast a luminance mechanism determines thresholds, but when, at lower contrasts, chromatic mechanisms support detection, they also support the spatial task. Physiological data were consistent with the different mechanisms originating at the retinal ganglion cell level.

Animals↗
PubMed1999

Normative data for onset VEPs to red-green and blue-yellow chromatic contrast.

OBJECTIVE: To better characterize the properties of chromatic VEPs to onset-offset of red-green and blue-yellow equiluminant patterns, and establish normative values for a set of stimuli able to elicit robust and reliable responses, suitable for the clinical application. METHODS: Chromatic VEPs have been recorded (Oz lead) from 28 normal subjects (age range 20-53 years) in response to monocular presentation of both red-green and blue-yellow equiluminant sinusoidal gratings. Stimuli were generated by a Cambridge VSG/2 card and displayed on a Barco CCID monitor (14x14 deg field size). Spatial frequency, chromaticity, contrast and onset-offset duration were varied. RESULTS: For both red-green and blue-yellow equiluminant stimuli, robust responses have been obtained with gratings of 2 c/deg, presented in onset (300 ms) offset (700 ms) mode, at contrasts ranging from 90 to 6%. In all observers, the VEP waveform consisted mainly of a negative wave at stimulus onset, with a latency rapidly increasing with decreasing contrast. For both red-green and blue-yellow stimuli, the VEP contrast threshold coincided with the psychophysical threshold. CONCLUSIONS: The results complement previous studies aimed at characterizing the properties of chromatic VEPs. In addition, normative data are provided for a set of stimulus characteristics suitable for the clinical routine.

Adult↗
PubMed1990

A neural and computational model for the chromatic control of accommodation.

Accommodation is more accurate with polychromatic stimuli than with narrowband or monochromatic stimuli. The aim of this paper is to develop a computational model for how the visual system uses the extra information in polychromatic stimuli to increase the accuracy of accommodation responses. The proposed model is developed within the context of both trichromacy and also the organization of spatial and chromatic processing within the visual cortex. The refractive error present in the retinal image can be estimated by comparing image quality with and without small additional changes in refractive state. In polychromatic light, the chromatic aberration of the eye results in differences in ocular refractive power for light of different wavelengths. As a result, the refractive state of the eye can be estimated by comparing image quality in the three types of cone photoreceptor. The ability of cortical neurons to perform such comparisons on image quality with a crude form of spatial-frequency analysis is examined theoretically. It is found that spatially band-pass chromatically opponent neurons (that may correspond to double opponent neurons) can perform such calculations and that chromatic cues to accommodation are extracted most effectively by neurons responding to spatial frequencies of between 2 and 8 cycles/deg.

Accommodation, Ocular↗
PubMedDate not supplied

Amplitude and phase variations of harmonic components in human achromatic and chromatic visual evoked potentials.

Occipital visual evoked potentials (VEPs) were recorded in response to low-contrast, low spatial-frequency chromatic, and achromatic gratings. Fast Fourier Transform (FFT) and time-domain analysis were used to reveal differences in harmonic content and amplitude of chromatic and achromatic response components over a wide range of temporal frequencies. The chromatic ON/OFF VEP is dominated by the fundamental component indicating that onset and offset responses are different. This type of response is typical of neurons with sustained type response characteristics. Conversely, the achromatic onset VEP contains a predominant second harmonic component in addition to the fundamental. This similarity between onset and offset responses suggests that transient mechanisms are responsible for the generation of achromatic components. Frequency analysis of VEPs elicited by phase-reversing stimuli reveals that all of the response energy is concentrated at the second harmonic of the stimulating frequency. The magnitude of the second harmonic component is maximal for achromatic stimuli and undergoes a distinct minimum for isoluminant, chromatic stimuli. This behavior indicates that under the stimulus conditions used, magnocellular neurons with transient characteristics dominate the reversal VEP.

Adult↗
PubMedDate not supplied

Chromatic properties of neurons in macaque area V2.

We recorded from single cells in area V2 of cynomolgus monkeys using standard acute recording techniques. After measuring each cell's spatial and temporal properties, we performed several tests of its chromatic properties using sine-wave gratings modulated around a mean gray background. Most cells behaved like neurons in area V1 and their responses were adequately described by a model that assumes a linear combination of cone signals. Unlike in V1, we found a subpopulation of cells whose activity was increased or inhibited by stimuli within a narrow range of color combinations. No particular color directions were preferentially represented. V2 cells showing color specificity, including cells showing narrow chromatic tuning, were present in any of the stripe compartments, as defined by cytochrome-oxidase (CO) staining. An addition of chromatic contrast facilitated the responses of most neurons to gratings with various luminance contrasts. Neurons in all three CO compartments gave significant responses to isoluminant gratings. Receptive-field properties of cells were generally similar for luminance and chromatically defined stimuli. We found only a small number of cells with a clearly identifiable double-opponent receptive-field organization.

Animals↗
PubMedDate not supplied

The chromatic input to global motion perception.

For over 30 years there has been a controversy over whether color-defined motion can be perceived by the human visual system. Some results suggest that there is no chromatic motion mechanism at all, whereas others do find evidence for a purely chromatic motion mechanism. Here we examine the chromatic input to global motion processing for a range of color directions in the photopic luminance range. We measure contrast thresholds for global motion identification and simple detection using sparse random-dot kinematograms. The results show a discrepancy between the two chromatic axes: whereas it is possible for observers to perform the global motion task for stimuli modulated along the red-green axis, we could not assess the contrast threshold required for stimuli modulated along the yellowish-violet axis. The contrast required for detection for both axes, however, are well below the contrasts required for global motion identification. We conclude that there is a significant red-green input to global motion processing providing further evidence for the involvement of the parvocellular pathway. The lack of S-cone input to global motion processing suggests that the koniocellular pathway mediates the detection but not the processing of complex motion for our parameter range.

Artifacts↗
PubMedDate not supplied

Infant color vision: influence of surround chromaticity on spontaneous looking preferences.

When infants are tested with stimuli of various chromaticities embedded in a dark or achromatic (white) surround, they show maximal preference for stimuli of maximal colorimetric purity, and minimal preference for achromatic stimuli. We investigated how this pattern of preferences changes with changes of surround chromaticity. Sixteen-week-old infants were tested in two experimental conditions. The surrounds in the first condition were red and white; and in the second condition green and white. The three test stimuli varied in colorimetric purity from white to red in the first condition, and from white to green in the second condition. A test stimulus that appeared achromatic to adults when viewed in the chromatic surround was included. Infant spontaneous looking preferences changed with changes of surround chromaticity. The changes were consistent with the conclusion that infant looking behavior is governed by a preference for the stimuli that differ maximally in purity from the surround. The implications of this pattern of results are discussed.

Adult↗
PubMedDate not supplied

A single mechanism for both luminance and chromatic grating vernier tasks: evidence from temporal summation.

Vernier thresholds are determined by luminance rather than chromatic contrast when both are present in vernier targets. The role of luminance and chromatic mechanisms in vernier performance under equiluminant conditions remains uncertain. Temporal summation functions for vernier thresholds with luminance and red-green equiluminant gratings were compared to those for detection thresholds with similar stimuli. Vernier thresholds showed similar temporal summation for luminance and chromatic gratings, which is consistent with a single mechanism underlying vernier performance in the two conditions. However, detection thresholds showed a shorter temporal summation duration for luminance gratings than for chromatic gratings, which suggests that two different mechanisms underlie detection thresholds. Analysis of physiological data supports the hypothesis that the frequency-doubled response of ganglion cells in the magnocellular pathway can provide accurate spatiotemporal information for vernier performance at equiluminance.

Animals↗
PubMedDate not supplied

Chromatic VEP assessment of human macular pigment: comparison with minimum motion and minimum flicker profiles.

To assess the effects of macular pigment optical density (MPOD) on isoluminant stimuli and to quantify MPOD electrophysiologically, MPOD distribution profiles were obtained in normal subjects using minimum motion and minimum flicker photometry. Isoluminance of VEP stimuli was determined using minimum flicker and tritan confusion lines were determined using a minimum distinct border criterion. Onset-offset and reversal VEPs to isoluminant red/green, blue/green, and subject-specific tritan gratings of different diameters were recorded from the same 14 subjects tested psychophysically. VEPs were additionally recorded to annular gratings. Chromatic VEP selectivity was assessed by Fourier analysis and as an index; onset negativity/(onset negativity + onset positivity). Peak MPOD varied between 0.2-0.8. Chromatic onset VEPs to all isoluminant 3-deg fields were predominantly negative. Larger blue/green and tritan stimuli elicited VEPs with additional positive, achromatic components; for 9-deg gratings, peak MPOD showed negative correlation with the power of the VEP fundamental (r = -0.70) and with the selectivity index (r = -0.83). Annular gratings elicited chromatic-specific B/G VEPs but only when isoluminance was determined for the annulus. Chromatic selectivity loss in VEPs to large B/G or Tritan gratings can be used to estimate subject-specific MPOD. An important implication is that isoluminant Tritan stimuli with short-wavelength components must be restricted in size in order to optimize koniocellular selectivity.

Color↗
PubMedDate not supplied

Inhibition or facilitation of return: Does chromatic component count?

Inhibitory effects have been reported when a target is preceded by a cue of the same color and location. Color-based inhibition was found using red and blue nonisoluminant stimuli (Law et al., 1995). Here we investigate whether this phenomenon depends on the chromatic subsystem involved by employing isoluminant colors varying along either the violet-yellow or purple-turquoise cardinal axis. Experiment 1 replicated Law et al.'s study: After fixating magenta, either a red or blue cue was presented, followed by a magenta "neutral attractor," and, finally, by a red or blue target. In Experiment 2, violet and yellow, cue or target, varied along a tritan confusion line in the CIE 1976 chromaticity diagram. In Experiment 3, purple and turquoise, cue or target, varied along a deutan confusion line in the CIE 1976 chromaticity diagram. Normal trichromats (n = 19) participated in all three experiments. In Experiment 1, color repetition indeed resulted in longer reaction times (RTs) (4.7 ms, P = 0.038). In Experiment 2, however, no significant color repetition effect was found; RTs to violet and yellow were not significantly different, though tending toward slower responses (2 ms) for violet repetition but faster (5 ms) for yellow. Experiment 3 also showed no color repetition effect (P = 0.58); notably, RTs were overall faster for purple than for turquoise (22 ms, P 0.05), but faster for turquoise (7 ms, P > 0.05). These findings demonstrate that color repetition is not always inhibitory but may turn facilitatory depending on the colors employed. The results indicate that disengagement of attention is an unlikely mechanism to be the sole explanation of previously reported color-based inhibition of return. We suggest a complementary, perceptual explanation: response (dis)advantage depends on whether the stimuli are isoluminant and on the opponent chromatic subsystem involved. The choice of the colors employed and the cue-attractor-target constellation also may be of significance.

Adult↗
PubMed1998

Temporal dynamics of chromatic tuning in macaque primary visual cortex.

The ability to distinguish colour from intensity variations is a difficult computational problem for the visual system because each of the three cone photoreceptor types absorb all wavelengths of light, although their peak sensitivities are at relatively short (S cones), medium (M cones), or long (L cones) wavelengths. The first stage in colour processing is the comparison of the outputs of different cone types by spectrally opponent neurons in the retina and upstream in the lateral geniculate nucleus. Some neurons receive opponent inputs from L and M cones, whereas others receive input from S cones opposed by combined signals from L and M cones. Here we report how the outputs of the L/M- and S-opponent geniculate cell types are combined in time at the next stage of colour processing, in the macaque primary visual cortex (V1). Some V1 neurons respond to a single chromatic region, with either a short (68-95 ms) or a longer (96-135 ms) latency, whereas others respond to two chromatic regions with a difference in latency of 20-30 ms. Across all types, short latency responses are mostly evoked by L/M-opponent inputs whereas longer latency responses are evoked mostly by S-opponent inputs. Furthermore, neurons with late S-cone inputs exhibit dynamic changes in the sharpness of their chromatic tuning over time. We propose that the sparse, S-opponent signal in the lateral geniculate nucleus is amplified in area V1, possibly through recurrent excitatory networks. This results in a delayed, sluggish cortical S-cone signal which is then integrated with L/M-opponent signals to rotate the lateral geniculate nucleus chromatic axes.

Action Potentials↗
PubMed1993

Antagonistic chromatic mechanisms in photoreceptors of the parietal eye of lizards.

Photoreceptors are the first in the chain of neurons that process visual information. In lateral eyes of vertebrates, light hyperpolarizes rod and cone photoreceptors that synapse onto bipolar and horizontal cells in the first synaptic layer of the retina. The sign of the photoreceptor signal is either conserved or inverted in bipolar cells, resulting in chromatically dependent depolarizing and hyperpolarizing responses to visual stimuli. Visual information is then conveyed to the second synaptic layer for encoding and transmission to the brain by ganglion cells. The parietal (third) eye of lizards does not contain bipolar cells or other interneurons. Photoreceptors synapse directly onto ganglion cells and yet, even in the absence of interneurons, antagonistic chromatic mechanisms modulate the ganglion cell responses. We report here that chromatic antagonism in the third eye originates in the chromatically dependent hyperpolarizing and depolarizing response of the photoreceptors to light. We also suggest that the antagonistic nature of these photoresponses may provide lizards with a mechanism for the enhanced detection of dawn and dusk.

Adaptation, Physiological↗
PubMed2005

Changes in chromatic and achromatic contrast sensitivities following tropicamide administration.

PURPOSE: To investigate the effects of tropicamide on chromatic and achromatic contrast sensitivities over the physiological range of spatial frequencies. METHODS: A total of 26 healthy volunteers, with a mean age of 32 years, were examined with and without one drop of 1% tropicamide being administered 30 min previously. On each occasion, acuity and pupil diameter were recorded, and chromatic and achromatic contrast sensitivities were examined using the Sussex Grating Machine. RESULTS: Following tropicamide administration mean pupil diameter increased from 4.1 mm to 7.2 mm (P<0.001), and mean BCVA was reduced by 0.07 LogMar units (P<0.001). Achromatic contrast sensitivity was significantly reduced following tropicamide administration at 2.20 cycles per degree (cpd) (P=0.01), 3.40 cpd (P=0.01), 10 cpd (P=0.04), 17 cpd (P=0.04), and 25 cpd (P<0.01). There was no difference in contrast sensitivity at lower spatial frequencies (0.33 and 0.66 cpd). Chromatic contrast sensitivity was not significantly altered when tested along the red-green and tritan confusion axes. CONCLUSIONS: Achromatic contrast sensitivity is significantly reduced following tropicamide administration at intermediate and high spatial frequencies. No significant changes were seen at low spatial frequencies and in chromatic contrast sensitivities.

Administration, Topical↗
PubMed1999

Both coloured overlays and coloured lenses can improve reading fluency, but their optimal chromaticities differ.

Some individuals read more fluently when the text is coloured: i.e., when coloured sheets of plastic (overlays) are placed upon the page, or when coloured lenses are worn. Overlays provide a surface colour whereas lenses mimic a change in the colour of a light source. The neural mechanisms that underlie colour constancy ensure that the chromaticity of overlays and lenses is processed differently by the visual system. We investigated (1) the relationship between the optimal colours of overlays and lenses, and (2) how reading rate is affected by a particular colour in overlays and lenses. In 100 patients we noted (1) the overlay(s) chosen from among the 29 combinations of the 10 IOO Intuitive Overlays which sample chromaticity systematically and (2) the chromaticity co-ordinates of the lenses subsequently chosen using the intuitive Colorimeter, a device providing a light source that can be adjusted in hue, saturation and luminance independently. The relationship between the chromaticities of the overlays and the lenses showed considerable variation. In a second study, patients attending the Specific Learning Difficulties clinic at the Institute of Optometry, London, were given overlays to use for two months. Seventeen who derived benefit were examined using the Intuitive Colorimeter. Patients were asked to read aloud randomly ordered common words (Wilkins Rate of Reading Test): (1) with no colour, (2) with the chosen overlay, (3) with lenses matching the chosen overlay and (4) with lenses matching the Colorimeter setting. The aids increased reading rate significantly only in conditions (2) and (4). There was no significant improvement when lenses matching the overlay colour were used, and under this condition the reading rate was significantly poorer than in conditions (2) and (4). The colour of a lens will improve reading only if it is selected under conditions that mimic a change in the colour of a light source: coloured overlays give no clinically reliable guide to optimal lens colour.

Color↗
PubMed1979

Chromatic induction effects in the Hermann grid illusion.

The chromatic Hermann grid illusion was investigated in sixteen subjects, with variation of the lightness contrast between the chromatic inducing squares and the background, and the saturation and hue of the inducing squares. Subjects made magnitude estimates of the sharpness and clarity of perceived dots at the intersections of the grid, and matched the appearances of the dots with Munsell chips. A chromatic induction effect was found to occur in the absence of lightness contrast, but the sharpness of the illusory dots increased with increasing lightness contrast (p less than 0.001). The saturation of the perceived dots increased with increases in the saturation of the inducing squares (p less than 0.05), and was higher for the longer wavelengths than for the shorter wavelengths (p less than 0.005). Neural units with center-surround arrangements responding differentially to light of the same color in the center and the surround, e.g. red off-centers and red on-surrounds, could account for the chromatic induction effect.

Color Perception↗
PubMed2006

Chromaticity, spatial complexity, and self-motion perception.

The effects of visual field color and spatial complexity on self-motion perception were investigated by placing observers inside a large rotating cylinder (optokinetic drum). Under optokinetic-drum conditions visually induced self-motion (vection) is typically perceived within 30 s, even though all forms of sensory input (eg vestibular, proprioceptive, auditory), except vision, indicate that the observer is stationary. It was hypothesized that vection would be hastened and vection magnitude increased by adding chromatic colors and spatial complexity to the lining of an optokinetic drum. Addition of these visual-field characteristics results in an array that shares more visual-field characteristics with our typical environment that usually serves as a stable frame of reference regarding self-motion perception. In the color experiment, participants viewed vertical stripes that were: (i) black and white, (ii) various gray shades, or (iii) chromatic. In the spatial complexity experiment, participants were presented with: (i) black-and-white vertical stripes, or (ii) a black-and-white checkerboard pattern. Drum rotation velocity was 5 rev. min(-1) (30 degrees s(-1)), and both vection onset and magnitude were measured for 60 s trials. Results indicate that chromaticity and spatial complexity hasten the onset of vection and increase its perceived magnitude. Chromaticity and spatial complexity are common characteristics of the environments in which our visual system evolved. The presence of these visual-field features in an optic flow pattern may be treated as an indicator that the scene being viewed is stationary and that the observer is moving.

Adult↗
PubMed2005

Perception of chromatic motion requires luminance interaction.

There is an ongoing debate related to whether chromatic motion perception arises as a consequence of a chromatic signal only (eg Wandell et al 1999 Neuron 24 901-909) or a signal that is essentially based on luminance processes (luminance artifacts) (Mullen et al 2003 Vision Research 43 1235-1247). These two views conform to the idea that colour and luminance processes are physiologically independent (Livingstone and Hubel 1988 Science 240 740-749), but according to other reports many primary cortical 'V1' cells respond to both colour and luminance contrast (eg Vidyasagar et al 2002 European Journal of Neuroscience 16 945-956). A psychophysical task was designed to test whether possible interaction between luminance and chromatic contrast could account for perception of chromatic motion. It is shown that subjects respond in a manner that reflects involvement of both processes.

Color Perception↗