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PubMed2003

Chromatic priming in hemianopic visual fields.

In three monkeys made hemianopic by unilateral striate cortical ablation, in one normal monkey and in a human hemianope (GY), we measured reaction times to chromatic targets presented in the normal hemifield as a function of prior chromatic primes in the blind field. The first of our three tasks showed an unspecific priming effect in that the colour of the here unpredictive prime was irrelevant. However, when contingencies were changed in the second task so that the prime was usually valid, its colour did significantly influence reaction times in two of the hemianopic monkeys as well as in the human subject. Even when the primes lost their predictive value again in the third task, this chromatically specific effect persisted. We conclude that chromatic processing in the cortically blind field can be revealed with indirect approaches that measure residual processing by its influence on the reaction to stimuli in the normal field, and that the validity of the prime (whether it predicts the colour of the target) is especially important.

Animals↗
PubMed2001

Detection of coloured patterns by honeybees through chromatic and achromatic cues.

We asked whether the detection range of two-coloured centre-surround patterns differs from that of single-coloured targets. Honeybees Apis mellifera were trained to distinguish between the presence and absence of a single-coloured disc or a coloured pattern at different visual angles. The patterns presented colours which were either different in chromatic and L-receptor contrasts to the background, equal in chromatic but different in L-receptor contrasts, or vice-versa. Patterns with colours presenting only chromatic contrast were also tested. Patterns with higher L-receptor contrast in its outer than in its inner element were better detected than patterns with a reversed L-contrast distribution. However, both were detected worse than single-coloured discs of the respective colours. When the L-receptor contrast was the same for both elements, the detection range of the two-coloured and single-coloured targets was the same. Patterns whose colours lacked L-receptor contrast were detected just as single-coloured targets of the same colours. These results demonstrate that both chromatic and L-receptor contrasts mediate the detection of coloured patterns and that particular distributions of L-receptor contrast within a target are better detected than others. This finding is consistent with the intervention of neurons with centre-surround receptive fields in the detection of coloured patterns.

Animals↗
PubMed1982

Effect of chromatic adaptation on the achromatic locus: the role of contrast, luminance and background color.

Two superposed annular test lights of complementary spectral composition were presented as 60-90' incremental test flashes on 480' steady backgrounds. Two observers adjusted the ratio of the two test lights to maintain an achromatic appearance under conditions of adaptation that varied with respect to background luminance, chromaticity and stimulus contrast. The shift in chromaticity of the achromatic point was in the direction of the chromaticity of the background, while the magnitude of the shift increased as an increasing function of background luminance and as a decreasing function of contrast. These data confirm and extend a model of chromatic adaptation that has the following properties: (1) non-additivity of transient test and steady background fields, in the sense that the background, although physically adding to the test flash, only affects its hue by way of altering the gain of cone pathways; (2) Vos-Walraven cone spectral sensitivities; and (3) adaptation sites in the cone pathways having the same action spectra as Stiles' pi 5, pi 4 and (modified) pi 1 mechanisms, and which generate receptor-specific attenuation factors (von Kries Coefficients) according to Stiles' generalized threshold vs intensity function, zeta (x).

Adaptation, Ocular↗
PubMed1990

Theory and measurement of ocular chromatic aberration.

We have determined the transverse chromatic aberration of the human eye by measuring the apparent offset of a two-color vernier viewed foveally through a displaced, pinhole aperture. For the same subjects, we also determined the longitudinal chromatic aberration for foveal viewing by the method of best focus. In both cases, the results were closely predicted by a simple, reduced-eye optical-model for which transverse and longitudinal chromatic aberration are directly proportional, with the constant of proportionally being the amount of displacement of the pinhole from the visual axis. Further measurements revealed that the natural pupil was closely centered on the visual axis for two subjects and slightly displaced in the temporal direction for three other subjects. One implication of these results is that, although the eye has substantial chromatic aberration, the pupil is positioned so as to minimize the transverse component of the aberration for central vision, thereby optimizing foveal image quality for polychromatic objects.

Color Perception Tests↗
PubMed1993

Does chromatic sensitivity develop more slowly than luminance sensitivity?

Chromatic sensitivity is very low in humans during the first few months of life. We examined whether low chromatic sensitivity reflects a deficiency among chromatic mechanisms or whether it is simply a manifestation of poor visual sensitivity in general. The sweep VEP was used to measure contrast sensitivity to gratings varying in the mixture of red and green components. For infants from 2 to 8 weeks of age, sensitivity to all mixtures was lower than color-normal adults' sensitivity, but infant and adult ratios of luminance/chromatic sensitivity were similar. This finding is consistent with the hypothesis that infants have functional MWS and LWS cones and the requisite post-receptor chromatic mechanisms to compare their signals.

Age Factors↗
PubMed1994

The time-course of chromatic facilitation by luminance contours.

The threshold for detecting an equiluminant chromatic spot is approximately halved by the presentation of a coincident, suprathreshold luminance pedestal flash. The dynamics of this facilitation were studied by varying the duration and temporal asynchrony of the chromatic test flash and luminance pedestal. Facilitation occurs in a narrow temporal window near the chromatic test presentation; masking may occur when the pedestal is temporally displaced from the test by longer times. The mechanism producing facilitation lags behind the chromatic signal by at least 20 msec.

Color Perception↗
PubMed1995

Cortical simple cells can extract achromatic information from the multiplexed chromatic and achromatic signals in the parvocellular pathway.

P cells, which carry both achromatic and chromatic information, are largely responsible for achromatic acuity and contrast sensitivity. The P cell achromatic information must be separated from the chromatic information to be useful. Cortical simple cells are well suited to the extraction of achromatic information by spatial bandpass filtering. Bandpass filtering of Type I P cells by cortical simple cells yields an achromatic signal with a residual chromatic response. The bandpass model makes predictions in accord with existing physiological data and explains the role of a heretofore puzzling class of cortical cells, which have bandpass tuning for both achromatic and chromatic modulations. The model is shown to be related to a previously postulated class of ideal detectors. Finally, the model is used to make a number of physiological and psychophysical predictions.

Color Perception↗
PubMed1996

Color contrast under controlled chromatic adaptation reveals opponent rectification.

Color contrast was assessed in the equiluminant plane using asymmetric matching. Test and surround stimuli lay on cardinal axes of a cone opponent chromaticity space, (l-lw, s-sw). Matches were made as a function of both test and surround chromaticity. Some matches showed constant maximal induction consistent with retinal adaptation to the surround; others showed constant minimal induction. These matches were separated by a hiatus in which color appearance did not vary greatly with test chromaticity. The results suggest that rectified retinal spectral opponent pathways do not form a unitary chromatic opponent pathway but are subject to pathway-specific interactions.

Adaptation, Ocular↗
PubMed1996

Specificity and selectivity of chromatic visual evoked potentials.

A paper by Rabin et al. (1994) Vision Research, 34, 2657-2671, claimed that spatially extensive grating stimuli could be used to generate chromatic-specific visual evoked potentials from subjects assumed to have standard spectral sensitivity and tritanopic confusion lines. Here we demonstrate that such spatially extensive stimuli may generate responses which are contaminated by luminance-contrast intrusions. Such intrusions are mainly due to chromatic aberrations and are compounded by the abovementioned assumptions. Claims regarding the chromatic selectivity of VEPs must, therefore, be substantiated by establishing correlations with the known properties of the chromatic system.

Color Perception↗
PubMed1994

Visual electrophysiology to achromatic and chromatic stimuli in premature and full term infants.

We have previously reported on the development of black/white pattern reversal VEPs in premature babies of more than 30 weeks post-menstrual age (PMA). Unlike the flash VEP, the pattern reversal VEP shows a similar morphology to that of the full term infant and the major positive component (P1) decreased in latency with increasing PMA. The N1 and N2 components were more likely to be present with increasing maturity. In our present study we are examining the development of the transient chromatic pattern VEP. In order to produce a purely chromatic stimulus it is necessary to remove luminance cues. Based on forced choice preferential looking we developed a method of determining the isoluminant point for infants. Preference was tested for a flickering sinusoidal red and green grating over the uniform field. As sensitivity for chromatic flicker is much poorer than for luminance flicker, sensitivity is expected to be least when the residual luminance variation in the stimulus is at a minimum. The red/green luminance ratio at which this occurs represents the isoluminant point. From this method we found the subjective isoluminant point for infants of 2-3 months of age to be very close to the objective measure of isoluminance. Using this information, pattern reversal VEPs to 20 chromatic red/green and achromatic checks were studied and it would appear that pattern reversal VEPs cannot be obtained to isoluminant stimuli before 7 weeks chronological age.

Aging↗
PubMed2005

Differential aging of chromatic and achromatic visual pathways: behavior and electrophysiology.

Aging of visual pathways was measured psychophysically and physiologically in subjects aged 20-89 years. Contrast thresholds for the chromatic pathways increased with age, but there were no significant changes for thresholds of the achromatic pathway at low spatial frequencies. For visual evoked potential (VEP) responses, again only the chromatic pathways were significantly affected by age. Age-related changes in chromatic responses were not apparent when stimuli were adjusted by age-related contrast thresholds. This suggests that the chromatic and achromatic visual pathways age differently and that the VEP accurately and objectively reflects behavioral changes with age.

Adult↗
PubMed2005

The detection of motion in chromatic stimuli: first-order and second-order spatial structure.

This study provides evidence for the existence of a low-level chromatic motion mechanism and further elucidates the conditions under which its operation becomes measurable in an experimental stimulus. Observers discriminated the direction of motion of amplitude modulated (AM) gratings that were defined by luminance or chromatic variation and masked with spatiotemporally broadband luminance or chromatic noise. The size and retinal location of the stimuli were varied and the effects of broadband noise and grating masks were both compared with the cohort of stimuli. Some significant disparities in the published literature were well explained by the results. In conclusion, evidence for a chromatically sensitive motion mechanism that evades the, detrimental effects of a luminance mask was found only at the fovea and only when the stimulus was small and centrally placed.

Color Perception↗
PubMed2001

Set-size and chromatic uncertainty in an accuracy visual search task.

Thresholds for chromatic differences were measured in a simple visual search task in which the target differed from the distractors in chromaticity only. In Experiment 1, the spatial separation between stimulus elements was varied. Slopes of threshold versus set-size (2-16) for elements in close proximity were somewhat elevated, suggesting non-independence of the stimulus elements. In Experiment 2, chromatic uncertainty was introduced to increase the attentional load beyond that accomplished with the set-size manipulation. The results were accounted for by a model assuming no limit in attention capacity. Furthermore, chromatic uncertainty was successfully modeled as a simple increase in the number of monitored signals.

Adult↗
PubMed2002

Effects of longitudinal chromatic aberration on accommodation and emmetropization.

PURPOSE: Less accommodation was found when human subjects read in blue (peak at about 440 nm) than when they read in red light (above 600 nm; [Kroger & Binder, British Journal of Ophthalmology 84 (2000) 890]). On the other hand, emmetropization in chickens did not appear to compensate for the chromatic defocus (385 nm versus 665 nm; [Rohrer, Schaeffel & Zrenner, Journal of Physiology 449 (1992) 363]). The apparently contradictory result was studied in more detail in humans and chickens. METHODS: Accommodation was measured with an eccentric infrared photorefractor, the PowerRefractor, in human subjects reading under quasi-monochromatic illumination conditions. Chickens were refracted in quasi-monochromatic ambient illumination but with no particular fixation target. In a second experiment, they were also raised in monochromatic light for two days and subsequently refracted both in complete darkness, in monochromatic light, and in white light, both without and with cycloplegia. RESULTS: Consistent with the initial report by Kroger and Binder [British Journal of Ophthalmology 84 (2000) 890], accommodation in human subjects was found to shift in accordance with the chromatic aberration function. An immediate shift in accommodation tonus was also found in the chickens when they were refracted under red and in blue ambient illumination (average difference between refractions in both conditions: 1.26+/-0.54 D, p<0.001 paired t-test). This value is close to the chromatic focus difference between the two wavelengths (1.5 D [Mandelman & Sivak, Vision Research 23 (1983) 1555]). When chickens were raised in blue or red light for two days, and their refractions were subsequently measured in complete darkness, they showed also a difference in refractions (1.41+/-1.00 D; ANOVA: p<0.0012, post hoc t-test: at least p<0.05 among different groups). This difference was no longer significant when they were refracted in white light but became again significant when they were cyclopleged (0.57+/-0.58 D, p=0.039, unpaired t-test). The latter observation makes it unlikely that the difference resulted just from a shift in the resting tonus of accommodation. CONCLUSIONS: (1) Imposed chromatic defocus produces a shift in accommodation tonus in both humans and chickens which is, in the case of the chicken, followed by a shift in cycloplegic refractive state into the same direction, (2) the difference to the previous study by Rohrer, Schaeffel and Zrenner [Journal of Physiology 449 (1992) 363] can be explained from the fact that shorter wavelengths were used than in the present study, at which emmetropization was no longer functional and, (3) the small amplitude and the variability of the shifts in refraction do not allow clear statements about the role of the "lag of accommodation" in refractive development but they show that several cone types contribute to emmetropization.

Accommodation, Ocular↗
PubMed1997

On the independence of chromatic and achromatic stereopsis mechanisms.

The extent to which the processing of stereoscopic depth information can take place separately in colour-contrast-sensitive and luminance-contrast-sensitive mechanisms has been investigated. Contrast thresholds for stereoscopic depth identification (front/back) were measured using 0.5 c/deg Gabor patches. The stimuli possessed different amounts of colour and luminance contrast ranging from isoluminance (red/green) to isochrominance (yellow/black) through intermediate values. Two models for combining chromatic and achromatic stereopsis information were tested. The first (single-pathway) model assumed colour and luminance contrast summation within a single luminance-contrast-sensitive mechanism before stereoscopic judgement. The second (dual-pathway) model assumed probability summation between independent chromatic and achromatic stereopsis mechanisms. The latter model provided the better fit to the data. In providing evidence in favour of an independent chromatic stereopsis mechanism, it was shown that luminance artifacts were unlikely to be the cause of maintained stereopsis at isoluminance. The possible neural substrates of chromatic stereopsis are discussed.

Color Perception↗
PubMed1997

Isoluminance and chromatic motion perception throughout the visual field.

Isoluminance and chromatic motion perception for red/green gratings were measured throughout an 80 deg visual field. Generally, the red/green isoluminance values changed with increasing eccentricity, i.e., observers increased the red luminance contrast for a fixed green luminance contrast. Enlarging the target size (to compensate for the cone density changes with eccentricity) and decreasing the spatial frequency (to compensate for receptive field property changes with eccentricity) did not change the isoluminance values within the central 20 deg, but the isoluminance ratios decreased beyond 20 deg. Our manipulations did not entirely compensate for a given eccentricity, which implies the need for a post-receptoral scaling function for the perception of drifting chromatic stimuli. Further, the results for isoluminance show heterogeneity between the visual field meridians where the red to green luminance ratio tends to be greater in the superior visual field. In our present conditions, chromatic motion was always perceived (up to 40 deg of eccentricity), but sensitivity generally decreased with increasing eccentricity. The inferior visual field was found to be the most sensitive to chromatic motion. We propose that the lower visual field and not the superior visual field is specialized for colour motion information.

Color Perception↗
PubMed1998

Reaction time to motion onset of luminance and chromatic gratings is determined by perceived speed.

We measured reaction times for detecting motion onset for sinusoidal gratings whose contrast was modulated in either luminance or chromaticity, for various drift rates and contrasts. In general, reaction times to chromatic gratings were slower than to luminance gratings of matched cone contrast, but the difference in response depended critically on both contrast and speed. At high image speeds there was virtually no difference, whereas at low speeds, the difference was pronounced, especially at low contrasts. At high image speeds there was little dependence of reaction times on contrast (for either luminance or colour), whereas at low speeds the dependence was greater, particularly for chromatic stimuli. This pattern of results is reminiscent of those found for apparent speed of drifting luminance and chromatic gratings. We verified the effects of contrast on perceived speed, and went on to show that the effects of contrast on reaction times are totally predictable by the perceived speed of the stimuli, as if it were perceived rather than physical speed that determined reaction times. Our results support that idea of separate systems for fast and slow motion (with separate channels for luminance and colour at slower speeds), and further suggest that apparent speed and reaction times may be determined at a similar stage of motion analysis.

Adult↗
PubMed1999

Effect of the tuberculostaticum ethambutol and stimulus intensity on chromatic discrimination in man.

In goldfish it has been shown that ethambutol shifts the threshold for wavelength discrimination without affecting the absolute sensitivity of the cones. In this study we demonstrate that a similar colour vision disturbance occurs in tuberculosis patients treated with ethambutol. After 2 months of ethambutol treatment, chromatic discrimination was measured with a computerized forced two choice (CD) test with isoluminant coloured stimuli and with three other colour vision tests: the Ishihara, the Oscar and the Lanthony Desaturated 15 Hue tests. The scores of the patient group (n = 19) on these four colour vision tests were compared with the scores of a group of control subjects (n = 33) and a group of congenital red/green colour-blind subjects (n = 5). A reduction of the stimulus intensity of 1 log unit caused a significant reduction in red/green chromatic discrimination, measured with the CD test in both, control subjects and patients. This intensity dependent reduction was significantly greater for patients than for controls. In this respect, man and goldfish behave similarly. Furthermore, the CD test showed the same ethambutol-induced reduction in chromatic discrimination at low intensity for the blue/green part of the spectrum. This has not been measured in goldfish. The origin of this ethambutol-induced colour vision disturbance must be at a post-photoreceptor site, because the Ishihara and Oscar tests, both designed to screen for photoreceptor-based, or primary red/green colour vision disturbances, did not discriminate between patients and control subjects. Thus, as in goldfish, we find that in patients ethambutol shifts the threshold for chromatic discrimination without changing the absolute sensitivity.

Adolescent↗