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At least 379 records · Page 21Linked to original sources
PubMed2000

Perception and the conditioning reflex: vector encoding.

Color perception is dependent on the generation of an excitation vector which, acting on a pool of color detectors (color detector map), produces a corresponding sensation. The generation of the color excitation vector starts at the retinal level, proceeds in the lateral geniculate body, and reaches color detectors at the cortical level. Following processing at the level of declarative memory and semantic maps, results in a verbal categorization of colors. Parallel to the excitation vector pathway, a network computing color differences is operating. The computation of color differences at the retinal level possibly takes place in phasic bipolar cells and progresses in the lateral geniculate body and at the cortical level. Detectors of color differences are assumed to be a basis of respective numerical estimations in humans. Data from frogs, fish, monkeys and humans are compared.

Animals↗
PubMed2006

Edge integration and the perception of brightness and darkness.

How do induced brightness and darkness signals from local and remote surfaces interact to determine the final achromatic color percept of a target surface? An emerging theory of achromatic color perception posits that brightness and darkness percepts are computed by weighting and summing the induction signals generated at edges in a scene. This theory also characterizes how neighboring edges interact to modulate the gain of brightness and darkness signals induced from one another. Here we assess evidence for this edge integration theory by means of computational modeling and a psychophysical experiment. We quantitatively show how local and remote edge induction signals in disk-ring displays give rise to either contrast or assimilation effects. Spatial integration of same-polarity edge signals supports a contrast effect, whereas integration of opposite-polarity signals supports an assimilation effect, particularly when the remote induction signal is much stronger than the local induction signal. The results confirm a key prediction of edge integration theory, namely, that strong assimilation effects can lead subjects to ignore the polarity of local edge information when setting achromatic color matches. The conditions necessary for strong assimilation effects are also associated with greater difficulty in setting matches, suggesting that caution is required when interpreting matching data in terms of gain control. We describe several avenues for further study of contrast, assimilation, and gain control.

Adult↗
PubMed2002

Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words.

In 'colored-hearing' synesthesia, individuals report color experiences when they hear spoken words. If the synesthetic color experience resembles that of normal color perception, one would predict activation of parts of the visual system specialized for such perception, namely the human 'color center', referred to as either V4 or V8. Using functional magnetic resonance imaging (fMRI), we here locate the region activated by speech in synesthetes to area V4/V8 in the left hemisphere, and demonstrate overlap with V4/V8 activation in normal controls in response to color. No activity was detected in areas V1 or V2, suggesting that activity in primary visual cortex is not necessary for such experience. Control subjects showed no activity in V4/V8 when imagining colors in response to spoken words, despite overtraining on word-color associations similar to those spontaneously reported by synesthetes.

Adult↗
PubMedDate not supplied

Chromatic detection and discrimination in the periphery: a postreceptoral loss of color sensitivity.

The peripheral visual field is marked by a deterioration in color sensitivity, sometimes attributed to the random wiring of midget bipolar cells to cone photoreceptors in the peripheral retina (Mullen, 1991; Mullen & Kingdom, 1996). Using psychophysical methods, we explored differences in the sensitivity of peripheral color mechanisms with detection and discrimination of 2-deg spots at 18-deg eccentricity, and find evidence for a postreceptoral locus for the observed loss in sensitivity. As shown before, observers' sensitivity to green was lower than to red in the periphery, although the magnitude of this effect differed across observers. These results suggest that the asymmetry in peripheral sensitivity occurs at a postreceptoral site, possibly a cortical one. In addition, noise masking was used to determine the cone inputs to the peripheral color mechanisms. The masked detection contours indicate that the red and green mechanisms in the periphery respond to the linear difference of approximately equally weighted L- and M-cone contrasts, just as they do in the fovea. Thus, if the midget retinal ganglion system is responsible for red/green color perception in the fovea, it is likely to be responsible at 18-deg eccentricity as well.

Choice Behavior↗
PubMed1972

Motion perception: a color-contingent aftereffect.

When observers who watched repeated alternations of a red contracting spiral and green expanding spiral were later shown stationary spirals, red and a green the red stationary spiral appeared to be expanding and the green stationary spiral appeared to be contracting. These color-contingent motion after effects complement reports of motion-contingent color aftereffects and suggest that both may reflect adaptation of detectors specific to color and motion.

Adolescent↗
PubMed2004

Cone inputs in macaque primary visual cortex.

To understand the role of primary visual cortex (V1) in color vision, we measured directly the input from the 3 cone types in macaque V1 neurons. Cells were classified as luminance-preferring, color-luminance, or color-preferring from the ratio of the peak amplitudes of spatial frequency responses to red/green equiluminant and to black/white (luminance) grating patterns, respectively. In this study we used L-, M-, and S-cone-isolating gratings to measure spatial frequency response functions for each cone type separately. From peak responses to cone-isolating stimuli we estimated relative cone weights and whether cone inputs were the same or opposite sign. For most V1 cells the relative S-cone weight was <0.1. All color-preferring cells were cone opponent and their L/M cone weight ratio was clustered around a value of -1, which is roughly equal and opposite L and M cone signals. Almost all cells (88%) classified as luminance cells were cone nonopponent, with a broad distribution of cone weights. Most cells (73%) classified as color-luminance cells were cone opponent. This result supports our conclusion that V1 color-luminance cells are double-opponent. Such neurons are more sensitive to color boundaries than to areas of color and thereby could play an important role in color perception. The color-luminance population had a broad distribution of L/M cone weight ratios, implying a broad distribution of preferred colors for the double-opponent cells.

Action Potentials↗
PubMed2001

Perception of forbidden colors in retinally stabilized equiluminant images: an indication of softwired cortical color opponency?

In color theory and perceptual practice, two color naming combinations are forbidden-reddish greens and bluish yellows-however, when multicolored images are stabilized on the retina, their borders fade and filling-in mechanisms can create forbidden colors. The sole report of such events found that only some observers saw forbidden colors, while others saw illusory multicolored patterns. We found that when colors were equiluminant, subjects saw reddish greens, bluish yellows, or a multistable spatial color exchange (an entirely novel perceptual phenomena); when the colors were nonequiluminant, subjects saw spurious pattern formation. To make sense of color opponency violations, we created a soft-wired model of cortical color opponency (based on winner-take-all competition) whose opponency can be disabled.

Color Perception↗
PubMed2006

Influence of random-dot textures on perception of suprathreshold color differences.

We have analyzed the way in which simulated textures made of random dots influence visual suprathreshold color tolerances. We considered 32 textures created by a systematic variation of the following variables: size, number, and color of the dots. Each texture was mapped on the five centers recommended by the International Commission on Illumination (CIE) in 1978 [Color Res. Appl.3, 149 (1978)]. A panel of five experienced observers determined the experimental tolerances using a CRT color monitor by the method of adjustment. At our observation distance, neither small differences in dot size (1 or 4 pixels) nor sparse number of dots (less than 20% of the surface of the sample) changed the tolerances found for homogeneous samples. For the textures that led to statistically significant differences with respect to homogeneous samples, the parametric factors of CIEDE2000 and CIE94 color-difference formulas were fitted. These simulated textures consistently reduced the color difference perceived in a pair or, equivalently, increased the tolerances, mainly (but not only) lightness tolerance. The results demonstrate that it is not simple to provide a unique set of parametric factors for all the potential textures.

Color↗
PubMedDate not supplied

Color vision in patients with the Hermansky-Pudlak syndrome.

PURPOSE: To study color vision in patients with oculocutaneous albinism (OCA) METHODS: We evaluated color vision in 42 patients with OCA using the HRR color plates. Sixty seven percent of the patients had the Hermansky-Pudlak syndrome (HPS), diagnosed genetically or clinically. The remaining patients had unknown mutations leading to OCA. RESULTS: 47.6 % of patients of OCA of all types included had a color vision defect. Of these, 55% were female and 45% were male patients. 50% of patients with the HPS (all types) had a color vision deficit. 42.9% of patients with OCA of unknown type had color weakness. 57.1% had normal color vision. CONCLUSIONS: Results suggest that many patients with OCA and the HPS have a mild red-green color perception deficiency that is not a sex linked trait. The prevalence of color vision deficits in our study population increased with decreasing visual acuity.

Adolescent↗
PubMedDate not supplied

[Trichromatic model of the constant perception of the color of objects].

A description is presented of a trichromatic model of colour constancy (MCC) of objects with concavities: folds, holes and so on. MCC involves the system of algorithmically independent processing blocks of the image heterarchically interacting and creating at its outlet a triparametric estimation of surface reflectance of each object of the picture. This estimation little depends on changes in the illumination colour. Results are presented and discussed of a computer experiment with MCC simulating the scene in which a plane with differently coloured regions and a conic hole in each are illuminated with a single chromatic light source.

Algorithms↗
PubMed1980

[Follow-up study of macular function after retinal detachment surgery (author's transl)].

Forty patients successfully operated on for retinal detachment involving the macula were followed up for periods ranging from six months to nine years. The follow-up examination included testing for visual acuity, color perception and metamorphopsias, as well as biomicroscopy and fluorescein angiography. Postoperative visual acuity was correlated with the preoperative duration of retinal detachment. As expected, visual outcome was reduced in cases with lesions demonstrated by biomicroscopy and/or fluorescein angiography. In some patients visual acuity and color perception were impaired and metamorphopsias were present, even though the macula appeared normal in funduscopy and fluorescein angiography. This may be due to a possible misalignement of the photoreceptors as demonstrated in the owl monkey by Machemer.

Adult↗