Biomedical subjects

S K Shevell

Publications and source records attributed to S K Shevell.

At least 19 recordsLinked to original sources

PubMed2000

A central mechanism of chromatic contrast.

The color appearance of a light can be altered by introducing a second, surrounding field. This phenomenon, called chromatic induction, is attenuated by chromatic variation within a remote region outside the surround [Shevell & Wei (1998). Vision Research, 38, 1561-1566]. We now consider the locus of the neural mechanism mediating the attenuation caused by the remote chromatic contrast. In the first experiment, the magnitude of chromatic variation within the remote region is changed either: (i) in the same eye that views the patch judged in color; or (ii) in only the opposite eye. The measurements are virtually the same in both cases, which implies attenuation of chromatic induction is mediated by a central, binocular mechanism. In the second experiment, the patch with its immediate inducing surround is changed in binocular disparity relative to the remote region with chromatic variation. The patch and surround, seen together in one depth plane, are perceived to be in front of, behind, or in the same plane as the remote region with chromatic variation. Attenuation of chromatic induction is strongest when the patch and surround are in the same depth plane as the remote region. This change of color appearance with disparity is consistent with a central binocular process. Overall, the color-appearance measurements are explained by monocular encoding of chromatic differences at edges, and a central binocular mechanism of chromatic-contrast gain control.

Color Perception↗

PubMed2000

Role of perceptual organization in chromatic induction.

Color matches between two small patches were made in a display containing ten larger regions of different chromaticities. The spatial organization of the ten regions was varied while keeping constant the immediate surround of each patch as well as the space-average chromaticity of the entire stimulus. Different spatial arrangements were designed to alter the perceptual organization inferred by the observer without changing the ensemble of chromaticities actually in view. For example, one arrangement of the ten regions was consistent with five surfaces under two distinct illuminations, with one edge within the display (an "apparent illumination edge") dividing the stimulus into two areas, one under illuminant A and the other under illuminant C. Another spatial arrangement had the ten regions configured to induce an observer to infer ten surfaces under a single illumination. When the ten regions were arranged with an apparent illumination edge, the patch within the area of illuminant C was perceived as bluer than when the same patch and immediate surround were presented without an apparent illumination edge. The results are accounted for by positing that observers group together regions sharing the same inferred illumination, with a consequent effect on color perception: A fixed patch-within-surround shifts in hue and saturation toward the perceived illumination. We suggest that the change in color perception in a complex scene that results from a difference in real illumination may be caused by the inferred illumination at the perceptual level, not directly by the physical change in the light absorbed by photoreceptors.

Adult↗

PubMed1999

Chromatic induction with remote chromatic contrast varied in magnitude, spatial frequency, and chromaticity.

Chromatic induction from a surround is attenuated by chromatic contrast within a remote region outside of the surround (Shevell & Wei, 1998, Vision Research, 38, 1561-1566). The present study reports hue-cancellation measurements that show the attenuation depends on the magnitude, spatial frequency and chromaticity of remote chromatic contrast. Spatial-frequency tuning is shown by maximal attenuation of induction with remote contrast elements of the same size as the test. Experiments with various chromaticities of remote contrast show that S-cone stimulation within the remote region has a much weaker effect than L-/M-cone chromatic contrast, and does not depend on whether the S-cone stimulation is uniform or uneven across the region. Overall, the results show that remote L/M contrast affects classical chromatic induction, with its effect depending on the spatial frequency and magnitude of contrast. The influence of remote S-cone stimulation, on the other hand, is relatively weak and depends on only the S-cone spatial average, at least when S-cone stimulation by the test and its immediate surround is minimal (as in all experiments here).

Color Perception↗

PubMed1999

Trichromatic color vision with only two spectrally distinct photopigments.

Protanomaly is a common, X-linked abnormality of color vision. Like people with normal color vision, protanomalous observers are trichromatic, but their ability to discriminate colors in the red-green part of the spectrum is reduced because the photopigments that mediate discrimination in this range are abnormally similar. Whereas normal subjects have pigments whose wavelengths of peak sensitivity differ by about 30 nm, the peak wavelengths for protanomalous observers are thought to differ by only a few nanometers. We found, however, that although this difference occurred in some protanomalous subjects, others had pigments whose peak wavelengths were identical. Genetic and psychophysical results from the latter class indicated that limited red-green discrimination can be achieved with pigments that have the same peak wavelength sensitivity and that differ only in optical density. A single amino acid substitution was correlated with trichromacy in these subjects, suggesting that differences in pigment sequence may regulate the optical density of the cone.

Amino Acid Substitution↗

PubMed1998

Relating color discrimination to photopigment genes in deutan observers.

Deutan observers are a heterogeneous group, varying nearly continuously from deuteranomalous trichromats with fine chromatic discrimination in the red/green range to deuteranopes who have none. We sought to relate chromatic discriminative ability among deutans measured psychophysically (phenotypes) to observers' separation between long-wave visual pigments inferred from visual pigment genes (genotypes). If middle-wave pigment genes are assumed not to be expressed in these deutan observers there is a clear relation between phenotype and genotype.

Color Perception↗

PubMed1998

Chromatic induction: border contrast or adaptation to surrounding light?

Chromatic induction from a surrounding light is measured with an additional remote field outside the surround. Chromatic induction from the surround into a central test field is found to be attenuated by a remote inhomogeneous 'checkerboard', composed of squares at two different chromaticities. A uniform remote field, on the other hand, either at the average or at the most extreme chromaticity of the 'checkerboard', has a weaker effect on chromatic induction than the inhomogeneous field, implying that chromatic contrast within the remote region is a critical factor. The complete set of experiments is accounted for by chromatic contrast gain control: chromatic induction, mediated by a neural signal for contrast at the edge of the test, is attenuated by contrast within the remote region. A contrast gain control set by variation in chromaticity over a broad area can contribute to the stable color appearance of surfaces embedded within complex scenes by minimizing chromatic induction from locally adjacent regions.

Adaptation, Ocular↗

PubMed1998

A year's memories: the calendar effect in autobiographical recall.

When asked to recall autobiographical events from the past year, students tend to recall more incidents from the beginning and the end of school terms than from other periods. We investigated this calendar effect in Experiment 1 by comparing free recall at schools with different academic calendars. The event distributions tracked the individual calendars, helping to eliminate the possibility that the calendar effect is due to seasonal, nonschool factors, such as holidays. In Experiments 2-4, we checked explanations based on the ideas that events at term boundaries are more important or distinctive than others, that events are incorrectly dated too near the boundaries, and that boundaries serve as implicit cues for recall. These experiments revealed no evidence that importance or errors in dating could explain the effect. Manipulating cues, however, did change the size of the effect, implicating retrieval from very long-term memory as the effect's source. We suggest that when people have to search episodic memory, they consider their own calendar rhythms (such as a student's academic schedule) and let the temporal structure of their personal context guide their search.

Achievement↗

PubMed1997

The visual photopigments of simple deuteranomalous trichromats inferred from color matching.

Deuteranomalous trichromacy is the most common form of inherited color-vision deficiency. A modern description of its cause is a single abnormality: the normal middle-wave cone photopigment (M) is replaced by a shifted middle-wave pigment (M) that is shared by all deuteranomalous trichromats. This explanation, however, fails to account for the individual differences in color vision observed even within the sub-group of deuteranomals with good chromatic discrimination. An ensemble of color matches is used here to test whether these individual differences reflect differences in the wavelength of peak sensitivity (lambda max) of individual deuteranomals' cone photopigments. The results show variation in both the lambda max and the effective optical density of their cone pigments. The individual differences found in lambda max are in accord with recent molecular biological research that shows individual differences in the genes thought to encode deuteranomalous photopigments.

Adult↗

PubMed1997

An account of brightness in complex scenes based on inferred illumination.

Achromatic brightness matches between two small patches were measured in a display containing ten larger regions of different luminances. The spatial organization of the ten regions was varied while keeping constant the immediate surround (and thus local contrast) of each patch as well as the average luminance of the entire stimulus. Various spatial arrangements were designed to alter the illumination inferred by the observer without changing the ensemble of luminances actually in view. Some spatial arrangements of the ten regions were consistent with five (simulated) surfaces under two distinct levels of illumination, with one luminance edge within the display (an 'apparent illumination edge') dividing the stimuli into an area of lower illumination and an area of higher illumination. In other spatial arrangements the ten regions were configured so that no luminance edge in the display could be interpreted as an ecologically valid illumination edge that provides a parsimonious interpretation of the ten regions; these conditions were designed to induce observers to infer ten surfaces under a single illuminant. When the ten regions were arranged with an apparent illumination edge, the patch within the area of lower perceived illumination was perceived as dimmer than when the same patch and immediate surround were presented with no apparent illumination edge. The results are interpreted by positing that the apparent illumination edge causes an observer to group together regions under the same perceived illuminant, with a consequent effect on brightness: lowering or raising the level of a perceived illuminant causes a patch of fixed contrast to be perceived as less bright or more bright, respectively, just as occurs when lowering or raising the level of real illumination. It is suggested that changes in brightness in a complex scene that result from a change in real illumination may be caused by a difference in inferred illumination at the perceptual level, not by simply a change in the amount of light absorbed by photoreceptors.

Adult↗

PubMed1996

Color perception with test and adapting lights perceived in different depth planes.

Adapting to a chromatic light can alter the color appearance of other lights in view. The chromatic adapting effect is measured here with the test and adapting field perceived in the same depth plane, or perceived in different depth planes (using stereo disparity). The measurements show only a weak, though consistent, shift in the appearance of the test when adapting field and test are perceived in different depth planes, compared to when they are in the same plane. Adding complexity to the adapting stimulus, in the form of a second chromatic light surrounding the background, alters the appearance of the test but shows no dependence on the depth relations. Overall, there is only a small difference in chromatic adaptation caused by introducing a three-dimensional representation of these stimuli.

Adaptation, Physiological↗

PubMed1996

Brightness contrast from inhomogeneous surrounds.

The luminance of a test within an inhomogeneous ("checkerboard") surround was adjusted to match the brightness of a comparison patch within a uniform surround. All stimuli were achromatic. Both surrounds had the same space-averaged luminance. With an incremental comparison patch, a test-within-checkerboard at a luminance between the luminances of the brighter and dimmer checks appears dimmer than if viewed within the uniform surround. A decremental comparison patch, however, is matched by a test luminance that is little affected by the inhomogeneity of the surround. In general, the brightness of the test is mediated neither by the space-averaged luminance of an inhomogeneous surround, nor by any equivalent uniform surround, regardless of luminance. We consider alternative models for the brightness of a region that is neither strictly an increment nor decrement with respect to contiguous surrounding surfaces.

Adaptation, Ocular↗

PubMed1996

Color memory and color constancy.

Color constancy is the perceived stability of the color of objects despite changes in the light illuminating them. An object's color is considered constant if the current perceived color is judged to be in accord with the remembered one. Thus the accuracy and precision of color memory are fundamental to understanding this classic problem. Two hypotheses of color memory are tested here: (1) the photoreceptor hypothesis, which states that the color recalled from memory reproduces the light absorbed by each type of cone and (2) the surface-reflectance hypothesis, which states that the color recalled from memory is based on an inferred spectral reflectance of a surface that does not depend on the spectral distribution of the illuminant. In the experiments a test color is surrounded by either (i) a complex pattern composed of several colored patches or (ii) a uniform "gray" field at the chromaticity of the illuminant. In a control condition the test color is presented on a dark background. Long-term memory of the test color is measured in a production task begun 10 min after the end of the learning phase. In general, the results with a complex surround are consistent with the surface-reflectance hypothesis but not with the photoreceptor hypothesis. Color memory with the "gray" surround, on the other hand, shows a much stronger effect of the illuminant used during learning. These results are consistent with computational models of color constancy that require three or more chromaticities in view.

Color↗

PubMed1995

Color appearance with sparse chromatic context.

We compared changes in the appearance of a test region caused by introducing an inhomogeneous chromatic background to changes caused by a space-averaged equivalent uniform background. Subjects adjusted a test field presented on a CRT so that it appeared neither reddish nor greenish. Sparse "white" or "green" dots, randomly scattered throughout a "red" background field, caused a large decrease (up to 15 nm) in the dominant wavelength of the red/green equilibrium setting, compared to measurements with a uniform "red" background. A uniform background with the same space-averaged chromaticity and luminance as the complex background had an effect similar to the uniform "red" background. These results contradict theories of color constancy that rely on the "gray world" assumption, and indicate the significance for color perception of individual chromaticities within discrete, noncontiguous regions.

Adaptation, Ocular↗

PubMed1995

Variation in color matching and discrimination among deuteranomalous trichromats: theoretical implications of small differences in photopigments.

Individual differences in abnormal color vision are well known. A fundamental unresolved problem is the great variation in color vision even among those classified as having the same color-vision defect. Several physiological hypotheses have been proposed to account for this variation but little consideration has been given to how (and how much) color matching and discrimination are affected by the posited physiological mechanisms. Advances in molecular genetics have renewed interest in this problem, which is at the foundation of the relation between genotype and phenotype. We report here theoretical Rayleigh ranges (chromatic discrimination) and quantal matches for deuteranomalous trichromats with photopigments in the red/green range that vary in their separation and optical density. The results show there is relatively little loss of discrimination with pigments of normal optical density separated by as little as 2-3 nm. With pigments separated by 4 nm or less, however, optical density can strongly influence discrimination when varied independently in the two types of cone. Moderately lower (or higher) optical density in only one cone-type affects discrimination by altering the shape of the cone's relative spectral sensitivity function. The lack of correlation between Rayleigh-match midpoint and range, which is reported in the literature, may be accounted for by independent variation in pigment separation and optical density.

Color Perception↗

PubMed1995

Color appearance under chromatic adaptation varied along theoretically significant axes in color space.

Changes in color appearance caused by chromatic adaptation were measured with a wide range of adapting fields. Observers viewed a 39'-55' annular test field composed of an admixture of lights from the red phosphor and the green phosphor of a CRT. The annular mixture field was centered and superimposed upon a 4.7 degrees steady, circular background field. After the observer was completely adapted to the background, the luminance of the red phosphor in the test was held fixed while the observer adjusted the luminance of the green phosphor until the test appeared neither reddish nor greenish. Twenty-two equiluminant backgrounds (4.5 cd/m2, approximately 50 Td) were systematically selected along two axes in Judd chromaticity space. One axis was along tritanopic confusion lines, with middle-wavelength-sensitive- (M-) and long-wavelength-sensitive- (L-) cone stimulation held constant. The other axis maintained constant short-wavelength-sensitive- (S-) cone stimulation. The results show that adapting backgrounds that were varied along tritanopic confusion lines do not have a differential effect on color appearance at high test levels (well above the adapting level). At lower test levels there is a systematic change in color appearance of the test light, which is quantitatively described by additive redness. Along constant S-cone-stimulation lines, adapting backgrounds differentially affect color appearance in a systematic way, reflecting changes in receptoral gain and the additive contribution. The measurements taken with adapting fields throughout color space are described by the two-process model of chromatic adaptation.

Adaptation, Ocular↗

PubMed1994

Color perception within a chromatic context: the effect of short-wavelength light on color appearance.

Light at the boundary of a uniform test field (contrast) has a qualitatively different effect on color perception than light in more remote noncontiguous regions (context). Basic properties of color perception with contextual short-wavelength light are assessed here with a 1 degree test field surrounded by either contiguous or noncontiguous 440 or 491 nm light (32 td). Contrasting stimuli are 3 or 5 degrees adapting fields, a thin 1 degree i.d.-2 degrees o.d. (0.5 degree wide) contiguous band, or a large 1 degree i.d.-5 degrees o.d. contiguous surround. Contextual stimuli are a remote 3 degrees i.d.-5 degrees o.d. ring or 0.5 degree wide noncontiguous bands at various distances from the edge of the 1 degree test field (2 degrees i.d.-3 degrees o.d., 3 degrees i.d.-4 degrees o.d., or 4 degrees i.d.-5 degrees o.d. bands). Contiguous surrounds have little influence on color appearance, but remote noncontiguous short-wavelength light strong affects the color of the test field, shifting it toward redness. The shift toward redness increases as a thin 440 nm band is moved farther from the test field (up to 5 degrees), unlike the effect of distance on remote middle- and long-wavelength bands. Measurements comparing the effects of 440 nm and luminance-equated 491 nm light indicate a contribution from S cones.

Color↗

PubMed1994

Individual differences in cone photopigments of normal trichromats measured by dual Rayleigh-type color matches.

Individual differences in color matches of normal trichromats are well documented. Recently, variants of the classical Rayleigh match have been measured to explore the cause(s) of these individual differences. Interobserver differences in the wavelength of peak sensitivity of photopigment (lambda max) are of primary interest because they are attributed to an X-chromosome-linked polymorphism. Color-matching equations, however, show the Rayleigh match cannot distinguish between interobserver differences in lambda max and interobserver differences in the optical density of photopigment. Further analysis of color-matching equations reveals that the ratio of two particular Rayleigh-type matches amplifies the effect of individual differences in the lambda max of L cones relative to the effects of optical density and pre-receptoral spectrally selective filtering. The ratio of these two color matches was measured for 17 color-normal males. The range of the results for the 17 observers is too large to be explained by only individual differences in photopigment optical density and pre-receptoral filtering. This implies there are interobserver differences in lambda max. The results are accounted for quantitatively by a small difference (3-5 nm) in the lambda max of the L-cone photopigment. The ratio of two Rayleigh-type matches is a rapid and convenient measurement for assessing the L-cone lambda max in the eye of an individual observer and therefore may be useful for classifying normal trichomats into phenotypic sub-types.

Color Perception↗

PubMed1994

Serine/alanine amino acid polymorphism of the L-cone photopigment assessed by dual Rayleigh-type color matches.

The dual Rayleigh-type color match is the ratio of 621 nm light to 550 nm light that in admixture matches 586 nm light, divided by the ratio of 667 nm light to 550 nm light that in admixture matches 586 nm light. Compared to the classical Rayleigh match, the dual-match procedure minimizes variation in color matching arising from differences in lens pigmentation and photopigment optical density, and thus amplifies individual differences due to shifts in L pigment lambda max. We hypothesized that the dual matches would provide a clearer distinction between subjects with serine and subjects with alanine than would the classical Rayleigh match because individuals with serine express L pigments with a lambda max shifted toward longer wavelengths than do those with alanine. Classical Rayleigh color matches were compared with dual Rayleigh-type color matches in 14 color-normal observers whose DNA had been analyzed previously for the presence of the amino acid serine or alanine at position 180 in the L opsin. The resulting distribution of dual-match measurements for the seven subjects with serine does not overlap the distribution of measurements for the seven subjects with alanine. The classical Rayleigh-match measurements for these two groups of subjects, on the other hand, overlap substantially. More than half of the subjects' classical Rayleigh matches are within the overlapping range. The dual Rayleigh-type matches, therefore, provide an improved psychophysical technique for assessing whether an individual observer has serine or alanine at position 180.

Adult↗