Biomedical subjects

Steven K Shevell

Publications and source records attributed to Steven K Shevell.

18 recordsLinked to original sources

PubMed2005

Chromatic assimilation measured by temporal nulling.

Chromatic assimilation is the shift in color appearance toward nearby light. Assimilation was measured using nearby light with time-varying chromaticity. This light induced time-varying assimilation within the test area. Assimilation was quantified by the amplitude of temporally varying test-area light--in counter-phase to the induced assimilation--required to null the assimilation. Unlike previous studies of assimilation, observers here judged only the steadiness of the test area, not its color. The inducing light was varied in luminance, temporal frequency and chromaticity. The measured assimilation could not be explained by only optical factors affecting receptoral quantal absorption. This implies a neural process contributes to assimilation. The nulling measurements showed also that assimilation was not induced independently within the L/M- and S-cone pathways.

Adult↗

PubMed2004

Color shifts from S-cone patterned backgrounds: contrast sensitivity and spatial frequency selectivity.

Patterned backgrounds that selectively stimulate the S-cones cause conspicuous color shifts. These shifts are accounted for by an S-cone antagonistic (+S/-S) center-surround receptive field [Monnier, P., & Shevell, S. K. (2004). Chromatic induction from S-cone patterns. Vision Research, 44, 849-856]. The present study tested two additional implications of the S-cone receptive field for color shifts: (1) proportionality of the shifts with respect to S-cone contrast within the inducing pattern and (2) bandpass selectivity of the shifts with respect to the spatial frequency of the inducing pattern. Measurements showed that the magnitude of the color shift was linear with S-cone contrast and that the largest color shift was observed with inducing patterns at an intermediate spatial frequency. These results further support an S-cone spatially antagonistic receptive field as the neural substrate mediating the large color shifts from S-cone patterns.

Color Perception↗

PubMed2004

Chromatic assimilation: spread light or neural mechanism?

Chromatic assimilation is the shift in color appearance of a test field toward the appearance of nearby light. Possible explanations of chromatic assimilation include wavelength independent spread light, wavelength-dependent chromatic aberration and neural summation. This study evaluated these explanations by measuring chromatic assimilation from a concentric-ring pattern into an equal-energy-white background, as a function of the inducing rings' width, separation, chromaticity and luminance. The measurements showed, in the s direction, that assimilation was observed with different inducing-ring widths and separations when the inducing luminance was lower or higher than the test luminance. In general, the thinner the inducing rings and the smaller their separation, the stronger the assimilation in s. In the l direction, either assimilation or contrast was observed, depending on the ring width, separation and luminance. Overall, the measured assimilation could not be accounted for by the joint contributions from wavelength-independent spread light and wavelength-dependent chromatic aberration. Spatial averaging of neural signals explained the assimilation in s reasonably well, but there were clear deviations from neural spatial averaging for the l direction.

Adult↗

PubMed2004

Brightness contrast and assimilation from patterned inducing backgrounds.

Theories of induction propose that the brightness of a test patch within a complex surround is explained by local contrast or by integrating contrasts from various regions within the surround, weighted inversely with the distance from the test. Results here corroborate that brightness induction from a patterned background depends on both contiguous and non-contiguous surrounding light, but the measurements were inconsistent with any linear integration of contrast at edges within the scene. In some conditions, assimilation rather than contrast to contiguous surrounding light was observed, depending on the luminance of the light in non-contiguous regions. This finding implies that brightness induction from patterned backgrounds depends on neural processes that can cause contrast and/or assimilation, depending on the luminance relation between contiguous and non-contiguous regions. A model in the literature postulating that the influence of a non-contiguous edge is regulated by the amount of contrast at the contiguous edge can accommodate brightness induced by these patterned backgrounds.

Adaptation, Ocular↗

PubMed2004

Chromatic induction from S-cone patterns.

Chromatic induction from patterned backgrounds depends on the spatial as well as the chromatic aspects of the background light. Color appearance with patterned and uniform backgrounds was compared using chromaticities distinguished by only the S cones; all backgrounds were equivalent to equal-energy white in terms of L-cone and M-cone stimulation. The measurements showed larger shifts in color appearance with a patterned chromatic background than with a uniform background at any chromaticity within the pattern. The measurements also showed that inducing light within different spatial regions could cause opposite shifts in color appearance: inducing light near a test field shifted appearance toward the inducing chromaticity (assimilation), while the same light some distance from the test shifted appearance away from the inducing chromaticity (simultaneous contrast). The shifts in color appearance were accounted for by a neural receptive field with S-cone spatial antagonism.

Color Perception↗

PubMed2003

Large shifts in color appearance from patterned chromatic backgrounds.

The perceived color of a light varies with the background on which it is seen. In the present study, patterned backgrounds composed of two different chromaticities caused larger shifts in perceived color than did a uniform background at either chromaticity within the pattern. Cortical receptive-field organization, but not optical factors or known retinal neurons, can account for the color shifts from patterned backgrounds.

Color Perception↗

PubMed2003

Surface color perception under two illuminants: the second illuminant reduces color constancy.

This study investigates color perception in a scene with two different illuminants. The two illuminants, in opposite corners, simultaneously shine on a (simulated) scene with an opaque dividing wall, which controls how much of the scene is illuminated by each source. In the first experiment, the height of the dividing wall was varied. This changed the amount of each illuminant reaching objects on the opposite side of the wall. Results showed that the degree of color constancy decreased when a region on one side of the wall had cues to both illuminants, suggesting that cues from the second illuminant are detrimental to color constancy. In a later experiment, color constancy was found to improve when the specular highlight cues from the second illuminant were altered to be consistent with the first illuminant. This corroborates the influence of specular highlights in surface color perception, and suggests that the reduced color constancy in the first experiment is due to the inconsistent, though physically correct, cues from the two illuminants.

Color Perception↗

PubMed2002

Simultaneous S-cone contrast.

Chromatic induction is the change in appearance of one light caused by a second, nearby light. We measured chromatic induction in a central test viewed within an inducing field that was varied in only short-wavelength-sensitive (S) cone stimulation. The observer matched the appearance of the central test by adjusting the chromaticity of a haploscopically presented comparison field, seen by the other eye on a dark background. When the central test weakly stimulated S cones, the S-cone level in the surround caused little change in the color appearance of the test. When the central test substantially stimulated S cones, on the other hand, the appearance of the center showed S-cone contrast: raising the level of S in the surround reduced the level of S set to match the central test. Further, a surround that weakly stimulated S cones raised the matching S-cone level above that required without a surround (dark-adapted condition). These results cannot be explained by S-cone sensitivity loss or by a two-process model of adaptation. A cortical mechanism is proposed to mediate S-cone antagonism.

Color Perception↗

PubMed2002

Stereo disparity improves color constancy.

Binocular disparity is an aspect of natural viewing. This research investigates whether disparity affects surface color perception. Achromatic settings were obtained and compared for two stereograms of a scene with specular reflections, one stereogram with binocular disparity and one without it (cyclopean view). Binocular disparity was found to improve color constancy. Next, the geometry of specular highlights, which is distorted without binocular disparity, was specifically examined. Measurements compared color constancy with specular reflections that were either normal (with stereo disparity) or distorted (cyclopean view of the specularities). No significant change in constancy was found due to the geometrical distortion of specular highlights that occurs without stereo disparity, suggesting that constancy depends on other features of the percept affected by disparity. The results are discussed in terms of illuminant estimation in surface color perception.

Color Perception↗

PubMed2002

Articulation: brightness, apparent illumination, and contrast ratios.

Luminance edges in the environment can be due to regions that differ in reflectance or in illumination. In three experiments, we varied the spatial organization of 10 achromatic (simulated) surfaces so that some arrangements were consistent with an ecologically valid and parsimonious interpretation of 5 surfaces under two different illuminants. A constant contrast-ratio along a luminance edge in the scene allows this interpretation. The brightness of patches in this condition was compared to their brightness with minimally different spatial arrangements that fail to maintain the constant contrast-ratio criterion. When the spatial arrangement of the 10 surfaces included a luminance edge satisfying the constant contrast-ratio criterion, brightness changed systematically, compared to arrangements without such a luminance edge. We account for the results by positing that a luminance edge with a constant contrast-ratio segments the scene into regions of lower and higher illumination, with the same effect as a difference in real physical illumination: all else equal, a given surface appears brighter under higher than under lower illumination.

Adult↗

PubMedDate not supplied

Brightness induction: unequal spatial integration with increments and decrements.

Modern theories of brightness induction include an influence from regions that do not share a border with the target. This study tested whether the spatial range of neural integration is the same with incremental versus decremental contrast edges in relatively remote parts of the background. Using an asymmetric matching task, observers set the brightness of a comparison ring, within its own uniform surround, to match the brightness of a test ring within a contiguous surround and a noncontiguous background. The measurements showed that the area of integration depended on the incremental versus decremental contrast polarity at the edge between the surround and background. This implies that brightness induction from an inhomogeneous background must consider the polarity of contrast edges within the whole scene.

Adaptation, Ocular↗

PubMedDate not supplied

Chromatic assimilation unaffected by perceived depth of inducing light.

Chromatic assimilation is a shift toward the color of nearby light. Several studies conclude that a neural process contributes to assimilation but the neural locus remains in question. Some studies posit a peripheral process, such as retinal receptive-field organization, while others claim the neural mechanism follows depth perception, figure/ground segregation, or perceptual grouping. The experiments here tested whether assimilation depends on a neural process that follows stereoscopic depth perception. By introducing binocular disparity, the test field judged in color was made to appear in a different depth plane than the light that induced assimilation. The chromaticity and spatial frequency of the inducing light, and the chromaticity of the test light, were varied. Chromatic assimilation was found with all inducing-light sizes and chromaticities, but the magnitude of assimilation did not depend on the perceived relative depth planes of the test and inducing fields. We found no evidence to support the view that chromatic assimilation depends on a neural process that follows binocular combination of the two eyes' signals.

Calibration↗

PubMedDate not supplied

Changes in color appearance caused by perceptual grouping.

How is chromatic induction affected by perceptual grouping? Chromatic induction has been studied extensively, as has grouping, but only a small number of experiments have connected them. Even fewer reports go beyond weakly controlled qualitative observations. We report here a new and substantial color shift caused by perceptual grouping: a shift in appearance due to chromatic induction in one part of the visual field occurs also in a separate region that belongs to the same group. The color appearance of a test square within various surrounds was measured by asymmetric matching. The test square was at the center of an "hourglass" structure formed by other elements in the surround. The test shifted in color appearance toward the appearance of these other elements, whose color was affected by local chromatic induction. Control experiments ruled out as explanations (1) direct chromatic induction from the other elements into the test area, and (2) the influence of background light occluded by introducing the elements forming the hourglass.

Adaptation, Ocular↗

PubMedDate not supplied

Influence of motion on chromatic detection.

Intense scrutiny has been focused on whether chromatic stimuli contribute to motion perception. The present study considers a related but different question: how does motion affect chromatic detection? Detection thresholds were measured for a disk that underwent a brief (13.3 ms) chromatic change in the L/(L+M) chromatic direction. The disk's presentation sequence and speed (0-16 deg/s) were manipulated. In the coherent presentation sequence, the disk moved smoothly along a circular path centered on the fixation point. In the random presentation sequence, the disk appeared randomly at positions along the circular path. In both types of sequences, the disk underwent a brief chromatic change midway through the temporal presentation sequence. Threshold was elevated in the coherent condition compared to the random condition, and threshold decreased with an increase in speed. The threshold elevation observed in the coherent presentation sequence can be accounted for by temporal integration. The decrease in threshold with an increase in speed can be accounted for by spatial integration. The results, therefore, can be explained by spatiotemporal integration, without invoking a neural mechanism specialized for motion.

Color Perception↗

PubMedDate not supplied

Resolution of binocular rivalry: Perceptual misbinding of color.

Is neural binding of color and form required for perception of a unified colored object? Individual cells selectively tuned to both color and orientation are proposed to moot the binding problem. This study reveals perceptual misbinding of color, thereby revealing separate neural representations of color and form followed by a subsequent binding process. Low luminance-contrast, rivalrous chromatic gratings were presented dichoptically. Each grating had alternating chromatic and gray stripes (e.g., red/gray in the left eye, green/gray in the right eye). Observers viewed the two rivalrous, 2 cpd gratings for 1 min. The duration of exclusive visibility was measured for four percepts: left-eye stimulus, right-eye stimulus, fusion of the two colors, or a two-color grating (e.g. a red/green grating). The percept of a two-color grating (misbinding) was observed with Michelson luminance contrast in the grating up to 20%. In general, for a given level of luminance contrast either misbinding (low luminance contrast) or color mixture (high luminance contrast) was observed, but not both of them. The perceived two-color gratings show that two rivalrous chromaticities are both represented neurally when color and form are combined to give a unified percept. "Resolution" of competing chromatic signals from the two eyes is not restricted to color dominance and color mixture. The transition from misbinding to color mixture by increasing luminance contrast shows that luminance edges have an important role in correct localization of color.

Color Perception↗

PubMedDate not supplied

Induced steady color shifts from temporally varying surrounds.

The color appearance of a physically steady central region can appear to vary over time if a surrounding chromatic light varies in time. The induced temporal variation, however, is strongly attenuated at surround temporal frequencies above approximately 3 Hz. At these higher temporal frequencies, the central region appears steady (De Valois et al., 1986). The posited explanation is a cortical low-pass temporal filter. Here, we investigate whether higher temporal-frequency surrounds induce color shifts in the steady appearance of the central test. Surrounds modulated in time along the l or s chromatic direction of MacLeod-Boynton color space were symmetric around equal-energy white (EEW). The temporal frequency of the surround was varied. If observers perceived the central test to be temporally modulating between two points in time, they set two separate matches to the extreme points of this modulation. If the central test appeared steady in time, then color matches were made to this steady appearance. Corroborating previous reports, measurements showed that surround temporal frequencies below approximately 3 Hz induced temporal modulation. At higher temporal frequencies, however, the surround induced steady color shifts, compared to a steady surround at its time average (EEW). The measurements imply that a nonlinear neural process affects chromatic induction from time-varying context.

Adaptation, Ocular↗

PubMedDate not supplied

Color shifts induced by S-cone patterns are mediated by a neural representation driven by multiple cone types.

This study investigated chromatic induction from inhomogeneous background patterns. Previous work showed that a background pattern detected by only S cones induced strong color shifts in a nearby test area (Monnier & Shevell, 2003). In that work, the S-cone patterns were composed with constant L- and M-cone stimulation over the entire background; in terms of L and M cones, therefore, the background was uniform. S-cone stimulation was varied over space to produce S-cone-isolated background patterns. These S-cone patterns, however, established spatial structure (the pattern) at both the receptoral level (S-cone stimulation) and the postreceptoral level (S/(L+M)). Here, these two levels of pattern representation were unconfounded to determine whether color shifts induced by S-cone patterns were due to spatial structure within an S-cone-specific neural pathway versus a pathway that combines responses from S cones and other cone types (e.g. S/(L+M)). The results showed that the induced color shifts were mediated by signals within a pathway that combines responses from multiple cone types. These results are consistent with a +s/-s spatially antagonistic neural receptive field, which is found in some neurons in V1 and V2.

Color↗

PubMedDate not supplied

Temporal nulling of induction from spatial patterns modulated in time.

Temporally varying chromatic-inducing light was used to infer receptive-field organization. Time-varying shifts in color appearance within a test field were induced by a surrounding chromatic pattern; the shifts were then nulled by adding a time-varying stimulus to the test area so the observer perceived a steady test. This method measured chromatic induction without requiring an observer to judge the color appearance of the test. The induced color shifts were consistent with a +s/-s spatially antagonistic neural receptive field, which also accounts for color shifts induced by static chromatic patterns (Monnier & Shevell, 2003, Monnier & Shevell, 2004). The response of this type of receptive-field, which is found only in the visual cortex, increases with S-cone stimulation at its center and decreases with S-cone stimulation within its surround. The measurements also showed a negligible influence of temporal inducing frequency in the range 0.5-4 Hz.

Color Perception↗