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Qasim Zaidi

Publications and source records attributed to Qasim Zaidi.

16 recordsLinked to original sources
PubMed2006

Lightness identification of patterned three-dimensional, real objects.

Conventional studies of lightness constancy have almost exclusively used flat plain stimuli and have shown that lightness matches across illuminants cannot be explained by physical matches of reflectance or luminance. The perceptual qualities that underlie lightness judgments still remain largely unknown. Real objects are often 3-D and patterned, giving additional cues for identification. We examine the perceptual strategies that underlie material identification of real objects. Stimuli were randomly crumpled papers printed with achromatic patterns with precisely calibrated mean reflectance and reflectance contrast, placed in backgrounds under varying levels of illumination. Observers were asked to identify objects based on physical reflectance differences. Reflectance identification functions were simulated by simple models that perform object identification based on dissimilarities in perceived brightness (luminance dissimilarity modified by light adaptation) or perceived contrast (contrast dissimilarity modified by mean luminance). The reflectance identification results were also recreated in two control experiments, using identical stimuli conditions, where choices were based explicitly on dissimilarities in perceived brightness or contrast. Rather than a reverse optics model of lightness perception where observers first estimate illuminant intensity and then extract relative lightness by discounting the illuminant, this study supports the use of simple percepts such as brightness and contrast.

Computer Simulation↗
PubMed2006

Specificity of cone inputs to macaque retinal ganglion cells.

The specificity of cone inputs to ganglion cells has implications for the development of retinal connections and the nature of information transmitted to higher areas of the brain. We introduce a rapid and precise method for measuring signs and magnitudes of cone inputs to visual neurons. Colors of stimuli are modulated around circumferences of three color planes in clockwise and counterclockwise directions. For each neuron, the projection of the preferred vector in each plane was estimated by averaging the response phases to clockwise and counterclockwise modulation. The signs and weights of cone inputs were derived directly from the preferred vectors. The efficiency of the method enables us to measure cone inputs at different temporal frequencies and short-wavelength-sensitive (S) cone adaptation levels. The results show that S-cone inputs to the parvocellular and magnocellular ganglion cells are negligible, which implies underlying connectional specificity in the retinal circuitry.

Action Potentials↗
PubMed2004

Three-dimensional shape from non-homogeneous textures: carved and stretched surfaces.

We examined the perception of 3D shape for surfaces folded, carved, or stretched out of textured materials. The textures were composed of sums of sinusoidal gratings or of circular dots, and were designed to differentiate between orientation and frequency information present in perspective images of the surfaces. Correct perception of concavities, convexities, saddles, and slants required the visibility of signature patterns of orientation modulations. These patterns were identical to those identified previously for developable surfaces (A. Li & Q. Zaidi, 2000; Q. Zaidi & L. Li, 2002), despite the fact that textures were statistically homogeneous on developable surfaces but not on carved or stretched surfaces. Frequency modulations in the image were interpreted as cues to distance from the observer, which led to weak but qualitatively correct percepts for some carved and stretched surfaces but to misperceptions for others, similar to the misperceptions for developable surfaces (A. Li & Q. Zaidi, 2003). Irrespective of whether texture on the surface is homogeneous or non-homogeneous, similar neural modules can be used to locate signature orientation modulations and thus extract shape from texture cues.

Cues↗
PubMed2004

Limits of lightness identification for real objects under natural viewing conditions.

We examined whether observers veridically perceive the reflectances of real objects under natural viewing conditions. A new forced-choice paradigm was used to measure observers' abilities to identify (not match) the reflectance of randomly crumpled gray papers across two levels of illumination, and also to simultaneously measure brightness discrimination thresholds for the same objects. Accuracy of lightness identification differed qualitatively among observers. By explicitly manipulating observer strategies, we show that when observers use brightness dissimilarity, their performance is similar to lightness identification. A brightness adaptation model simulates how instead of extracting lightness, observers can rely on perceived relative brightness to achieve the measured degrees of lightness identification.

Adaptation, Ocular↗
PubMed2004

Colour constancy in context: roles for local adaptation and levels of reference.

By determining the locations of boundaries between colour categories, we measured changes in the colour appearance of test-reflectances as a function of the simulated illumination. Test-reflectances were displayed against a variegated background of reflectance samples. Under prolonged adaptation to each illuminant, observers demonstrated a high degree of appearance-based colour constancy. By using backgrounds that consisted of chromatically biased sets of reflectances, we tested whether this stability depends on estimates of the illuminant's cone-coordinates based on simple scene statistics. The chromatic bias of the background had only a small effect on the classification of test materials. To compare the roles of spatially local and spatially extended estimation processes, we then (unknown to the observer) simulated different illuminants on the test and on the background. Observers continued to demonstrate reasonable colour constancy. To examine the relative roles of automatic adaptation and perceptual strategies, we reduced the duration of exposure to the test compared to exposure to the background (under the conflicting illuminant). The results suggest that mechanisms that preserve information across successive test-presentations (e.g. spatially local adaptation with a time course of a few seconds, and perceptual adjustments to levels of reference) are key determinants of the stability of colour appearance.

Adaptation, Ocular↗
PubMed2004

Illuminant color perception of spectrally filtered spotlights.

The color perceived to belong to the illumination of objects is often based on cues from the scene within which the objects are perceived, instead of being based on any view of the source itself. We present measurements of illuminant color estimation by human observers for moving, spectrally filtered spotlights. The results show that when only one illuminant is in the field of view, estimates of illuminant color are seriously biased by the chromaticities of the illuminated surfaces. When the surround of the spotlight is illuminated by a dimmer second light, spotlight matching moves toward veridical in most conditions. Simulations show that a gray-world model cannot be rejected as an adequate explanation for illuminant color estimation and provides as good a fit as a model that gives greater weights to the brightest surfaces. When the surrounding illuminant is brighter than the spotlight, the situation is similar to that of a moving filter. Spotlight matches are close to veridical, and the results can be fit by a model based on estimating both illuminants.

Color Perception↗
PubMed2004

Perceived transparency of neutral density filters across dissimilar backgrounds.

We examine how the luminance distributions of overlaid surfaces affect the perception of transparency of neutral density filters. Pairs of neutral density filters were generated overlying variegated backgrounds of varying luminance distributions, and observers adjusted a single parameter of one filter until the pair appeared equally transparent. Physically identical filters appeared equally transparent on similar backgrounds, but did not appear equally transparent when backgrounds differed in luminance or contrast. Reducing luminance or contrast of the background decreased perceived transparency of the overlaying filter by a multiplicative factor. Observers matched perceived transparency of physically dissimilar filters by applying a linear trade-off between reflectivity and inner transmittance. In a second experiment, filters had their spatial structure altered in order to abolish the perception of transparency and appeared as patterned opaque disks, and observers equated perceived contrast of the two overlaid areas. Match settings gave results similar to the previous experiment, indicating that, in general, perceived transparency corresponds closely to the perceived contrast of the overlaid region.

Contrast Sensitivity↗
PubMed2003

Observer strategies in perception of 3-D shape from isotropic textures: developable surfaces.

We document the limitations of isotropic textures in conveying three-dimensional shape. We measured the perceived shape and pitch of upright and pitched corrugated surfaces overlaid with different classes of isotropic textures: patterns containing isotropic texture elements, isotropically filtered noise patterns, and patterns containing ellipses or lines of all orientations. Frequency modulations arising from surface slant were incorrectly interpreted as changes in surface distance, resulting in concavities being misclassified as convexities, and right and left slants as concavities. In addition, images of pitched surfaces exhibited oriented flows that confound surface shape and surface pitch. Observers related oriented flow patterns to particular surface shapes with a bias for perceiving convex surfaces. When concave and convex curvatures were concurrently visible, the number of correct shape classifications increased slightly. Isotropic textures thus convey correct 3-D shapes of developable surfaces only in some conditions, and the same perceptual strategies lead to non-veridical percepts in other conditions.

Computer Simulation↗
PubMed2003

Time-course of S-cone system adaptation to simple and complex fields.

We examine the temporal nature of adaptation at different stages of the S-cone color system. All lights were restricted to the S-cone-only (a constant L and M) cardinal axis in color space passing through mid-white (W). The observer initially adapted to a steady uniform field with a chromaticity on the -S end of the axis or on the +S end of the axis or a complex field composed of chromaticy -S and +S (+/-S adaptation). The observer then readapted to a steady uniform field of chromaticity W for a variable length of time (i.e., 0, 0.1, 0.25, 0.5, 1.0, or 2.0 s). A probe-flash technique was used to measure S-cone discrimination at various points along the S-cone-only cardinal axis. This allowed estimation of the response of the S-cone system over an extended response range. Following exposure to the -S and +S uniform fields, sensitivity was maximal at or near the chromaticity of the initial adaptation field and decreased linearly away from the adapting point. The shift from +S to W occurred more rapidly than the shift from -S to W; both of these shifts can be described by a multiplicative scaling of the S-cone signal. Following +/-S adaptation the threshold curve initially had a shape similar to that measured following -S adaptation, but returned rapidly to the W adaptation state. The shift following +/-S adaptation cannot be described by the multiplicative model, but can be explained by a change in the shape of the non-linearity. The results suggest the existence of fast post-receptoral processes.

Adaptation, Ocular↗
PubMed2003

Interactions between color and luminance in the perception of orientation.

At the early stages of visual processing in humans and other primates, chromatic signals are carried to primary visual cortex (V1) via two chromatic channels and a third achromatic (luminance) channel. The sensitivities of the channels define the three cardinal axes of color space. A long-standing though controversial hypothesis is that the cortical pathways for color and form perception maintain this early segregation with the luminance channel dominating form perception and the chromatic channels driving color perception. Here we show that a simple interaction between orientation channels (the tilt illusion) is influenced by both chromatic and luminance mechanisms. We measured the effect of oriented surround gratings upon the perceived orientation of a test grating as a function of the axes of color space along which the gratings were modulated. We found that the effect of a surround stimulus on the perceived orientation of the test is largest when both are modulated along the same axis of color space, regardless of whether that is a cardinal axis. These results show that color and orientation are intimately coupled in visual processing. Further, they suggest that the cardinal chromatic axes have no special status at the level(s) of visual cortex at which the tilt illusion is mediated.

Color Perception↗
PubMed2002

Limitations on shape information provided by texture cues.

This paper uses visual, empirical and formal methods (Li & Zaidi, Vision Research, 40 (2000) 217; Li & Zaidi, Vision Research, 41 (22) (2001a) 2927) to examine the roles of oriented texture components in conveying veridical percepts of concave and convex surfaces that are pitched towards or away from the observer. The results show that pairs of components, oriented symmetrically around the axis of maximum curvature, combine to provide the geodesic orientation modulations that are critical for veridical shape perception. The degree of pitch determines the orientations of the critical pair of components. Perspective is crucial to the veridical perception of concavities and convexities, regardless of the degree of pitch. The results of this paper reconfirm that veridical shape perception depends on extracting critical patterns of oriented energy, but also show that the class of textures capable of conveying veridical percepts of developable shapes in general views is even more restricted than that identified by Li and Zaidi (Journal of Optical Society of America A, 18 (2001b), 2430).

Cues↗
PubMed2002

Cues and strategies for color constancy: perceptual scission, image junctions and transformational color matching.

The identification of objects, illuminants, and transparencies are probably the most important perceptual functions of color. This paper examines the effects of perceptual scission, image junctions, color adaptation, and color correlations on identification. Simulations of natural illuminants, materials, and filters were used in a forced-choice procedure to simultaneously measure thresholds for identifying filters and objects across illuminants, and discrimination thresholds within illuminants. In the vast majority of the cases, if observers could discriminate within illuminants they could identify across illuminants. Since results were similar for identical color distributions, whether transparency cues like X-junctions were present or not, the primary cues for color identification were systematic color shifts across illuminants. These color shifts can be well described by three-parameter affine transformations, and the parameters can be derived from differences and ratios of mean chromaticities. A strategy based on post-transformation color matching predicts generally accurate identification despite perceptible color shifts, and also provides plausible reasons for those few conditions where identification thresholds are significantly higher than discrimination thresholds.

Color Perception↗
PubMed2002

Similarities between visual processing of shear and uniform motion.

A number of papers have claimed that at moderate to high contrasts, sensitivity is higher for shear motion than for uniform motion. We show in a 2 x 2AFC task, designed to minimize any potential artefacts due to criterion level or response bias, that sensitivities are essentially equal for shear and uniform motion under general conditions. It has also been claimed that position tracking enhances sensitivity for shear motion. We added moving sinusoidal gratings to stationary sinusoidal gratings of the same spatial frequency and orientation, to create stimuli in which position changes and motion energy have opposite directions, to show that shear and uniform motion are both subserved by motion-energy mechanisms at speeds above 2.0 deg/s and by position tracking at slower speeds.

Artifacts↗
PubMed2002

Sensory and physical determinants of perceived achromatic transparency.

What are the physical and sensory determinants of perceived transparency? To explore this question, we simulated pairs of physically different neutral density filters on a CRT and asked observers to match their perceived transparency. Matching was accomplished by adjusting one of two physically independent filter properties, reflectivity and inner transmittance. Results show that observers can make reliable matches through a linear trade-off of these two properties. In a separate experiment, observers matched the perceived contrast of the overlaid regions. The reflectivity and inner transmittance values for contrast matches are similar to those of perceived transparency matches, suggesting that perceived image contrast is the sensory determinant of perceived transparency. In variegated displays, neither Michelson contrast nor other standard contrast metrics predicts contrast appearance. When perceived transparency is plotted in terms of filter reflectance and filter transmittance, perceived transparency corresponds closely to filter transmittance.

Contrast Sensitivity↗
PubMed2002

Accuracy of color scission for spectral transparencies.

When surfaces are overlaid by a transparent filter, color scission refers to the perceptual separation of the colors of the image into the colors of the underlying surface and the color of the overlaying layer. We used filter matching to measure the accuracy of color scission for simulated physical filters and materials. Standard filters were placed on various sets of chromatic materials and match filters on achromatic materials. In the majority of cases, filter matching was close to veridical. The spectral effects of filters are complex, but with respect to the visual system, they can be closely approximated by 3-D affine transformations of cone absorptions or chromaticities. Veridical filter matches can be predicted by neural strategies that match ratios of mean cone absorptions or match mean chromatic contrasts between filtered and exposed regions. However, when the shape of a filter transmittance differed significantly from the shapes of background reflectances, the overlaid region had lower saturation than the surround, and filter matches had broader transmittance spectra than veridical.

Color Perception↗