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Results for “Form Perception”

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At least 1,243 records · Page 69Linked to original sources

PubMed1993

Lightness and brightness judgments of coplanar retinally noncontiguous surfaces.

Several experiments reveal that judgments of lightness and brightness of an achromatic surface depend, in part, on the luminances of other surfaces perceived to share the same depth plane, even if the surfaces are well separated on the retina. Two Mondrians, simulated on a CRT, were viewed through a haploscope. The more highly illuminated Mondrian contained a comparison patch and appeared nearer than the more dimly illuminated Mondrian, which contained the test patch. By independently varying the disparity of the test patch, observers could make the test patch appear to be in the depth plane of either the dimly or the highly illuminated Mondrian. Observers set the luminance of the test patch to match that of the comparison patch. The test was set as high as 15% more luminous when it was perceived in the depth plane of the highly illuminated rather than the dimly illuminated Mondrian. Both brightness and lightness judgments were affected by the perceived depth of the test, although the lightness judgments of inexperienced observers sometimes were dominated by local-contrast matching.

Adult↗

PubMed1998

Color constancy in the nearly natural image. 2. Achromatic loci.

Most empirical work on color constancy is based on simple laboratory models of natural viewing conditions. These typically consist of spots seen against uniform backgrounds or computer simulations of flat surfaces seen under spatially uniform illumination. In this study measurements were made under more natural viewing conditions. Observers used a projection colorimeter to adjust the appearance of a test patch until it appeared achromatic. Observers made such achromatic settings under a variety of illuminants and when the test surface was viewed against a number of different backgrounds. An analysis of the achromatic settings reveals that observers show good color constancy when the illumination is varied. Changing the background surface against which the test patch is seen, on the other hand, has a relatively small effect on the achromatic loci. The results thus indicate that constancy is not achieved by a simple comparison between the test surface and its local surround.

Adult↗

PubMed2001

Veridicality of three-dimensional shape perception predicted from amplitude spectra of natural textures.

We show that the amplitude spectrum of a texture pattern, regardless of its phase spectrum, can be used to predict whether the pattern will convey the veridical three-dimensional (3-D) shape of the surface on which it lies. Patterns from the Brodatz collection of natural textures were overlaid on a flat surface that was then corrugated in depth and projected in perspective. Perceived ordinal shapes, reconstructed from a series of local relative depth judgments, showed that only about a third of the patterns conveyed veridical shape. The phase structure of each pattern was then randomized. Simulated concavities and convexities were presented for both the Brodatz and the phase-randomized patterns in a global shape identification task. The concordance between the shapes perceived from the Brodatz patterns and their phase-randomized versions was 80-88%, showing that the capacity for a pattern to correctly convey concavities and convexities is independent of phase information and that the amplitude spectrum contains all the information required to determine whether a pattern will convey veridical 3-D shape. A measure of the discrete oriented energy centered on the axis of maximum curvature was successful in identifying textures that convey veridical shape.

Contrast Sensitivity↗

PubMed2005

Border-ownership-dependent tilt aftereffect.

Single-cell recordings from macaque visual cortex have shown orientation-selective neurons in area in V2 code for border ownership [J. Neurosci. 20, 6594 (2000)]: Each piece of contrast border is represented by two pools of neurons whose relative firing rate indicates the side of border ownership. Here we show that the human visual cortex uses a similar coding scheme by demonstrating a border-ownership-contingent tilt aftereffect. The aftereffect was specific for the adapted location, indicating that the adapted neurons have small receptive fields. We conclude that figure-ground organization is represented by border-ownership-selective neurons at early stages in the human visual cortex.

Adaptation, Ocular↗

PubMed1991

Texture segregation by chromatic and achromatic visual pathways: an analogy with motion processing.

We present results to show that texture segregation can be obtained through the so-called coherent spatial grouping of local shape (orientation) and of local color under both nonequiluminant and equiluminant conditions. Color grouping entails texture segregation independent of orientation grouping, while the reverse is not true under equiluminant conditions. The experiments permit the isolation of chromatic- and luminance-oriented mechanisms, as well as of chromatic nonoriented mechanisms, all of which contribute to texture discrimination. As a general rule, the present results (including the asymmetry between color and orientation grouping) are similar to those obtained by us in a series of motion-perception experiments. This similarity suggests that the perceptual rules governing spatial grouping are analogous (if not identical) to those governing spatiotemporal grouping. As in the case of directional discrimination, texture-discrimination performances may be accounted for by the activation of higher-order units receiving inputs from subunits, all of which display similar tuning properties within a multidimensional space.

Color Perception↗

PubMed1992

Asymmetric color matching: how color appearance depends on the illuminant.

We report the results of matching experiments designed to study the color appearance of objects rendered under different simulated illuminants on a CRT monitor. Subjects set asymmetric color matches between a standard object and a test object that were rendered under illuminants with different spectral power distributions. For any illuminant change, we found that the mapping between the cone coordinates of matching standard and test objects was well approximated by a diagonal linear transformation. In this sense, our results are consistent with von Kries's hypothesis [Handb. Physiol. Menschen 3, 109 (1905) [in Sources of Color Vision, D. L. MacAdam, ed. (MIT Press, Cambridge, Mass., 1970)]] that adaptation simply changes the relative sensitivity of the different cone classes. In addition, we examined the dependence of the diagonal transformation on the illuminant change. For the range of illuminants tested, we found that the change in the diagonal elements of the linear transformation was a linear function of the illuminant change.

Color Perception↗

PubMed1964

ERRORLESS ESTABLISHMENT OF VISUAL DISCRIMINATION USING FADING PROCEDURES.

A visual discrimination task involved presenting a triangle briefly as a sample. When it was withdrawn, this triangle and two others differing slightly in degree of rotation were presented in different positions, with S required to locate the sample that had been presented. Discrimination proved difficult for preschool children. When only the correct triangle was illuminated, discrimination was readily established. The brightness difference between correct and incorrect matches was gradually faded out by increasing the intensity of the incorrect matches, until they were equal in brightness to the correct match. The discrimination established by brightness difference was maintained in its absence, thereby transferring stimulus control from brightness to form, in an almost errorless sequence.

Child↗

PubMed1964