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Biomedical subjects

L E Arend

Publications and source records attributed to L E Arend.

16 recordsLinked to original sources

Simultaneous color constancy: paper with diverse Munsell values.

Arend and Reeves [J. Opt. Soc. Am. A 3, 1743 (1986)] described measurements of color constancy in computer simulations of arrays of colored papers of equal Munsell value under 4000-, 6500-, and 10,000-K daylight illuminants. We report an extension of those experiments to chromatic arrays spanning a wide range of Munsell values. The computer-simulated scene included a standard array of Munsell papers under 6500-K illumination and a test array, an identical array of the same papers under 4000 or 10,000 K. Observers adjusted a patch in the test array in order to match the corresponding patch in the standard array by one of two criteria. They either matched hue and saturation or they made surface-color matches, in which the test patch was made to "look as if it were cut from the same pice of paper as the standard patch." The test and the standard patches were surrounded by a single color (annulus display) or by many colors (Mondrian display). The data agreed with those of our previous equal-value experiment. The paper matches were often approximately color constant. The hue-saturation matches were in the correct direction for constancy but were always closer to a chromaticity match (no constancy) than to the chromaticity required for hue-saturation constancy.

Adaptation, Ocular

Lightness and brightness over spatial illumination gradients.

We extended our studies of lightness and brightness in complex scenes to cathode-ray-tube simulations of an array of 35 gray reflective patches under spatially varying illuminants. There were three illuminance profiles, an abrupt step, a linear gradient, and a simulation of side illumination, with nine steepnesses of each. In half the sessions observers adjusted a test patch at one end of the illumination gradient in order to match the lightness of a standard patch at the other end of the gradient. In the remaining sessions they matched the brightness of the test patch to that of the standard. For all three illuminance profiles the lightnesses of the patches matched when they had approximately the same simulated reflectance; i.e., there was excellent lightness constancy even though the illuminance gradients were clearly visible.

Adaptation, Ocular

What is psychophysically perfect stabilization? Do perfectly stabilized images always disappear?: reply to comment.

We acknowledge the earlier work cited by Ditchburn in his comment [R. W. Ditchburn, J. Opt. Soc. Am. A 4, 405-406 (1987).] and point out a crucial difference between that work and our novel approach [L. E. Arend and G. T. Timberlake, J. Opt. Soc. Am. A 3, 235 (1986)]. Our procedure provides measurements uncontaminated by residual errors of stabilization. The visual system's extreme sensitivity to small temporal changes and the indirectness of the evidence cited by Ditchburn leave the meaning of stabilized-image reappearance unclear.

Humans

Simultaneous constancy, lightness, and brightness.

An achromatic surface in a complex scene has both an apparent reflectance attribute (lightness) and an overall intensitive attribute (brightness). We studied changes of these two attributes as a function of changes in illumination level and pattern complexity. Subjects observed simultaneously two arrays of simulated achromatic surfaces with identical reflectance distributions. The left-hand array (standard) was always illuminated at a moderate level. The right-hand array (test) had different illuminances from trial to trial. The subjects adjusted patches in the test array to match the corresponding patches in the standard array in either lightness or brightness. In complex patterns (32 grays) lightness constancy was nearly perfect; test reflectance settings were invariant over illuminance. In disk/annulus patterns (two grays), the lightness-match data confirmed previously published reports. At high illuminances, the standard patches could be matched with a smaller range of test-array reflectances than at low illuminances, i.e., lightness constancy was imperfect. Brightness matches varied substantially as a function of illuminance in all conditions.

Color Perception

What is psychophysically perfect image stabilization? Do perfectly stabilized images always disappear?

High-contrast luminance gratings stabilized on the retina with a Purkinje image eyetracker do not disappear completely. This could be due to small errors of stabilization, or the visual system could include mechanisms capable of responding to temporally constant images. We examined the visual system's sensitivity to small movements of gratings. We (1) replicated previous measurements of contrast sensitivity for gratings with controlled retinal-drift velocities, (2) developed a method for calculating sensitivity to small oscillations of gratings using thresholds for flickering stabilized gratings, and (3) examined the calculations empirically. We calculated that movements of only 8 sec of arc peak to peak produce detectable temporal changes. Since existent stabilization systems cannot eliminate movements this small, residual stabilized-grating detectability does not require detectors sensitive to temporally constant images.

Humans

Reading with a macular scotoma. I. Retinal location of scotoma and fixation area.

To investigate how patients with macular scotomas use residual functional retinal areas to inspect visual detail, a scanning laser ophthalmoscope (SLO) was used to map the retinal locations of scotomas and areas used to fixate. Three patients with dense macular scotomas of at least 20 months duration and with no explicit low vision training were tested. SLO stimuli were produced by computer modulation of the scanned laser beam, and could be placed on known retinal loci by direct observation of the retina on a television monitor. Videotaped SLO images were analyzed to produce retinal maps that are corrected for shifts of stimulus position due to fixational eye movement, thus showing the true retinal locations of scotomas and fixation loci. Major findings were as follows: 1) each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma to fixate, and did not attempt to use the nonfunctional foveola, 2) fixation stability with the eccentric fixation locus was as good as, or better than, that of ocularly normal subjects trying to fixate at comparable eccentricities, 3) fixation stability was not systematically related to clinical visual acuity, and 4) there is good agreement as to the shape and overall size of SLO and standard clinical tangent screen scotoma maps for these three patients.

Adult