Biomedical subjects

D T Lindsey

Publications and source records attributed to D T Lindsey.

At least 19 recordsLinked to original sources

PubMed2001

Direction repulsion in unfiltered and ring-filtered Julesz textures.

Perceived directions of motion were measured for each of two superposed two-dimensional dynamic random patterns consisting of unfiltered or ring-filtered dense random-check (Julesz) textures. One pattern always moved in a cardinal direction (up, down, left, or right), and the other texture always moved in an oblique direction separated from the cardinal component by 20 degrees-80 degrees. Several cardinal/oblique speed ratios were tested. In Experiment 1, the textures were unfiltered. In Experiment 2, the textures were ring filtered and had the same center frequency (1, 2, or 4 cpd). In Experiment 3, a 1-cpd ring-filtered texture was paired with a 2-, 4-, or 8-cpd texture. Subjects consistently misperceived the directions of component motion in these experiments; the angular separation of movement of the two textures was perceptually exaggerated, a phenomenon referred to as direction repulsion (Marshak & Sekuler, 1979). The results show that (1) direction repulsion occurs across at least a fourfold range of spatial frequencies and a sixfold range of speed ratios, (2) direction repulsion varies systematically with speed ratio, and (3) across most conditions, direction repulsion is anisotropic--direction repulsion is more evident in the oblique directions than in the cardinal directions. These findings suggest that the spatiotemporal range of inhibitory interactions involved in motion transparency is much greater than previously appreciated.

Acceleration↗

PubMed1998

Opponent motion interactions in the perception of transparent motion.

Interactions in the perception of motion transparency were investigated using a signal-detection paradigm. The stimuli were the linear sum of two independent, moving, random-check "signal" textures and a third texture consisting of dynamic random "noise." Performance was measured as the ratio of squared signal and noise contrasts was varied (S2/N2). Motion detectability was poorest when the two signal textures moved in opposite directions (180 degrees), intermediate when they moved in the same direction (0 degrees), and best when the textures moved in directions separated by 90 degrees in the stimulus plane. This pattern of results held across substantial variations in velocity, field size, duration, and texture-element size. Motion identification was also impaired, relative to 0 degrees, in the 180 degrees but not in the 90 degrees condition. These results are consistent with the idea that performance in the opponent-motion condition is limited by inhibitory (or suppressive) interactions. These interactions, however, appear to be direction specific: little, if any, inhibition was observed for perpendicular motion.

Humans↗

PubMed1996

On the relative contributions of motion energy and transparency to the perception of moving plaids.

Stoner, Albright and Ramachandran [(1990) Nature, 344, 153-155] found that moving rectangular-wave plaid patterns that admitted a transparency interpretation appeared to segment that "slide" past one another as the plaids were translated, while the components of plaids that did not admit a transparency interpretation appeared to unify and move rigidly in the direction of translation of the plaid. In experiment I, we show that the magnitude of the effect reported by Stoner et al. is due largely to their repeated-trials experimental protocol, in which plaids moving in a particular direction, upward or downward, are repeatedly presented. This protocol leads to a direction-of-motion-specific adaptation that diminishes the effectiveness of processes that are presumably involved in the unification of the various sensory signals evoked by a moving plaid. In the second experiment, we measured frequencies of nonrigidity for a larger class of moving plaid-like patterns that moved either upwards or downwards on a pseudorandom schedule identical to that employed by Stoner et al. Some of the patterns admitted a transparency interpretation, while others did not. The overall pattern of results could not be accounted for within the context of Kim and Wilson's [(1993) Vision Research, 33, 2479-2489] model of motion integration that considers only the oriented motion energy present in a moving plaid stimulus. The results indicate that additional factors, distinct from though perhaps related to the visual analysis of transparency, must also be incorporated into models of perceived plaid motion.

Adaptation, Ocular↗

PubMed1996

Spatial interactions in perceived speed.

Previous research has shown that the perception of motion within a local region is influenced by other motions within neighboring areas (eg induced motion). Here, a study is reported of the perceived speed of dots moving within a circular target region, which was surrounded by other motions within a larger surrounding area. The perceived speed of the central dots was found to be fastest when the surround was stationary; it became slower as the speed of motion in the surround was increased. This decrease in the perceived target speed with increases in surround velocity occurred regardless of whether the direction in which the surround moved was the same as or opposite to the motion of the target region. This result cannot be explained by using simple models of perceived speed that depend only upon such factors as the magnitude of relative motion between center and surround. The spatial area over which these motion interactions occur was also investigated.

Analysis of Variance↗

PubMed1995

Infant luminance and chromatic contrast sensitivity: optokinetic nystagmus data on 3-month-olds.

Infant color vision is poor, and most psychophysical experiments agree that infant color vision emerges between ages 3 weeks and 3 months. Presumably, the color vision of infants is poor during the months immediately after it has emerged. We have tested two alternative explanations for the poor color vision of infants: (1) there is a special critical immaturity within the color pathways of infants; (2) infants have poor infant color vision because they are insensitive to contrast. Luminance and chromatic contrast thresholds were measured on 3-month-olds using optokinetic nystagmus (OKN), and adult luminance and chromatic contrast thresholds were measured using OKN and two forced-choice methods: direction-of-motion discrimination and grating detection. The infant chromatic-to-luminance contrast threshold ratio shows that infants are as sensitive or even more sensitive than adults to color, depending on the testing method used on adults. This result suggests that the general contrast insensitivity hypothesis is correct. Conservative "worst-case" quantitative analysis strongly suggests that this result is not the consequence of a luminance artifact.

Adult↗

PubMed1993

Serine/alanine amino acid polymorphism of the L and M cone pigments: effects on Rayleigh matches among deuteranopes, protanopes and color normal observers.

In a first experiment, groups of deuteranopes and protanopes were characterized psychophysically by the slopes of regression lines fitted to yellow intensity settings from their Rayleigh matches. In a second experiment, color normal male subjects were characterized by their 2 and 8 deg Rayleigh match points. All subjects had been previously characterized genetically by the presence of the amino acid serine or alanine at position 180 on their L cone or L/M hybrid opsins. Dichromats and color normal subjects with serine had greater sensitivity to the red primary than did those with alanine. Calculations based on psychophysical results suggest that the substitution of serine by alanine in the L cone opsin or L/M hybrid opsin produces a shift in lambda max of the expressed pigment toward shorter wavelengths by an amount varying between 2.6 and 4.3 nm, with the shifts in lambda max for the dichromats being larger than those for the color normal subjects.

Adult↗

PubMed1993

Measuring the effect of attention on simple visual search.

Set-size in visual search may be due to 1 or more of 3 factors: sensory processes such as lateral masking between stimuli, attentional processes limiting the perception of individual stimuli, or attentional processes affecting the decision rules for combining information from multiple stimuli. These possibilities were evaluated in tasks such as searching for a longer line among shorter lines. To evaluate sensory contributions, display set-size effects were compared with cuing conditions that held sensory phenomena constant. Similar effects for the display and cue manipulations suggested that sensory processes contributed little under the conditions of this experiment. To evaluate the contribution of decision processes, the set-size effects were modeled with signal detection theory. In these models, a decision effect alone was sufficient to predict the set-size effects without any attentional limitation due to perception.

Adult↗

PubMed1993

Motion at isoluminance: motion dead zones in three-dimensional color space.

Under some conditions, moving isoluminant stimuli perceptually slow down or even appear to stop. The purpose of the experiment was to explore the shape of the motion dead zone, the region of color space over which the perception of stopped motion occurs. Subjects viewed a small patch of moving grating (2.3 deg x 2.3 deg, 1.3 cycles/degree, 2.9 deg/s), that was spatially modulated in chromaticity, luminance, or both, presented either foveally or at 2-deg eccentricity. The bars of the grating moved from both edges inward toward the center of the patch. Subjects set perceptual motion boundaries by adjusting the contrast of the luminance-modulation component of the grating. Over most or all of the available gamut of chromatic contrasts, the upper and lower boundaries of the motion dead zone formed two parallel planes near the V lambda-isoluminant plane in three-dimensional color space. The data thus suggest that under the conditions of the experiment, perceptual-motion boundaries are determined largely or entirely by the luminance contrast of the stimulus. The data also provide the most extensive evidence available to date for the additivity of motion photometry.

Adult↗

PubMed1992

Polymorphism in red photopigment underlies variation in colour matching.

Genetic variation of human senses within the normal range probably exists but usually cannot be investigated in detail for lack of appropriate methods. The study of subtle perceptual differences in red-green colour vision is feasible since both photopigment genotypes and psychophysical phenotypes can be assessed by sophisticated techniques. Red-green colour vision in humans is mediated by two different visual pigments: red (long-wavelength sensitive) and green (middle-wavelength sensitive). The apoproteins of these highly homologous photopigments are encoded by genes on the X chromosome. Colour matches of males with normal colour vision fall into two main groups that appear to be transmitted by X-linked inheritance. This difference in colour matching is likely to reflect small variations in the absorption maxima of visual pigments, suggesting the presence of two common variants of the red and/or green visual pigments that differ in spectral positioning. We report that a common single amino-acid polymorphism (62% Ser, 38% Ala) at residue 180 of the X-linked red visual pigment explains the finding of two major groups in the distribution of colour matching among males with normal colour vision.

Amino Acid Sequence↗

PubMed1992

Infant temporal contrast sensitivity at low temporal frequencies.

The data on infant temporal contrast sensitivity functions (TCSFs) are scarce and contradictory. Earlier studies suggest that critical flicker frequency (CFF) is adultlike at 2-3 months postnatal (Regal, D. M., 1981 Vision Research, 21, 549-555), while contrast sensitivity at low temporal frequencies remains poor. If both of these findings are true, then infant TCSFs are much flatter than those of adults. In the present study, we have re-investigated 2-month-olds' contrast thresholds at low temporal frequencies. To match the conditions of Regal's CFF study, test fields were embedded in a luminance-matched surround. As in previous studies, low contrast sensitivities were found. Models of infants' flat TCSFs are discussed.

Contrast Sensitivity↗

PubMed1992

Defective colour vision associated with a missense mutation in the human green visual pigment gene.

All red/green colour vision defects described so far have been associated with gross rearrangements within the red/green opsin gene array (Xq28). We now describe a male with severe deuteranomaly without such a rearrangement. A substitution of a highly conserved cysteine by arginine at position 203 in the green opsins presumably accounted for his colour vision defect. Surprisingly, this mutation was fairly common (2%) in the population but apparently was not always expressed. In analogy with nonexpression of some 5'green-red hybrid genes in persons with normal colour vision, we suggest that failure of manifestation occurs when the mutant gene is located at a distal (3') position among several green opsin genes. This mutation might also predispose to certain X-linked retinal dystrophies.

Amino Acid Sequence↗

PubMed1992

Genotype-phenotype relationships in human red/green color-vision defects: molecular and psychophysical studies.

The relationship between the molecular structure of the X-linked red and green visual pigment genes and color-vision phenotype as ascertained by anomaloscopy was studied in 64 color-defective males. The great majority of red-green defects were associated with either the deletion of the green-pigment gene or the formation of 5' red-green hybrid genes or 5' green-red hybrid genes. A rapid PCR-based method allowed detection of hybrid genes, including those undetectable by Southern blot analysis, as well as more precise localization of the fusion points in hybrid genes. Protan color-vision defects appeared always associated with 5' red-green hybrid genes. Carriers of single red-green hybrid genes with fusion in introns 1-4 were protanopes. However, carriers of hybrid genes with red-green fusions in introns 2, 3, or 4 in the presence of additional normal green genes manifested as either protanopes or protanomalous trichromats, with the majority being protanomalous. Deutan defects were associated with green-pigment gene deletions, with 5' green-red hybrid genes, or, rarely, with 5' green-red-green hybrid genes. Complete green-pigment gene deletions or green-red fusions in intron 1 were usually associated with deuteranopia, although we unexpectedly found three carriers of a single red-pigment gene without any green-pigment genes to be deuteranomalous trichromats. All but one of the other deuteranomalous subjects had green-red hybrid genes with intron 1, 2, 3, or 4 fusions, as well as several normal green-pigment genes. The one exception had a grossly normal gene array, presumably with a more subtle mutation. Amino acid differences in exon 5 largely determine whether a hybrid gene will be more redlike or more greenlike in phenotype. Various discrepancies as to severity (dichromacy or trichromacy) remain unexplained but may arise because of variability of expression, postreceptoral variation, or both. When phenotypic color-vision defects exist, the kind of defect (protan or deutan) can be predicted by molecular analysis. Red-green hybrid genes are probably always associated with protan color-vision defects, while the presence of green-red hybrid genes may not always manifest phenotypically with color-vision defects. Four subjects who were found to have 5' green-red hybrid genes in addition to normal red- and green-pigment genes had normal color vision as determined by anomaloscopy. These were discovered among a group of 129 Caucasian males who had been recruited as volunteers for a vision study.(ABSTRACT TRUNCATED AT 400 WORDS)

Base Sequence↗

PubMed1990

Motion at isoluminance: discrimination/detection ratios for moving isoluminant gratings.

Subjects viewed a 2.3 x 2.3 deg patch of a moving 1.3 c/deg, 3.75 Hz sinusoidal grating, centered 1.8 deg from fixation. Two-alternative forced-choice contrast thresholds were measured along the luminance axis and 10 chromatic axes at isoluminance for three tasks: detection (D), form discrimination (F), and discrimination of upward from downward motion (M). F/D threshold ratios averaged approx. 1:1 on all axes. M/D ratios were approx. 1:1 on the luminance axis, but varied from 3:1 to indeterminately large with chromatic axis at isoluminance. We conclude that under the present conditions there are large, highly specific losses of direction-of-motion information at isoluminance. The results imply the existence of chromatic channels that are labeled for form but not for direction of motion at threshold. The pattern and significance of variations in M/D ratios within the isoluminant plane is also discussed.

Contrast Sensitivity↗

PubMed1989

Influence of variations in edge blur on minimally distinct border judgments: a theoretical and empirical investigation.

Minimally distinct border (MDB) settings were made for white-chromatic borders that varied from 1 to 32 arcmin in the space constant of Gaussian blur. The spectral characteristics, additivity, and variability of the MDB judgments remained essentially unchanged across all degrees of edge blur up to and including a space constant of 8 arcmin. For space constants of 16 and 32 arcmin, the variability of the settings increased dramatically, but no consistent trends toward changes in spectral characteristics or additivity were found. Predictions of visual response to edge blur were derived from Wilson and Gelb's six-channel model of spatial vision [J. Opt. Soc. Am. A 1, 124 (1984)]. The fits of these calculations to the data suggest that either a single low- to mid-spatial-frequency-tuned channel mediates MDB judgments over a wide range of variations of edge blur or else the channels that jointly mediate these judgments are importantly similar in their chromatic characteristics.

Adult↗

PubMed1989

Motion nulls for white versus isochromatic gratings in infants and adults.

The relative sensitivity of infants and adults to luminance modulations of 2.6-cd/m2, 0.3-cycle/deg broadband isochromatic red, green, or blue test gratings was measured with a motion-nulling technique. Optokinetic nystagmus was used as the response measure. Each test grating was pitted against a standard, 2.6-cd/m2, 50%-contrast white grating, and the contrast of the test grating required for a motion null was determined. The results were similar for both age groups. Both infants and adults required approximately 50% contrast in the red and green gratings, but only about 25% contrast in the blue grating, to produce a motion null. Quantitative analysis of the results suggests that a peripheral photopic luminance mechanism, with or without a small scotopic contribution, controls optokinetic nystagmus responses in all subjects under these conditions.

Adult↗

PubMed1986

Phase-dependent sensitivity to heterochromatic flicker.

We measured modulation sensitivity to a pair of equally luminous sinusoidally modulated lights (568 and 630 nm) as a function of their relative phase. Measurements were made for 2, 3, 6, and 12 Hz at a retinal illuminance of 100 Td. The data indicated that two processes were active and their outputs combined by a vector summation rule. There was a phase shift of -18 degrees to -20 degrees (630 nm leads 568 nm) at 6 Hz, no phase shift at 12 Hz, an equivocal shift at 2 Hz, and an indeterminate shift at 3 Hz. At frequencies where a phase shift was observed, our analysis indicated that the phase shift affected sensitivities measured at all relative phase settings. These results are inconsistent with models postulating equal contributions of long-wavelength- and middle-wavelength-receptors to centers and surrounds of processes responsible for the detection of luminance flicker.

Color Perception↗

PubMed1985

Electroretinographic studies following vitrectomy and intraocular silicone oil injection.

Vitrectomy was performed bilaterally in pigmented rabbits and followed by silicone oil injection in one eye only. Electroretinographic studies were performed preoperatively and at selected postoperative intervals. In the early postoperative period both eyes showed an initial reduction in a and b wave amplitudes, followed by a return to baseline values. The amplitudes were symmetrically and equally affected. Sequential studies over a 20-month period showed no deterioration in electroretinographic response in eyes with and without silicone oil injection.

Animals↗

PubMed1983

VISUAL EFFICIENCY--a microcomputer program for the analysis of action spectra.

A microcomputer program for the analysis of complex action spectra is described. This program can take data gathered in a series of spectral ranges and fit them to a model consisting of multiple pigments, varying pigment concentrations, and absorption before the photosensitive pigment. Chi square and correlation analysis are performed on the final fit.

Animals↗