Biomedical subjects

H R Wilson

Publications and source records attributed to H R Wilson.

At least 37 records · Page 2Linked to original sources

PubMed1999

Effects on commercial broiler chicks of constant exposure to ultraviolet light from insect traps.

Constant exposure of newly hatched Avian x Avian broilers to ultraviolet light from insect traps for 42 d resulted in no significant differences in mortality, weight gain, feed consumption, or feed conversion. Birds were exposed to greater intensities of ultraviolet light for longer periods than could be expected under commercial conditions. Although house flies are rarely a problem in broiler houses, our results indicate that insect traps with ultraviolet light as an attractant would not be detrimental to production of broilers. The need for additional testing of light traps for nuisance fly control in commercial broiler houses is discussed.

Animal Husbandry↗

PubMed1999

Factors limiting peripheral pattern discrimination.

Previous reports indicate that some foveally discriminable compound gratings are indiscriminable in peripheral vision, even when they are scaled by the ratio of peripheral to foveal grating acuity. To determine the stimulus properties that limit peripheral discrimination, we used Gaussian derivatives of various orders. These patterns are spatially localized and have intrinsic even or odd symmetry. Our results show that certain odd symmetric patterns are discriminable in the periphery, while others are not. Furthermore, certain even symmetric patterns are not peripherally discriminable. These data are consistent with three limitations on peripheral pattern discrimination: (1) Patterns that produce different maximum neural responses will be peripherally discriminable. (2) Positional uncertainty and undersampling degrade discrimination of high spatial frequency patterns in the periphery. (3) Patterns generating substantial neural activity within a constrained region are processed as textures in peripheral vision so that pattern details within that region are no longer available for discrimination. A neural model incorporating inhibition of simple cells by complex cells implements a transition between contour analysis and texture analysis in peripheral vision and explains the experimental data.

Fovea Centralis↗

PubMed1998

Dynamics of a divisive gain control in human vision.

Evidence for a divisive contrast gain control in human vision was obtained using a contrast version of the probe-on-flash technique that has been employed in the light adaptation literature. Thresholds were measured for a briefly flashed (30 ms), vertical test pattern superimposed on a cosine mask as a function of time after mask onset (SOA). Threshold elevations declined monotonically for SOAs up to 150 ms. and exhibited an exponential time course with an average time constant of 51 ms. Increment thresholds for the test as a function of mask contrast provide direct evidence that these effects are due to operation of a divisive gain control within the first 150 ms after stimulus onset. Experiments to measure the spatial spread of this gain control show it to be localized to a region of no more than 45 arc min radius.

Adaptation, Ocular↗

PubMed1998

Detection and recognition of radial frequency patterns.

Detection thresholds for radial deformations of circular contours were measured using a range of radii and contour peak spatial frequencies. For radial frequencies above two cycles, thresholds were found to be a constant fraction of the mean radius across a four-octave range of pattern radii and peak spatial frequencies (mean Weber fraction: 0.003-0.004). At low radial frequencies, thresholds were unaffected by contrast reduction. In 167 ms presentations, subjects were able to identify radial frequencies of six cycles and below with an accuracy of over 90% correct even when phase was randomized. The extreme sensitivity of subjects to these radial deformations (as low as 2-4 s of arc) cannot be explained by local orientation or curvature analysis, and points instead to the global pooling of contour information at intermediate levels of form vision.

Female↗

PubMed1998

Detection of global structure in Glass patterns: implications for form vision.

Glass (Nature 1969;223:578-580) patterns are random dot stimuli that generate a percept of global structure. To study the mechanisms underlying this global form perception, concentric, radial, hyperbolic, and parallel Glass patterns were constructed. Thresholds for detecting each type of pattern were measured by degrading the patterns through the addition of noise. Concentric patterns yielded the lowest thresholds for all subjects, while radial and hyperbolic patterns produced somewhat higher thresholds. For all subjects the parallel patterns produced the highest thresholds. Threshold measurements as a function of the area containing pattern structure provided evidence for global pooling of orientation information in the detection of radial and concentric Glass patterns but only local pooling in the detection of parallel patterns. Monte-Carlo simulations demonstrate that plausible neural models can accurately predict the data. These models indicate that the visual system contains networks that pool orientation information within regions 3.5-4.5 degrees in diameter in central vision. This pooling is organized to extract cross-shaped, X-shaped, and quasi-circular forms from the retinal image. The results are in good agreement with recent single unit physiology of primate area V4, an intermediate level of the form vision pathway.

Computer Graphics↗

PubMed1998

Measurement of the texture-coherence limit for bandpass arrays.

Casual observation suggests that when the elements of a visual array are packed at a sufficiently high density they cohere to generate the percept of a texture. This 'texture-coherence limit' has been quantified by using arrays composed of Gabor functions, sixth Gaussian derivatives, or differences of Gaussians. In all cases the texture-coherence limit was a power-law function of the size of the elements as quantified by their space constants with an exponent averaging 0.7. Furthermore, the texture-coherence limit was independent of both element spatial frequency and contrast over a considerable range. A quantitative fit to the data is provided by a model in which the texture-coherence limit is determined by activation of complex cells, which pool a spatial range of subunit inputs, throughout the stimulus region. Possible extensions to two dimensions are considered.

Female↗

PubMed1998

Apparent contrast and spatial frequency of local texture elements.

We measured the apparent contrast and spatial frequency of a parafoveal Gabor signal located at the center of an array of Gabor signals as a function of both element density and the direction of contrast and spatial frequency of the surrounding elements. The target Gabor appeared lower in contrast and higher in spatial frequency when the elements were in close proximity, regardless of the direction of contrast and spatial frequency of the surrounding elements. Overall, the evidence suggests that the appearance of a parafoveal target is strongly affected by its visual context. These findings provide additional support for the existence of spatial interactions among neurons implicated in textural processing.

Contrast Sensitivity↗

PubMed1997

Translational repression by a transcriptional elongation factor.

One of the classical positive regulators of gene expression is bacteriophage lambda N protein. N regulates the transcription of early phage genes by participating in the formation of a highly processive, terminator-resistant transcription complex and thereby stimulates the expression of genes lying downstream of transcriptional terminators. Also included in this antiterminating transcription complex are an RNA site (NUT) and host proteins (Nus). Here we demonstrate that N has an additional, hitherto unknown regulatory role, as a repressor of the translation of its own gene. N-dependent repression does not occur when NUT is deleted, demonstrating that N-mediated antitermination and translational repression both require the same cis-acting site in the RNA. In addition, we have identified one nut and several host mutations that eliminate antitermination and not translational repression, suggesting the independence of these two N-mediated mechanisms. Finally, the position of nutL with respect to the gene whose expression is repressed is important.

Bacteriophage lambda↗

PubMed1997

Motion integration over space: interaction of the center and surround motion.

Motion integration occurs over a restricted range of visual space. However, there have been studies suggesting interactions among motion detectors operating on widely separated spatial regions. To understand these lateral spatial interactions beyond motion pooling regions, we examined the effect of surrounding motion on the direction of the center stimulus under several stimulus conditions. We have found that there is a motion direction shift of the center stimulus caused by surrounding motion depending on its motion direction, spatial proximity to the center stimulus, contrast, speed, and the extent of motion area. This effect was observed both for monocular and dichoptic presentations of the pattern. However, the perceived direction shift decreased when the spatial frequency ratio of the center and surround stimuli varied, or a non-Fourier motion pattern was used for both center and surround stimuli. We present a model consisting of lateral inhibitory interactions between pattern motion unit networks to explain the direction shift observed in the experiments.

Contrast Sensitivity↗

PubMed1997

Concentric orientation summation in human form vision.

Psychophysical data demonstrate that orientation information in concentric, random-dot Glass patterns is summed linearly to extract a global form percept. Surprisingly, no such global pooling was found for Glass patterns with parallel structure. A simple neural model explains these results and agrees with recent V4 single unit physiology. As V4 provides the major input to IT, global concentric units may play an important role in analyzing complex images such as faces. In support of this possibility, deficits in the perception of concentric Glass patterns have recently been linked to prosopagnosia.

Humans↗

PubMed1997

Evolving concepts of spatial channels in vision: from independence to nonlinear interactions.

By the 1960s it was evident from neuroanatomy that there were extensive recurrent interactions, both excitatory and inhibitory, among visual cortical neurons. Nevertheless, the psychophysical discovery of 'spatial-frequency channels' gave rise to a decade in which parallel, independent channels were thought to subserve early spatial vision. Recent work, however, has clearly demonstrated that early visual channels do not perform a Fourier or wavelet decomposition of the image. Instead, they interact through a variety of nonlinear pooling mechanisms. Such nonlinear interactions perform important computations in texture perception, stereopsis, and motion and form vision.

Depth Perception↗

PubMed1997

Effects of maternal nutrition on hatchability.

The effects of dietary factors on the development and viability of avian embryos have been extensively documented. A good nutritional status of the parent birds is crucial to the transfer to the egg of an adequate, balanced supply of nutrients required for normal development of the embryo. The consequences to the embryo may be lethal if the egg contains either inadequate, excessive, or imbalanced levels of nutrients. As nutritional deficiencies or excesses occur, it is common for the effects on the embryo to also become more severe and to occur at earlier stages of development. The type of nutritional stress signs visible in the embryo often depend upon the severity of the maternal nutritional stress. Diseases, parasitic infections, toxins, poisons, or drugs may also cause nutritional or pseudonutritional problems with hatchability.

Animal Nutritional Physiological Phenomena↗

PubMed1997

Effect of strain and age of the broiler breeder female on incubation time and chick weight.

Two experiments were conducted to evaluate effects of strain [five in Experiment (Exp.) 1 and six in Exp. 2)] and age (29, 47, and 57 wk in Exp. 1 and 29, 41, and 52 wk in Exp. 2) of commercial broiler breeders on incubation time and chick weight. Highly significant differences in egg weight were found among strains in both Exp. After adjusting for effects of egg weight, significant effects of strain, age, and their interactions were found on incubation time, egg weight at transfer, and chick weight at hatch in Exp. 1, but not in Exp. 2. Mean incubation times varied among strains from 496.6 to 498.8 h in Exp. 1 and from 499.3 to 501.9 h in the second experiment. In Exp. 1, incubation time decreased from 498.6 h when breeders were 29 wk to 494.8 at 47 wk, whereas in Exp. 2, it decreased from 510.5 h at 29 wk to 495.1 h at 41 wk. This decrease also resulted in a negative correlation between egg weight and incubation time. Differences due to strain and age were found for yolk and albumen percentage and yolk: albumen ratio. Percentage yolk was 27.2 and 32.7% and percentage albumen was 60.1 and 55.9% in eggs from 29 to 52 wk breeders, respectively. Shell percentage was significantly affected by strain. Strain by age interactions were found for each response in Exp. 1 but only for set and chick weight in Exp. 2. Differences among incubators were found only for incubation time; interactions of incubation time and strain and age were also detected. Results indicate that genotype, age of the female breeder, and incubator should be considered along with their interactions to obtain optimum hatching performance.

Aging↗

PubMed1997

Effects on white Leghorn hens of constant exposure to ultraviolet light from insect traps.

Constant exposure of Hy-Line W-36 White Leghorn hens to ultraviolet light from insect traps resulted in no significant differences in egg production, fertility, hatchability of fertile eggs, or total hatchability. Also, there were no apparent effects on the eyes of the birds. Results were the same when either blacklight or blacklight blue tubes were used. The need for additional testing of light traps for nuisance fly control in commercial caged layer houses is discussed.

Animal Husbandry↗

PubMed1997

Lateral interactions in peripherally viewed texture arrays.

A horizontal array of vertically oriented Gabor elements was used to examine lateral masking in the near periphery (1.9 degrees-5.7 degrees eccentricity). Thresholds were assessed for detecting changes in the contrast, the spatial frequency, and the orientation of the central element within the array. The presence of surround elements induced marked threshold elevations that increased in strength as interelement spacing decreased and as retinal eccentricity increased. A model incorporating spatial summation by complex cells and reciprocal inhibition between simple and complex cells is shown to provide a quantitative fit to the data. This model suggests that complex cells analyze highly redundant textures, whereas simple cells function predominantly in the presence of isolated contours.

Contrast Sensitivity↗

PubMed1996

High-affinity inhibitors of dihydrofolate reductase: antimicrobial and anticancer activities of 7,8-dialkyl-1,3-diaminopyrrolo[3,2-f]quinazolines with small molecular size.

A series of 7,8-dialkylpyrrolo[3,2-f]quinazolines were prepared as inhibitors of dihydrofolate reductase (DHFR). On the basis of an apparent inverse relationship between compound size and antifungal activity, the compounds were designed to be relatively small and compact. Inhibitor design was aided by GRID analysis of the three-dimensional structure of Candida albicans DHFR, which suggested that relatively small, branched alkyl groups at the 7- and 8-positions of the pyrroloquinazoline ring system would provide optimal interactions with a hydrophobic region of the protein. The compounds were potent inhibitors of fungal and human DHFR, with K(i) values as low as 7.1 and 0.1 pM, respectively, and were highly active against C. albicans and an array of tumor cell lines. In contrast to known lipophilic inhibitors of DHFR such as trimetrexate and piritrexim, members of this series of pyrroloquinazolines were not susceptible to P-glycoprotein-mediated multidrug resistance and also showed significant distribution into lung and brain tissue. The compounds were active in lung and brain tumor models and displayed in vivo activity against Pneumocystis carinii and C. albicans.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1996

Direction repulsion between components in motion transparency.

We measured the perceived direction of one motion component as a function of the contrast and speed of a second component for three pattern classes: plaids with two different spatial frequency components, multi-aperture patterns, and contrast-modulated (CM) patterns. The components were moving at +/- 63.4 or +/- 71.6 deg to the vertical, angles where motion transparency always occurred under our conditions. For multi-aperture and CM patterns on a single spatial scale, the components were perceived to deviate from the component motion directions by up to 20 deg at high contrasts or high speeds of the second component. However, for plaids with components on different spatial scales, the test components were perceived moving in the component directions regardless of the contrast or the speed of the second component. Our data show that this direction repulsion between components occurs within a single spatial scale but not between widely separated spatial scales. This implies that two different mechanisms are involved in motion transparency.

Contrast Sensitivity↗

PubMed1996

Fourier and non-Fourier pattern discrimination compared.

Various studies indicate the existence of non-Fourier visual mechanisms that can extract non-luminance cues (e.g., contrast modulation) as well as a Fourier mechanism that deals with luminance variation only. We compared the pattern discrimination performance of the non-Fourier mechanism with that of the Fourier mechanism by using orientation discrimination and spatial-frequency discrimination tasks. The Fourier patterns used were D6s, the sixth spatial derivative of a Gaussian function multiplied by another Gaussian function in the orthogonal dimension. The corresponding non-Fourier patterns were D6 contrast-modulated cosine gratings. Our results showed a similar trend for the non-Fourier and the Fourier performance at various peak spatial-frequencies or orientations of D6. However, the discrimination threshold of the non-Fourier mechanism was about two-fold higher than that of the Fourier mechanism. The oblique effect was also stronger for non-Fourier patterns. In addition, worse performance for non-Fourier patterns at short stimulus durations (around 33.3 msec) was consistent with the prediction of the two-stage non-Fourier model, which requires more time for the additional rectification and filtering operations.

Cues↗