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Biomedical subjects

H R Wilson

Publications and source records attributed to H R Wilson.

At least 55 records · Page 3Linked to original sources
PubMed1996

Stereopsis from interocular spatial frequency differences is not robust.

Based on data obtained using one-dimensional noise patterns, Tyler & Sutter (1979). (Vision Research, 19, 859-865) concluded that stereoscopic tilt can result from an interocular spatial frequency difference in the absence of consistent horizontal disparity. We tested stereopsis using two-dimensional random-dot patterns that were bandpass filtered to contain 1.0 octave bands of spatial frequency with means that differed between the two eyes. With vertical, one-dimensional stimuli we replicated the results of Tyler and Sutter. However, stereoscopic tilt was not perceived based on spatial frequency differences alone when the monocular images contained as little as a +/- 14 deg range of orientation variation. In addition, model simulations demonstrate that the modest steroscopic performance produced by interocular spatial frequency differences in one-dimensional noise patterns are predicted by random disparity correlations at the pattern edges. The observations lead to the conclusion that stereopsis from frequency differences in the absence of pointwise disparity correlations does not reflect a special processing capability of human vision but is an artifact associated with one-dimensional stimuli. As such, it plays no role in steroscopic analysis of the natural environment.

Convergence, Ocular↗
PubMed1996

Low temperature effects on embryonic development and hatch time.

A series of experiments was conducted to estimate phenotypic correlations between incubation characteristics, and to evaluate the effects of cold stress and genotype during incubation on chick weight, egg weight loss, hatching time, and embryonic mortality. Eggs were cooled at 18 or 24 C, for 12, 24, 36, 48, or 72 h beginning on Day 8, 12, 14, 16, or 18 of incubation. Other eggs were cooled intermittently for 6 h every 48 h or 12 h every 96 h. A control group in each experiment was not cold stressed. Results indicated a low and negative correlation between hatching time and chick weight, and a low and positive correlation between hatching time and weight loss from transfer to hatching when variability due to egg weight was removed. Chick weights at hatching were lower in chicks from cooled eggs than those of chicks from eggs incubated under normal temperature. The chicks from cooled eggs were more susceptible to dehydration during holding in the hatcher. Incubation times were delayed approximately as long as the times of embryonic cooling. Embryonic mortality was significantly increased under continuous (single period) cold stress, but not under intermittent cooling (6 h every 48 h). Significant genotype by environment interactions were found in the response of embryos of various strains to cold stress. Exposure for 36 h or longer had detrimental effects on chick weight and embryo viability, but these effects were modified by interactions among the factors involved. The results indicated that embryos from cooled eggs lose more weight during incubation and that the neonatal chicks are more susceptible to dehydration during holding time, and have a longer incubation period, and a greater embryonic mortality.

Animals↗
PubMed1995

Enhanced antitumor activity for the thymidylate synthase inhibitor 1843U89 through decreased host toxicity with oral folic acid.

The purpose of this investigation was to determine whether antitumor selectivity of the third generation thymidylate synthase inhibitor 1843U89 could be enhanced by a combination of the drug with folic acid. The effects of folic acid on toxicity of 1843U89 to the dog and mouse and on antitumor efficacy of 1843U89 in the mouse were studied. These data were compared to the effect of folic acid on the in vitro cell culture antitumor activity of 1843U89. The sensitivity of eight cancer cell lines (three ovarian, one colon, one ileocecal, one epidermoid, one osteosarcoma, and one breast line) to 1843U89 was tested in vitro in the presence and absence of folic acid. Folic acid concentrations greater than 100 microM were required to decrease 1843U89 activity in seven of the cell lines. Only the activity in HCT-8, the ileocecal line, was reserved at folic acid concentrations below 100 microM. Oral folic acid given 30 min prior to an i.v. dose of 1843U89 increased the maximally tolerated dose and the lethal dose of 1843U89, both in dogs and in thymidine-depleted mice. In mice, oral folic acid produced little or no effect upon the antitumor efficacy of 1843U89 in two of three tumor cell lines in vivo. HCT-8, the line that was sensitive to folate reversal in vitro, was also sensitive in vivo. The results show that an oral dose of folic acid 30 min prior to i.v. 1843U89 can block mouse and dog intestinal toxicity without decreasing efficacy of 1843U89 in two of three human tumor lines in the nude mouse. Thus, the data reported here indicate that the antitumor selectivity of 1843U89 may be enhanced through a combination of 1843U89 with oral folic acid.

Animals↗
PubMed1995

Detection of Salmonella enteritidis in eggs and chicken with enzyme-linked immunosorbent assay.

An ELISA previously developed for the rapid detection of Salmonella enteritidis (SE) in environmental samples was modified and applied to food samples. A sandwich ELISA was designed that employs affinity-purified rabbit polyclonal antibodies for the capture stage and highly specific monoclonal antibodies for the detection stage. Thirty-nine species of bacteria other than SE, including 32 Salmonella species, were included in cross-reactivity testing with ELISA. Results showed no reactivity with any species tested besides SE. Salmonella enteritidis was added to homogenized food samples (chicken skin, meat, and eggs) to test ELISA sensitivity. The lower limit for ELISA detection of SE was 10(4) cells/mL for pure cultures and in 10% meat (wt/vol), 10(5) cells/mL in 10% skin (wt/vol), and 10(7) cells/mL in 10% eggs (wt/vol). Salmonella enteritidis detection with ELISA was confirmed with the Food and Drug Administration (FDA) Bacteriological Analytical Manual (BAM) method. Results were obtained within 24 h for ELISA method compared to 96 h for the BAM procedure. Results show that sensitivity of ELISA can vary with the type of food tested for detection of SE.

Animals↗
PubMed1994

The role of second-order motion signals in coherence and transparency.

Second-order or non-Fourier motion was originally demonstrated in the laboratory by the use of contrast-modulated stimuli of various types. As these stimuli seem somewhat artificial, it might be questioned whether they play a significant role in the processing of natural motion. An affirmative answer is provided by the key observation that the non-Fourier motion apparatus analyses the motion of texture boundaries in two-dimensional patterns. Given this, a wide range of motion phenomena can be explained by a neural model that combines both Fourier and non-Fourier motion signals in a neural network that computes the vector sum direction. The existence of a vector sum computation is supported by two observations: patterns composed of two cosine gratings ('plaids') appear to move in the vector sum direction during brief presentations and plaids composed solely of non-Fourier components always move in the vector sum direction. The neural model predicts these results and also agrees with much of area MT physiology. Within this context, many instances of motion transparency result when the angle between Fourier motion directions is large (> 90 degrees) and the non-Fourier motion signal is minimal or absent.

Animals↗
PubMed1994

Chaos in percepts?

Multistability in perceptual tasks has suggested that the mechanisms underlying our percepts might be modeled as nonlinear, deterministic systems that exhibit chaotic behavior. We present evidence supporting this view, obtaining an estimate of 3.5 for the dimensionality of such a system. A surprising result is that this estimate applies for a rather diverse range of perceptual tasks.

Eye Movements↗
PubMed1994

A two-process analysis of pattern masking.

By comparing the masking effects of cosine gratings and uniform fields on spatially narrow-band test patterns, we obtain evidence that pattern masking is mediated by two stages of visual processing: an early process of point-wise luminance adaptation and a late process of spatial-frequency and orientation-selective filtering. The early (presumably receptoral) component of masking is not affected by the polarity (incremental or decremental) or spatial frequency of test patterns, and may either increase or decrease pattern sensitivity based on local light level. The late masking process is orientation and spatial-frequency dependent, implying a cortical origin. For this cortical process, we find competitive interaction between parallel ON and OFF visual mechanisms: on a bright bar of a mask, threshold shift is greater for decremental than incremental tests, and the opposite is true on a dark bar of the mask. We suggest that ON-OFF interactions in pattern masking serve to normalize the gain of ON and OFF mechanisms simultaneously in order to preserve the relative contrast in the image.

Adaptation, Ocular↗
PubMed1994

Disparity averaging across spatial scales.

To determine whether disparity averaging occurs across spatial scales, a series of depth matching experiments was performed using stimuli composed of the sum of two cosine gratings of different spatial frequency and retinal disparity. Although there were large individual differences in the data, averaging was observed when the relative frequency of the cosines were separated by 3.5 octaves or less and when the relative orientation was less than 30 deg. Beyond these limits, averaging never occurred and the stimulus components were perceived transparently in depth. Furthermore, as the relative contrast of the gratings was varied, perceived depth varied smoothly in the direction of the higher contrast component. This dependence on contrast can be accounted for by a multi-channel model in which separate disparity estimates are generated on each spatial scale and then combined to determine the overall perceived depth of the stimulus.

Contrast Sensitivity↗
PubMed1994

Perceived motion in the vector sum direction.

The visual system is known to extract both Fourier (e.g. contour) and non-Fourier (e.g. texture boundary) motion signals from moving patterns. Recent evidence suggests that both Fourier and non-Fourier motion signals are involved in the analysis of rigid two-dimensional image motion. To determine how these signals are combined, we have measured perceived directions for plaids composed of either two non-Fourier components or one Fourier plus one non-Fourier component. In all cases the visual system combines these components to produce perceived motion in the vector sum direction, even when this deviates by as much as 53 deg from the intersection of constraints direction. This result was predicted by a model for pattern motion that combines Fourier and non-Fourier component motion signals using a vector sum operation.

Humans↗
PubMed1994

Effect of removing trace minerals from the diet of hens laying eggs with heavy or light shell weight.

Hy-Line W36 hens, 48-wk-old, were used to study the effect of removing either supplemental Fe, Cu, Zn, Mn, or all of them from the diet of hens laying eggs with heavy shell weight (HSW) or light shell weight (LSW). The experiment was conducted for 10 wk. Egg production (EP), egg mass (EM), feed consumption (FC), feed conversion efficiency (FE, grams feed:gram egg), egg weight (EW), shell weight (SW), percentage shell (PS), specific gravity (SG), percentage fertility, percentage hatchability of fertile eggs (HF), and percentage hatchability of total eggs set (HT) were measured. Removing trace minerals from the diet had no significant effect on EP, EM, FC, FE, EW, PS, SG, HF, or HT with either HSW or LSW hens. However, removing Mn from the diet significantly reduced SW with HSW hens. Diet by shell classes (HSW and LSW) interaction was not significant for any variable measured. The HSW hens had significantly higher EW, SW, PS, SG, and EM and lower EP, HF, and HT. However, there were no significant differences on FC or percentage fertility between the two groups of hens.

Animal Feed↗
PubMed1993

Nonlinear processes in visual pattern discrimination.

Visual performance on many simple pattern discrimination tasks can be accurately predicted by quasi-linear models composed of oriented linear filters followed by simple contrast nonlinearities. However, many complex discrimination tasks require highly nonlinear processes for their explanation. Evidence is provided for two nonlinear processes in pattern discrimination: (i) one process involves a sequence of filtering, rectification, and subsequent filtering to extract texture boundaries; (ii) the second process results from contrast gain-control processes. It is suggested that quasi-linear processes and nonlinear texture-boundary processes, each with an appropriate contrast gain control, may operate in parallel to provide the basis for all higher-level visual analyses.

Discrimination, Psychological↗
PubMed1993

Baquiloprim, a new antifolate antibacterial: in vitro activity and pharmacokinetic properties in cattle.

During examination of the half-lives in cattle of a series of 5-substituted diaminobenzyl-pyrimidines, it was found that replacement of the phenyl ring of trimethoprim (TMP) by bicyclic structures, particularly a quinolyl group, led to increases in half-life. The presence of a dimethylamino group on the quinolyl ring of the compound baquiloprim (BQP) conferred a half-life of about 10 hours. In contrast to TMP (half-life about one hour), BQP was well absorbed from the gastrointestinal tract in all ages of cattle, plasma concentrations reaching a plateau on the day after dosing followed by a slow decline. BQP showed the same high broad spectrum antibacterial activity as TMP, with marked synergy with sulphonamides. Its differential binding of the dihydrofolate reductases of Escherichia coli and rat liver predicted a margin of safety similar to that of TMP. The results of efficacy studies in mice in comparison with TMP showed that the longer half-life of BQP was associated with greater efficacy, and therapeutic properties superior to those of TMP in cattle were therefore predicted for BQP.

Administration, Oral↗
PubMed1993

Illusory motion of texture boundaries.

We have measured the perceived direction of motion of rigid patterns containing texture boundaries at various orientations. Texture boundaries are defined here as the locus of 180 degrees phase shifts in a square-wave grating. When these patterns are moved parallel to the orientation of the grating, the perceived direction of motion in foveal viewing differs by as much as 45 degrees from the true direction of rigid motion as defined by the intersection of constraints (IOC) construction. At 8.0 degrees in the periphery the perceived direction was closer to the true direction of rigid motion. Thus, the peripheral visual system analyzes motion of these stimuli more accurately than does the fovea. These results are consistent with a current model of motion analysis that combines responses of Fourier and non-Fourier motion pathways using a vector sum operation.

Humans↗
PubMed1993

Peripheral temporal frequency channels code frequency and speed inaccurately but allow accurate discrimination.

Perceived temporal frequency is vastly underestimated in the peripheral visual field, as all temporal frequencies above 10 Hz are perceived as flickering at 10 Hz even after scaling for acuity. Varying the contrast and spatial frequency of the peripheral pattern four-fold have negligible effects on the perceived flicker rate. Speed and auditory matching experiments also support this finding. Despite the saturation of perceived temporal frequency, frequency discrimination beyond 10 Hz was as accurate as in the fovea. By using a temporal masking paradigm, we obtained threshold elevation data that could be accounted for by three overlapping, broadly tuned temporal channels peaking at 5.5, 12 and 22 Hz. Based on these temporal frequency channels, we proposed that the visual system uses a line-element scheme for mediating temporal frequency discrimination, but adopts a weighted-average method for determining perceived temporal frequency. In the peripheral visual field, the weight assigned to the highest temporal channel is much larger than those assigned to the lower frequency channels.

Contrast Sensitivity↗
PubMed1993

Dependence of plaid motion coherence on component grating directions.

We measured motion coherence for plaids composed of two different spatial frequency (SF) cosine grating components moving at various relative angles. The component SFs were in a ratio of 6:1, and several component motion directions were chosen to produce small to large angular differences. For angles of less than +/- 45 deg all subjects perceived rigid coherent motion, while for angles of +/- 45 deg or greater the components were perceived to slide transparently. The results were not altered when we changed the ratio of component contrasts or speeds over a factor of 2 and varied the SF ratio up to 9:1. These results show that transparency or coherence of moving plaids in different spatial scales depends on the relative component motion directions and is relatively independent of contrast, speed, and SF difference between the components. This angular dependence also explains recent data previously thought to be based on a visual computation of multiplicative transparency. A quantitative model in which the resultant motion on each scale provides a facilitative biasing signal to units tuned to similar directions (within +/- 30 deg) on other scales explains the experimental results.

Contrast Sensitivity↗
PubMed1993

Spatial frequency adaptation and contrast gain control.

Spatial frequency adaptation is shown to cause a decrease in the exponent of the power law describing contrast increment thresholds. Next we show that spatial frequency adaptation produces almost no threshold elevation at a 30 msec test duration but a normal threshold elevation at 500 msec. Control experiments rule out an explanation in terms of sustained and transient mechanisms. Together these results show that spatial frequency adaptation cannot result from neuronal fatigue. The data can be explained by a contrast gain control network in which unit responses provide feedback signals that divide input contrast. Adaptation is hypothesized to produce an increase in the strength of this divisive feedback via temporary synaptic modification. A simple network model fits the data and also predicts both the magnitude and temporal dependence of the tilt aftereffect.

Adaptation, Ocular↗
PubMed1993

Nature of coarse-to-fine constraints on binocular fusion.

Several stereo-matching algorithms posit that processing on coarse (low spatial frequency) scales constrains processing on fine (high spatial frequency) scales by shifting the disparity range over which fine mechanisms operate. If this were the case, stereo increment thresholds for high frequencies in the presence of low frequencies would be constant, regardless of base disparity. In contrast, we find that in the presence of low spatial frequencies, increment thresholds for high spatial frequencies increase with base disparity, as do increment thresholds for high frequencies presented alone. As a further test of whether information on coarse scales enhances processing on fine scales, diplopia thresholds were measured. In the presence of low frequencies, fusion ranges for high frequencies were severely reduced. These constraints were found to exhibit a power-law dependence on low frequency contrast and to operate over relatively localized areas of the visual field.

Algorithms↗